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Below is a collection of Jim Grant's Tech Tips sorted by Vehicle Make. These Tech Tips were answers to questions submitted to Jim by ALLDATAdiy.com users over the course of many years.
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  Jim Grant's Tech Tips

'93 Pontiac Grand Prix, Won't Idle Right

Q: I have a ‘93 Pontiac Grand Prix with a 3.4 V-6 twin cam engine. The problem I have is the vehicle wont idle when I am able to get it to start. I’ve already replaced the EGR valve because it was bad and had a blown gasket and the Throttle Position Sensor as well. When I am able to get the vehicle to start the RPMs run at 2000 to 2500. I press the accelerator and the engine runs real rough at 3800 RPM. The rev limiter isn’t set that low that I know of and it keeps bogging down. When the engine gets to operating temperature the roughness of the idle kind of disappears, but the idle is still at 2000 to 2500. My friend and I couldn’t find any vacuum leaks. We don’t really know where to really begin to look. Somebody mentioned that the intake manifold gasket could have blown, (the lower one). Should we start the project of tearing apart the engine to replace the gasket, or start looking elsewhere? Any suggestions on how to fix the problem or where to begin?

- Adam

A: The lower intake manifold gasket on your vehicle’s engine is renowned for failing, resulting in a list of drivability woes. In addition, you should be aware of a totally unrelated component that will cause tortuous idle speed problems with the ability to send the vehicle’s computer off into la-la land. That component is the Air Conditioning system’s pressure transducer (sensor). This sensor seems to have the uncanny ability to short/fail only when the engine is being cranked over. The power supply for this sensor just happens to supply several important sensors to the computer. When the A/C pressure sensor fails, it causes all of the other sensors to report bad information to the computer. The computer stores this information and uses it a baseline to control the engine. Here’s where it gets fun! Once the engine starts, the A/C pressure sensor gets better. This causes the affected sensors to report totally different values to the computer. The computer wigs out and the engine will have so many running problems you won’t have a clue where to begin diagnosing. To diagnose this problem requires some special equipment and training. I would suggest having a qualified technician diagnose your running problem. Once you know what’s broken you can perform the repairs yourself.

 
     
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