Please Help me with this Idle problem I have on my 92 Firebird.
It is 5 speed 3.1 ltr V6. The car has been in the shop all week and
the Mechanics still have not figured out what the heck the problem is with the Idle. The symptoms of the problem are very specific. After the car warms up which doesn't take but a minute the idle will drop to about 500 rpms and the car sounds like crap like it is only runing on four or five cylinders. When I am driving as soon as I get over 12 mph the rmp's jump to 3000 - 3500 rpms. I don't even have to touch the accelerator to be doing 65 on the freeway. Trying to stop is tricky. Everytime I clutch to shift WHAM the rpms jump to the above mentioned range. But as soon as I get to the 12 mph Range while slowing down for a stop or anything else the rpms just drop almost to the point of the car dying. At that time the check enging light comes on and the code is for a bad MAP Sensor. Here is what the mechanic has done to fix the problem. Complete tune-up, Cleaned the IAC, cleaned Inside the Throttle Body, Replaced the MAP Sensor. And the problem is still there. The guy that has been working on the car is off on Monday so there will be a new set of eyes and hands working on the car I need to some info from my fellow 3rd gens as to what he should be looking for. I would appreciate all the help I can get from you guys. Could it be the fuel pump, fuel filter, bad computer, Vacuum leaks. I just don't know!!!!! Also the only thing that I have done to the Car is put a K & N air filter on it. The car also has been very well maintained for all of its life and it has a little over 134k on the odometer, and the engine has always run perfect. Up till now.

 

I'm going with a large vacuum leak.
At idle, leak may be bad enough to lean out a cyl or 2 so bad it/they won't fire. When car accelerates, computer gives eng more gas and cyls that were too lean to fire, start firing, reving eng up.
May explain the MAP sensor code also. Was the old sensor tested after removal?
Was some work done previously, or did it just start on its' own?

 

IAC

 

The only work done on the car before I took it into the shop was an attempt to replace the water pump. But all I managed to do was remove some of the bolts and the coolant, but then I realized it was going to be a bigger job to do than it was on my 305 Firebird. So I just put the bolts back in and filled it back up with coolant and water and the following Monday took it to the garage. If there is a Vacuume leak would they be able to here the sucking noise of the air or are they just going to have to look real hard for it. Another question if they have replaced the MAP sensor and didn't test the old one to ensure that it was faulty what should I do about that. Also as I stated the car was completely tuned up and the IAC was cleaned plus the inside of the Throttle body. Thanks guys for the assist on this

 

Sometimes a vacuum leak can be heard, but usually not.
Water can be used to find the leak. Make-up a squirt bollte with a piece of vacuum line on it so you control where the water goes. Wet everywhere there is a gasket starting at throttle body and work your way down to the intake with eng running.
Some folks use carb cleaner (or some other type of aerosol) to find vacuum leaks. I caught an eng on fire doing that once.
I only use water now.
When water hits the leak, you'll hear the eng sputter, and a lot of times, you'll hear the water being sucked into the eng.
Pull the vacuum hoses one at a time and plug the vac port, or pinch the vacuum line closed with some smooth jaw pliers.
Listen for eng rpms to change.

As Marc mentioned, the IAC (idle air control) could be the problem. If it's stuck open, it would be the same as a big vacuum leak.

On my car, I can close the IAC(s) using my scanner. Not sure abt your eng.

After IAC has been commanded closed, I squirt carb cleaned in the hole where the IAC gets fresh air. If IAC is not closing or leaking , it will suck the carb cleaner thru the hole. It it's closed, it will remain a puddle.

There is a hand held tester for the IACs, but it costs more than I wanted to spend. Your mechanic should have one.

You probably have no recourse for the old MAP sensor being tested or not. I'm sure they will say they tested it.

If you get the old one back, you can test it, but you'll need an o-scope and a vacuum pump to check it right. You can use a multimeter, but you can't see if the signal is noisy or not. Computers hate elec noise.

 

I may be wrong but I had a similar problom with my car, turns out my timing was way off. It was set at 30 by a dumb mechanic, I set back to 10 and now it runs like a charm.

 

Well I finally got my car back from the Dealer. (note to self always take car to Dealer even if it is more expensive.) The other garage had my car for a week and a half, and still did not solve the Problem. I didn't have to pay for all of the poor dumb *** trouble shooting because they never found the problem. Although he was close. And Mark 85Z28 is the winner. He pegged it with the IAC problem. But that still would not have fixed the dang thing because the computer was shot also. I also got a brand new MAP Sensor from the other garage for free because they threw my other one away and it was not bad. Grand total for all of the repairs on my car the last two weeks. $1,103. this includes water pump replace, AC serviced, tune-up, new IAC, new Computer, Fuel injector cleaning,. Oh well there goes most of this months Navy Retainer pay.