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.
Distributor Advance
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.
Hei Module
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...
The oxygen sensor is key for fuel economy and emissions. If your Pontiac is running rich, check the sensor output with a digital voltmeter or scan tool. It should bounce between 0.100 and 0.900 volt. If it's fixed below 0.500 volt and the engine is rich, the sensor is faulty.
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.
Dis Coil
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.