19 Ways to Diagnose Car Engine Problems - Shade Tree M.D. (Master Of Diagnostics)

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.

Alternator
Symptoms: Glimmering ALT or GEN Light On Dashboard

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.

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.

Fuel Injectors
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.