Even "smart" guys can have difficulties diagnosing a variety of engine problems. Follow al
Within the Pontiac community, there are many who are very good at physically working on their cars. Changing parts, swinging motors, and the like comes very naturally to them. But when chasing a problem, you need to first determine what has to be changed.
The art of diagnosing is a gift that not everyone is blessed with. In much the same manner that the medical field has surgeons (those good at swapping parts in and out) and diagnosticians (the ones who tell the surgeons what to do), so does the auto business. When your Pontiac isn't running correctly, you first need to be a diagnostician before you can be a surgeon.
The reason for the problem may be obvious, such as a ruptured radiator hose or a burned ignition wire. All too often, the cause of the engine performance issue is not so easy to spot. It can be very elusive. An engine has a synergy between its operating parts, and a failure in one area can induce a false symptom in another. In dentistry, they call this sympathetic pain. With cars, we call it an exercise in frustration.
Many feel that a proper method of diagnosing is to simply replace everything in sight and hopefully stumble upon what is wrong. Though that theory is eventually effective, it's very costly and time consuming, and doesn't speak well for the knowledge of the mechanic.
This isn't meant to say that at times it isn't a proper diagnostic step to replace a part with a known good component. This, however, should only be done when proper protocol points in that direction, and if the component is not serviceable. For example, it's not necessary to identify which circuit in the HEI module is causing a no-start condition, but it is important to accurately determine that it's the module and not the coil. Since there is no means or reason to try and repair the ignition module, it's replaced as a unit after it's determined to be faulty.
Diagnosing requires a very good knowledge of the working theory of the engine or individual part. If one is chasing a cold-start issue, a carbon-clogged EGR valve isn't where the proper diagnostics will lead you: Pinging will bring you there. But if you aren't familiar with the function of the EGR system, it can become suspect. Thus, the first step in finding a problem is to reference the proper Pontiac shop manual to see if it provides a flow chart with some helpful hints on where to look.
Another aspect of diagnosing is that the procedure often defines what isn't broken instead of what is. For example, if you are chasing down a rough idle problem and don't find a vacuum leak, you still don't know the cause, but you have eliminated an air leak as a possibility. The list of potential reasons has just got shorter.
Having made a career of diagnosing mechanical problems, I have a term that I use when I teach seminars for the auto industry: SAT, Stop And Think.
The first step in finding a problem begins in your mind. Did I disturb something accidentally while I was working on another part of the car? Does the condition only appear under certain temperatures, operating loads, engine speed, and so on? Can another area be impacting or causing the problem, such as the alternator?
A few minutes looking under the hood and studying the engine while thinking the problem through will usually lead to a more proficient diagnostic, and may very well quickly reveal the cause. Even the best and most experienced diagnostician has been led astray by a series of events, or by taking shortcuts. The best results are usually realized with a thorough and analytical approach.
Another common mistake is to not assign actual values and just assume that the part is working properly. This is very common with fuel pressure and voltage. A test light is a good tool when used properly, but it doesn't replace a voltmeter. Similarly, just because the carburetor or EFI system has gasoline going to it doesn't mean the pressure is correct. There is a reason why Pontiac provides operating specifications in the shop manual. Nowhere does it replace numbers with an assumption.
The craft of diagnosing is a skill that will grow over time. Use your and others' experiences as learning tools. Show interest when someone else finds a problem with their Pontiac, and ask them about the symptoms and what eventually was determined to be wrong. When you find the problem with your own car, don't just be satisfied that it's finally fixed. Identify where you went wrong in your procedure, and think about and study the component or circuit to gain a better understanding for the next time. The best diagnostic people use lessons learned from something else to help fix the problem at hand.
To this cause, HPP has prepared this primer to help you get into the right area when diagnosing a problem with your Pontiac. Some of the information you may have seen before in HPP when discussing a particular component or theory, but not as a diagnostic lesson as presented here. Our goal is to make you into the Dr. House of the Pontiac world. And if you don't know who he is, check your local TV listings and watch an episode. The only thing we don't want you to emulate is his arrogant attitude!
Symptoms: Rough Idle, Stalling, Hesitation, Bog, Ping
The exhaust-gas recirculation valve is an emissions-control device that reintroduces spent gas into the combustion chamber to limit the amount of combustible mixture, and thus, the flame temperature. Elevated combustion temperature creates an emission called oxides of nitrogen (NOx). An EGR valve is usually vacuum controlled, though later OBD-II versions may use an electric solenoid instead of differential pressure.
A common problem with the system is carbon buildup. If this occurs under the pintle of the valve, it will hang open and the engine will idle roughly, and possibly stall or bog when the throttle is opened slightly. If the transfer passage is carboned closed or partially closed, the engine may ping at light throttle openings no matter how much you retard the timing.
To identify a carbon-laden passage, while wearing insulated gloves, reach under the valve and push up on the diaphragm while the engine is running. It should run very rough and possibly stall. If it has little impact on the idle quality, the passage is carbon-clogged and will need to be cleaned out.
To check for a poor sealing pintle or carbon under it, the valve will need to be removed and inspected. Pour a liquid, such as WD-40, on the pintle to see if it leaks. It should not.
Mass Air Flow Sensor
Symptoms: Stalling, Check Engine Light, Poor Performance
Early MAF sensors were very susceptible to premature failure, especially the Bosch-produced unit used on TPI engines. It employed a burn-off cycle that would glow the sensing wire at 1,000 degrees F when the key was turned to Off and the ECM was in closed loop in order to remove contaminants. If the burn-off relay sticks closed, the sensor will fail because the wire will break. If the relay isn't replaced, then the new sensor will fail because the burn-off is on all the time. This type of failure illuminates the CE/SES light.
A more elusive problem is a sensor that is beginning to fail which has a loose solder joint in it. This will cause the car to buck and possibly stall, but it will usually idle fine.
The diode trio sits over the rectifier bridge in the Delcotron alternator and can easily b
To test for this, gently tap the MAF sensor with the handle of a screwdriver while the engine idles. The engine should be indifferent to the sensor being shocked. If the engine reacts at all (with a miss, stumble, or other), the sensor is going bad and needs to be replaced.
Symptoms: Glimmering ALT or GEN Light On Dashboard
Worn alternator brushes can cause a high-rpm charging circuit issue, since they lose conta
A common problem when a Pontiac gets older is a glimmering or flickering charging light on the dashboard. Sometimes it is only visible at night or when the system is loaded with the headlights or rear window defroster on. If a voltage check is performed and the specification is in range, this becomes confusing. The problem is actually caused by a weak diode trio in the alternator.
A diode is an electrical one-way valve. An alternator produces an AC current, but to charge the battery, the output is chopped and modified by a series of diodes. The diodes allow only rectified current to be sent to the battery. When the diode trio begins to fail, unrectified AC current is sent to the electrical circuit and the alternator light glimmers very slightly. The diode trio is a service item and can be replaced easily by taking the alternator apart.
Under the diode trio is a series of diodes called the rectifier bridge. If the bridge fails, the alternator will not charge at all or may work, but will kill the battery when the ignition is shut off. It will backfeed to ground and deplete the battery in a short period of time. When looking for a draw (dead battery), do not forget to unplug the alternator and recheck. Often the draw is from the rectifier bridge.
Black smoke after a cold start is usually the result of a worn or failed choke pull-off.
Symptoms: Loads up after a cold start--carburetor applications
The common complaint is the Pontiac starts fine, but as it warms up (this is called the intermediate stage) the engine performs poorly, bucks, stalls and/or may blow black smoke from the tailpipe(s). This is most likely caused by a poor performing choke break or pull-off.
A choke pull-off is designed to open the choke plate slightly against spring tension, as a function of engine vacuum and fast idle speed. The pull-off is designed to create enough airflow to allow for a clean transition from cold to operating temperature.
The vacuum diaphragm in the pull-off can either rupture, or more commonly, swell. When it swells, it holds vacuum, but does not open the choke plate enough, and the engine loads up and runs rich. Most Rochester pull-offs have an adjustment screw to set the angle. Older Pontiacs used a drill bit index for the specification, while from the late '70s on an angle measurement is supplied, which requires a special tool with a magnet that affixes to the choke plate.
If the engine starts fine when cold and then runs poorly, the choke pull-off is the cause, but only if it is running rich. If the intermediate performance issue is not from being rich, the choke spring tension needs to be adjusted one notch richer. The engine needs the choke on longer, until the manifold can vaporize the fuel properly.
Pontiac EFI injectors come in many different flow rates and often look nearly the same on
Symptoms: Rough Idle, Poor Performance, Poor Fuel Mileage
Electronic fuel injectors live under terrible conditions of heat and dealing with fuel deposits. In addition, a small O-ring is all that seals the injector to the intake manifold. If this seal dries out, the engine will suffer from an air leak, but possibly on only one cylinder.
An EFI Pontiac should be treated to a good quality in-tank fuel injector cleaner, such as Chevron Techron, on a regular basis to help keep the injector pintle clean. When the pintle gets dirty, the idle quality will degrade and the engine may even misfire, but runs fine at higher speeds. As engine rpm increases, so does the length of time the injector stays open, making it less influenced by deposits.
There may come a time when the injectors in your Pontiac need to be replaced, since they can no longer be chemically cleaned. If this occurs, make sure you obtain the proper size in fuel flow for your application. Too large an injector will not make any more power, but will cause an overly rich running condition.
If compression, engine vacuum, and ignition all check fine and there are no air leaks, the rough idle is most likely from dirty injector pintles.
Symptoms: Pinging At High Engine Speeds
Delco's distributors (either breaker points or HEI) are excellent designs, but can cause a high-speed engine ping. Their centrifugal weight mechanism has a nylon stop that, if damaged or dried out from age, will allow the distributor to add additional advance at extremely high rpm. The common procedure is to set the ignition timing/curve so that it is all in by a certain rpm, such as 2,500. But the timing needs to be checked at a speed way beyond that figure to make sure the weights are not traveling further. This can be done at home with an advance timing light, but it is best performed at a shop with a distributor machine. It is not advisable to free spin an engine at 5,000 rpm to check the full advance curve. The few dollars a distributor tuner charges is much less than a new engine when yours throws a rod through the block.
Symptoms: Rough Idle, Pinging, Poor Performance, Stalling
When one part of the HEI module fails, the engine has no spark and does not run. But if another internal circuit goes bad, the engine will run but act poorly. The system is designed to have an expanding dwell (coil saturation) period.
To check the dwell on the HEI module, connect a standard dwell meter (green lead) to the TACH terminal on the distributor cap. On an eight-cylinder Pontiac, the dwell at idle should be between 5 and 10 degrees and expand to 30-35 degrees by 2,000 rpm. If it does not expand or only moves slightly, the module is defective.
A common occurrence is a module fails and causes a no-start condition. Then a cheap aftermarket replacement is installed and the engine runs, but does so poorly. Many off-shore modules do not have the proper expanding dwell period and can cause this condition. Always check the dwell on a new module when it is installed.
An ignition module can also go open under high heat conditions if the dielectric heat-sink grease is old. Once cooled down, the module will work fine until it reaches a critical temperature again.
The oxygen sensor is key for fuel economy and emissions. If your Pontiac is running rich,
Degraded Oxygen Sensor
Symptoms: Poor Fuel Mileage, Failed Emissions Test
The oxygen sensor is a consumable that will need to be replaced at some point. As the sensor degrades, its output voltage drops. The ECM reads this as the mixture being lean and adds more fuel. There is a good deal of correction available to the oxygen sensor before the CE/SES light recognizes the condition. A degraded oxygen sensor comes about over time--just like gray hair. It usually does not go bad overnight. The only time this would occur is if the engine had a head gasket failure and anti-freeze was burned in the combustion chamber. The coolant will then coat the sensor with silica (it will have a white appearance) and it needs to be replaced. A properly tuned engine will have a very light gray coating on the oxygen sensor shell. The sensor cannot be cleaned, and, once degraded, needs to be replaced.
Symptoms: Stalling, Rough Running, Bucking Under Load When TCC Is Engaged
The GM distributorless ignition system can experience a premature ignition coil failure, especially if the engine has a bad ignition wire. To check the coil, use an ohm meter. With the secondary wires removed, go across each pair of coil terminals. A Type I coil has the three packs in one enclosure. The proper resistance of that coil is under 13,000 ohms. Type II coils are individual designs with the secondary leads on one side. Their resistance should be under 6,000 ohms. If the coil checks bad, replace it along with the ignition wires.
One of the tools you will need is a good volt-ohm meter. An older Pontiac can get by with
Symptoms: Any Electrical Problem That Does Not Make Sense
A poor ground circuit in a Pontiac can wreak all kinds of havoc. This is especially true with a newer EFI version, but it's just as important on an older model.
The only accurate method to check a ground is a voltage drop test using a voltmeter. This simple test measures the amount of electrons caught in a traffic jam because they can't pass through the poor ground.
To perform this test, connect the positive end of the voltmeter to the ground in question and the negative lead to the battery negative terminal; then power up (turn on) the circuit.
The voltage drop on the ground should be no more than 2/10 of a volt. If it's higher, the ground is faulty and usually just needs a cleaning. A visual inspection will show the reason it failed the voltage drop test.
Don't assume the manifold bolts are all tight. Confirm this with a wrench. A minor air lea
Manifold Vacuum Leaks
Symptoms: Rough Idle, Slight Pinging
An intake manifold vacuum leak that only affects one cylinder will act differently than a major leak that impacts all the bores. An intermittent miss or roughness that may feel like a bad spark plug or even very light detonation is the result of the mixture being too lean, but only in the offending bore(s).
All that is usually required is a good snugging of the bolts with a wrench to cure this condition. It is very common with an aluminum intake manifold and a cast iron cylinder head due to the varied expansion rates.
Idle Air Control (IAC) Motor
Symptom: Stalls on deceleration
GM EFI systems, both TBI and port designs, use an idle air control (IAC) motor as a controlled air leak around the throttle plate. It works in similar fashion to an old carburetor dashpot but without moving the throttle plate. The throttle plate, and in turn, the IAC pintle are prone to getting carboned up from gasoline fumes and the exhaust gas that is introduced into the manifold. When this occurs, the engine may stall or undershoot and struggle to idle when the throttle is released quickly, such as when coming to a stop sign.
To remedy this situation, clean the throttle plates with throttle body cleaner and do the same with the IAC pintle.The IAC will need to be removed to clean its pintle. Reinstall everything and take the Pontiac for a ride. On some early TBI EFI models ('82-'84), the idle may stay high for a few miles until the system reteaches itself. After a few miles and one ignition key cycle on and off, everything will be fine.
Symptom: Many Misunderstand Them
A common misconception when using a scan tool to access ECM troubles codes is the assumption that the failure is in the sensor. GM makes it very clear that a trouble code identifies the circuit that is being affected. The problem can be due to wiring; a mechanical issue, such as a jumped timing chain, which would impact engine vacuum; or some other malady. For this reason, when a code is set, it's imperative that you follow the Pontiac shop manual instructions, and not just replace the sensor. If the engine does not run properly due to a mechanical or ignition issue, there may be no codes set, since the failure is not with an engine management circuit.
Don't forget the basics such as performing a compression and vacuum test. Examine each spark plug electrode and insulator--they shouldn't have oil or fuel on them. To determine if a problem is fuel or ignition related, try richening the mixture temporarily by choking the carburetor slightly with your hand. If the engine responds even slightly in a positive manner, there is a good chance that the problem is fuel related. When diagnosing driveability issues, determine if the condition is impacted by speed, rpm, etc., with a test drive.
Here are few quick diagnostic tips.
Editor's note: Special thanks to our lead photo/cover models, Eric Orban and Kevin Clarkin, for participating in our photo shoot and for having a good sense of humor about it.
Often an engine may misfire after new spark plugs are installed. This is due to a piece of
The carbon button under the coil in an HEI and needs to be examined for wear. A misfire at
The TPI system is very prone to throttle plate and IAC coking since the EGR is introduced
Proper float level is essential for the carburetor to work correctly and should be high on
If the fuel system seems to act up without reason and then corrects itself, it usually mea
To check for an electrical draw, connect a test light in series with the battery ground ca