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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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