There are a number of factors involved in deciding if your engine needs a rebuild. This chapter covers the symptoms associated with a worn-out engine and the simple diagnostics involved in making an informed decision as to whether your engine needs to be rebuilt.
While engines with 100,000 or more miles will certainly benefit from a rebuild, many high-mileage engines that have had regular oil changes while being kept in good tune may be less in need of rebuilding than a low-mileage engine that hasn’t been properly maintained. Mileage alone isn’t a determining factor in the need for a rebuild; maintenance history is just as important.
This Tech Tip is From the Full Book, HOW TO REBUILD BIG-BLOCK FORD ENGINES. For a comprehensive guide on this entire subject you can visit this link:
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Excessive Oil Consumption
As a general rule, if your engine uses a quart of oil for every 1,000 miles or less, it is consuming an excessive amount. Take note that I’m referring to an engine that “uses” oil, as opposed to ”burning” it. You should devote the time to determine the cause for excessive oil consumption before assuming that your engine is burning it. Check carefully for external oil leaks, as a leaking rear main seal, intake manifold gasket, or valve cover gasket that leaves a few drops of oil on your driveway can contribute greatly to overall oil consumption since the leak will be constant when the engine is running.
Internal oil loss may be the result of worn or cracked valvestem seals or guides, problems that are correctible without completely rebuilding the engine. All engines have clearance between the stems of the valves and their guides for lubrication and, over time, the clearance increases, allowing oil to pass down the valvestem into the combustion chamber. Likewise, the rubber seals on the valvestems may become brittle and crack, allowing an excessive amount of oil to flow past the valveguides. Valvestem seals are replaceable without removing the cylinder heads from the engine and may greatly reduce internal oil consumption. A common indicator that an engine is suffering from a problem with valveseals is a puff of blue smoke from the exhaust upon engine startup that dissipates within a short time.
Some years back I encountered a 351W Ford engine that was using oil at a prodigious rate and smoking heavily from its exhaust, giving the appearance of a terminal engine problem yet not making any unusual noises. The culprit in this case turned out to be a cracked fuel-pump diaphragm that was allowing motor oil into the fuel.
Drop in Oil Pressure
Sadly, at some point, someone in the automotive industry got the foolish idea that real oil pressure gauges were unnecessary, thus beginning the era of the “idiot light.” I have heard more than once: “How can I be three quarts low on oil—the light never came on?” As it turns out, the average person went blissfully through life secure in the thought that if there were any problem relating to engine lubrication, the magic light would come to the rescue, while the rest of us installed aftermarket gauges in our cars at a furious pace. And since I am covering vehicles equipped with Ford’s Lima series engine, I can’t think of one that rolled off the assembly line sporting an oil pressure gauge as standard equipment. This tells me that in most cases a drop in engine oil pressure will go unnoticed until something catastrophic occurs and the oil light glows brightly from the dash.
Note: As you read on, you’ll learn the engine we are rebuilding for this book came out of a limited-production 1970 Torino. These particular cars received modifications for use as pace cars in the NASCAR racing series, one of which was the installation of a mechanical oil pressure gauge.
Usually the first sign of a loss in oil pressure in cars equipped with the original equipment (OE) light will be the ominous tapping of hydraulic valve lifters as they begin to collapse. Even if you don’t want to invest in a full-time, dedicated mechanical or electrical oil-pressure gauge, there is an alternative available for diagnostic purposes. For many years I have kept an inexpensive aftermarket mechanical oil pressure gauge and an assortment of adapter fittings in my toolbox for just such occasions. If I encounter an idiot-light-equipped car I merely remove the oil pressure switch from the engine (on Lima series engines this switch is located on top of the block just behind the intake manifold) and use my adapters to tap in the mechanical gauge and take oil pressure readings.
Oil pressure readings should be taken with the engine cold and then again at normal operating temperature, at idle and off-idle conditions. Pressure readings of over 20 psi at idle with the engine at normal operating temperature are usually considered adequate and you should note an immediate increase in the pressure reading without fluctuation as engine RPM is increased.
If your engine has low oil pressure, the causes may run the gamut from dirty oil with decreased viscosity to a clogged filter or passage in the engine’s lubrication system, to something more critical such as excessive main or rod bearing wear. Even though the rotors in an oil pump can be subject to wear or failure, I have found that the oil pump is very seldom at fault when an engine has lost oil pressure. Of course, I still make it a habit to swap out the oil pump for a high-volume pump anytime I rebuild an engine.
Decrease in Performance
Just because fuel economy and engine performance have decreased does not necessarily mean your engine has worn out and is in need of a rebuild. There are various causes for loss of power and fuel economy, some of which do not directly relate to the condition of the engine. A clogged or restricted exhaust system could be the cause. Ascertain if the exhaust manifold heat riser is stuck in the closed position, there is a collapsed or kinked pipe in the system, or the catalytic converter is faulty. Something as simple as a dirty fuel filter, carburetor, or fuel injector will restrict flow to the engine resulting in changes to the air/fuel mixture, which will cause a loss of power and fuel mileage. Slippage due to a wornout clutch or faulty automatic transmission are also potential causes for a decrease in power and mileage.
The engine’s ignition system is often the culprit behind decreased performance and fuel mileage. If your engine is equipped with a points-style ignition system, breaker point gap, a faulty condenser, faulty spark plug wires, or a dirty or cracked rotor or distributor cap can cause symptoms that would indicate a severe problem when one does not exist. An electronicignition- equipped engine can suffer similar maladies with the exception of problems with points or condenser.
A stretched timing chain and worn timing gears will cause a change in the valve timing of the engine and affect its performance even to the point that the engine will no longer run. Burned valves or a build up of carbon deposits in the combustion chamber, weak valvesprings, or excessive wear to camshaft lobes will also adversely affect engine performance. Engines equipped with solid valve lifters will react to changes of thousandths of an inch in adjustment by losing power and in extreme cases may suffer damage to related valvetrain parts. A blown head gasket is a common engine problem that will manifest itself through an immediate loss of power. While this is a serious engine problem, when detected and repaired in a timely fashion it should not be a direct indication that the engine needs rebuilding.
Your engine is full of moving parts and all of them have the potential to make noises that signal a problem, but due to the harmonics involved, engine noises can be very difficult to locate. Your first determination should be if the noise is at engine speed or half engine speed. As a general rule, noises that are at half engine speed usually emanate from the valvetrain (with the exception of fuel pump noises or a condition called piston slap). Noises at engine speed normally indicate a problem in the bottom end or crankshaft area of the engine.
A timing light is an easy way to determine the speed of the noise. Hook up your timing light to a spark plug wire and start the engine. If you hear the noise once for each time the light flashes, the noise is at half engine speed, and if the noise occurs twice for each flash of the light, it indicates that the noise is at engine speed. Once the speed of the noise has been determined you can get about locating its source.
The cylinder block can transmit internal noises harmonically, so it may be difficult to locate the source without a listening device. A mechanic’s stethoscope is often used to pinpoint engine noises, but if you do not have one at your disposal, a length of plastic tubing or a long screwdriver will get the job done quite well. I remember as a teenager learning how to use a screwdriver to locate the source of engine noises. A mechanic had diagnosed a noise in the engine of a friend’s car as emanating from a faulty wrist pin. Lucky for us, a local hot-rodder schooled me on the technique of placing the end of a long screwdriver against the engine while touching the handle end to my ear to pinpoint internal noises. In that instance, the fuel pump eccentric was the source of the noise and not a wrist pin as initially diagnosed. A length of plastic hose also transmits sound well. The technique here is to move your probe from place to place until the source of the sound is located.
If the noise is from a particular cylinder, it could be caused by the piston and rod assembly. You can tell by placing the tip of your probe next to a spark plug. If the noise is coming from the top of the engine, it is very likely valvetrain related and may be isolated and pinpointed by removing a valve cover and visually inspecting the valvetrain both with the engine running and shut off. With the engine off, check for signs of a bent pushrod or broken valvespring. With the engine running, ascertain that the pushrods are rotating and check for a rocker arm that may not be opening its valve as far as the others. Should either be the case, you may have a wiped (worn) camshaft lobe(s). In engines equipped with hydraulic valve lifters, a collapsed lifter(s) will result in a persistent tapping sound at half engine speed.
Piston slap is caused by excessive piston-to-wall clearance and manifests itself as a hollow noise that is most prominent when the engine is cold and under a load. The causes of piston slap include wear due to poor lubrication or high mileage, or in extreme cases a collapsed or broken piston skirt. If the sound goes away soon after the engine warms up, the condition is not that severe. Note that engines equipped with forgedaluminum pistons, as opposed to the more common cast type, are more prone to slap until the engine has warmed up due to the increased piston- to-wall clearance required when using forged pistons. An easy method of determining if the noise you are hearing is piston slap is to retard the ignition timing a few degrees while the engine is running. Remember that Ford engines have a counterclockwise rotation, so to retard the timing, slowly turn the distributor in a counterclockwise direction. By retarding the timing, you are reducing the load on the pistons caused by combustion. If piston slap is the culprit, the noise should diminish.
Wrist pin noise will be most prominent at idle or low speed and will manifest itself with multiple knocking sounds that are quite distinct. If your engine has developed wrist pin noise, the bushing at the small end of the connecting rod may be failing or the pin may have come loose from the piston.
Engine bearings with excess clearance will cause a knocking sound that will be most pronounced when the engine is first started, either hot or cold, before a sufficient level of oil pressure has been reached. Bearing noises will also often manifest themselves under hard acceleration, but should not be confused with detonation, a condition that produces more of a rattling sound. Main bearings will knock at half engine speed with the somewhat muffled noise coming from deep within the block.
Connecting rod bearings will also knock when clearances are excessive or if there is insufficient oil pressure. A rod-bearing knock is most prominent upon deceleration after the engine has been run at a constant speed for a period of time. Piston rings that are broken, or have lost the tension required to hold them against the cylinder wall, will create a chattering sound that is most noticeable under acceleration. The easiest way to diagnose a piston ring problem is to conduct a compression check on the engine.
Diagnostic Tools and Steps
Now that I have covered some of the problems that your engine may be experiencing, it’s time to discuss how to diagnose the overall condition of the engine and the tools used to pinpoint potential problems. Using this information, you should be able to confidently assess the need for a rebuild.
Here is a good tip: start simple and do not overlook the obvious. There is no need to go high-tech immediately, as some of your engine’s simplest parts will reveal information that will speak volumes about what has been taking place under the hood.
Spark plugs are a window to what is occurring in each of the engine’s cylinders, so it’s smart to keep them in order as removed to assist in pinpointing potential problem areas. First check that the proper heat range spark plugs for your application are in the engine. Something as simple as incorrect spark plugs can adversely affect performance and fuel economy. Once you have verified that your engine is fitted with the correct spark plugs, conduct a visual inspection of each spark plug. There are several things to look for:
- A wet, black insulator indicates excessive amounts of oil in the combustion chamber or a plug that is not firing and has been fouled by fuel. You can ascertain the latter by holding the plug to your nose and conducting a sniff test to determine the presence of raw gasoline. • Bubbling or blistering of the insulator is an indication of excessive heat in the combustion chamber and is usually attributed to an overly lean fuel mixture.
- If the plugs show a dry black or dark gray coating, the fuel mixture may be too rich or there could be a problem with the ignition.
- A serious problem such as a blown head gasket may also exist if two spark plugs from adjacent cylinders show a white foamy deposit while the other plugs are clean.
Check Ignition Timing
You will notice references to “ignition timing,” “setting your engine on top dead center (TDC),” and “rotating 90 degrees in a clockwise direction,” more than once in this book so I think this is a good time to cover how to make these readings on an engine.
The Lima series engine’s timing increments are machined into the face of the vibration damper. You can read from the passenger side of the engine when they align with a pointer affixed to the timing cover. First get the damper clean so that the marked increments will be easier to read. I use a good cleaning solvent to remove years of dirt and grime from the outside circumference of the damper (the area that is marked with the timing increments).
You may find that it is easiest to access the damper from under the car and that it will be necessary to turn the engine over in order to clean the entire surface of the damper. While you can disable the ignition and bump the starter over using the ignition switch or a remote starter switch, I often choose to turn the engine manually using a socket and breaker bar on the crankshaft bolt since this gives me more control of where the damper stops in its rotation. Note that if you choose to turn the engine manually it is easier to accomplish by first removing the spark plugs so that you are not fighting compression.
With the damper cleaned up, you can make a number of marks with two different paint colors so that they will be easier to distinguish later. You will find TDC marked on the damper as either TDC or the numeral 0 (zero).
In the Appendix, refer to TDC chart and the timing increments marked as 3-6-9-12. I begin with the TDC line and highlight the line using a red marker. Automotive touch-up paint also shows up well and does not tend to wear off easily.
Next, I use a straightedge to divide the damper into sections 90 degrees apart and place a short paint mark at each point. This makes it easy to move the engine through its rotation 90 degrees at a time. The final step is to mark the timing increment that corresponds with the manufacturer’s instructions for your particular year and model engine. For this step, I prefer to use a white marker or paint so that it will contrast with the TDC and 90-degree markings and cause no confusion. With the vibration damper now clearly marked, you are ready to check ignition timing, verify TDC, and make accurate 90-degree rotations of the crankshaft.
If your engine is equipped with a points-style ignition system, which early Lima series engines are, you must first check breaker point gap/dwell angle prior to checking the timing. If the gap on the points is too close, a condition most often caused by normal wear on the rubbing block that contacts the distributor cam, ignition timing will be retarded. Use a dwell meter or simple feeler gauge to set the point gap/dwell angle.
Of course, if your engine is equipped with an electronic ignition system, you can disregard the previous step. With the points set, you are ready to check timing. First hook the timing light to the number-1 spark plug lead. The number-1 cylinder in a Ford engine is the first one on the passenger side farthest from the firewall and closest to the grill.
Disconnect and plug the vacuum advance hose from the distributor, loosen the hold-down clamp at the base of the distributor, and then make a thorough check to ensure that nothing is too near any moving parts before starting the engine. With the engine running at normal idle speed, trigger the timing light to initiate the flash and aim it at the timing pointer. The strobe effect of the light will cause a stop-action view of the vibration damper as it rotates, allowing you to see the relationship between the timing increments on the damper and the pointer. If the correct incremental mark is not in line with the pointer, rotate the distributor slightly until they are aligned. Carefully tighten the distributor hold-down clamp and recheck the timing to make sure it has not moved. Now, reattach the vacuum advance hose to the distributor, activate the timing light again, and you should see the timing advance slightly on the damper. If there is no advance in the timing, there may be a problem with the distributor’s vacuum advance or low engine vacuum that can result in a loss of power and fuel economy.
Later-model vehicles equipped with Lima series engines and automatic transmissions may call for the ignition timing to be set with the vehicle idling while in gear for emissions reasons. If this is the case, do not attempt to do this alone by depending on the parking brake to hold the vehicle in place. Enlist the assistance of a friend to hold the car in place with its brakes, but do not forget to activate the parking brake and chock the wheels as well, for a little added insurance. Some 30 years ago, I was setting the timing on a car that was in gear and held in place only by the parking brake. I advanced the distributor too far, causing the car to lurch forward and nearly pin me against my workbench.
Conduct a Vacuum Test
A simple vacuum gauge is a valuable diagnostic tool and can reveal a number of engine problems from minor to major. When running, an engine creates vacuum in the intake manifold. A vacuum test will reveal any time one or more cylinders are not operating at peak power.
To check engine vacuum, connect the vacuum gauge to a port on the intake manifold and start the engine. A normal vacuum reading at idle should be between 16 and 18 degrees of mercury. Keep in mind that vacuum readings will be lower at high altitude or in cases where a camshaft with a more radical profile (more overlap) than stock has been installed in the engine.
A low initial vacuum reading may be indicative of nothing more serious than incorrect ignition timing, so get out that timing light and check and/or adjust timing as needed. If you are getting slow fluctuations on the vacuum gauge, it may indicate a fuel mixture that is too rich. Try increasing idle speed or turning the idle mixture screws on the carburetor to correct the problem.
If the gauge shows a consistently low vacuum reading, the engine could have a blown head gasket or air leak that will require further diagnostic work.
Revving the engine with a vacuum gauge hooked up should result in a drop in vacuum with a steady reading on the gauge. If the needle shows a fluctuation, it is indicative of weak valvesprings.
With the engine at normal operating temperature you can perform cranking vacuum tests. First disable the ignition so that the engine will not start, and then crank the engine over with the assistance of a helper or a remote starter. The reading on your vacuum gauge should remain steady. A fluctuation of the needle indicates a problem in one or more of the cylinders. In this case, the problem may be as simple as valve adjustment in an engine equipped with mechanical lifters or a collapsed lifter in those with hydraulics. It could also indicate a wiped camshaft lobe, leaking valves, worn piston rings, a damaged piston, or blown head gasket. The following simple test will assist you in locating the problem cylinder(s).
You can pinpoint a problem cylinder(s) by checking the power balance between them. Most engine analyzers have the capability of conducting a power balance test, but few of us own this expensive piece of equipment. Luckily there is an alternative means for analyzing power balance. A pad, pencil or pen, and a tachometer (a dwell tachometer will suffice) are the only tools needed for this test.
First start the engine and set the RPM at 1,000, and then disable the cylinders one at a time by removing the spark plug wire. Be careful not to contact the live ends of the plug wires as they will continue to carry current even when disconnected. Using a pair of insulated-handle pliers is not a bad idea here. Note the RPM drop and then reconnect the plug wire. Repeat this process until you have disabled each cylinder and noted the RPM on each. The theory here is that the greater the RPM drop when the cylinder is not firing, the more that particular cylinder is contributing to the engine’s power. On the other hand, the lower the drop in RPM, the less that cylinder is contributing to engine power. Thus, any cylinder that is down on power will reveal itself by a smaller decrease in RPM.
A word of caution here: This method should only be used to disable cylinders on engines that have conventional points-type ignitions, as disconnecting a plug wire in an electronic-ignition-equipped vehicle may cause a power surge that can damage the ignition. There are commercially available test kits that allow you to disable cylinders in an engine equipped with an electronic ignition by shorting out the plug wire without risking a power surge.
A compression test is the simplest and least-expensive means of determining how well a cylinder is sealing. Be certain the engine has reached its normal operating temperature prior to checking compression. For this test you will need paper, pen or pencil, and a compression gauge.
Be aware that when you are conducting a compression check, a combustible mixture of air and fuel is blowing out of the cylinders under pressure and any spark or flame could result in an explosion. Make sure the area is well vented and free of any ignition sources. Disable the ignition, remove the spark plugs and block the throttle in the open position, then install the compression gauge in the cylinder to be tested. Using a remote starter, or an assistant, crank the engine over a minimum of three full revolutions and note the highest reading on the gauge. Repeat this process until all the cylinders are checked and the readings recorded.
The maximum reading in this case is not as important as the percentage of difference between the readings for each cylinder. All cylinders should read above 100 psi, with 180 psi being the norm for a Lima series engine. Each reading should be within 75 percent of the highest, while 90 percent or better is optimum. If you find two adjacent cylinders that read considerably lower than the rest, chances are that a head gasket has blown between the two cylinders. If you experience a single cylinder with a low compression reading, an easy way to determine if the cause is piston-ring or valve related is to squirt approximately a teaspoon of oil into the cylinder via the spark plug hole and repeat the test. If the pressure comes up, the piston rings are not seating. But if there is no change, it is likely that a valve is not seating properly or you have a blown head gasket.
Perform Leak-Down Test
A leak-down test is a more sophisticated means of checking how a cylinder is sealing, using external pressure to test the rate at which a cylinder loses pressure. Since a leak-down tester is a more sophisticated tool, you won’t find it in too many home shops, but you can perform this test at home using an air tank and spark plug hole adapter (with the exception of reading percentage of leak down).
To perform a leak-down test, the piston in the tested cylinder must be at TDC on its compression stroke, so both valves for that cylinder are closed. This test is easiest to perform following the engines firing order, so I start by bringing the number-1 cylinder to TDC on the compression stroke.
Step one is to disable the ignition, remove the spark plugs, and install a compression gauge in the spark plug hole. Then, using a remote starter or a friend, turn the engine over until the compression gauge indicates the cylinder has compression. Don’t be fooled by the weaker exhalation that occurs on the exhaust stroke. Then verify that the timing pointer is at TDC on the damper. Next, take off the radiator cap and engine breather cap and block the throttle in the open position to assist in identifying what is leaking if a leak is present.
Fill the cylinder with compressed air using an adapter between the hose and the spark plug hole. Caution: Keep hands away from the fan, belts, or pulleys during a leak-down test because if the piston is not at TDC, introducing air under pressure into the cylinder may cause the engine to turn over. The cylinder should hold pressure and not leak down at a rate of more than 5 to 10 percent. More leak down than that indicates a sealing problem in that cylinder.
If you are using the homemade leak-down tester as previously described and do not have the gauge to determine percentage of leak down, you can still make a fairly accurate determination for the cause for any leakage by performing a few simple checks. Listen for air escaping from the carburetor, breather, oil filler, dipstick tube, exhaust pipe, or radiator. Note that a small amount of air escaping through the breather is common in worn engines. If you hear air escaping through the carburetor, it means that an intake valve is leaking. Air coming from the exhaust pipe means an exhaust valve is at fault. To be doubly certain that either of the valves is leaking, go back and ascertain that the cylinder being checked is still at TDC on the compression stroke.
If a blown or leaking head gasket is the problem, air will leak from an adjacent cylinder to the one that is pressurized, or through the cooling system via the radiator filler neck if the gasket has blown between the cylinder and the cooling system. Air leaks at the dipstick tube and breather are indications that the piston rings are not sealing properly against the cylinder walls.
Once you have completed the test on cylinder number-1 and noted the results, you can proceed through the firing order 1-5-4-2-6-3-7-8 (Refer to firing-order charts in the Appendix), testing each cylinder as you go by rotating the engine clockwise to the next 90-degree mark on the damper. Verify the cylinder by removing the distributor cap and confirm that the rotor is facing toward the position on the cap that holds the wire for that cylinder.
Test Cooling System Pressure
Head gaskets that have blown or are leaking into adjacent coolant passages in the cylinder block will also show up during a cooling system pressure test. A head gasket problem will reveal itself as air and/or coolant escaping into the affected cylinder. This simple diagnostic test will also identify other leaks in the cooling system.
The test is performed by obtaining a simple pressure tester designed for this task. Simply install the tester in place of your radiator cap, pump up the pressure, and check for leaks. This tool is available at quality auto parts stores.
Written by Charles R. Morris and Republished with Permission of CarTech Inc
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