94 Grand Prix cold start problem

n758bb · #1 12-09-2009, 11:36 AM

The car is a GTP with the 3.4 liter engine. On cold mornings, when we start the car, it starts up right away but races to 3500 rpm for a few seconds, then falls down to less then 400 rpm, and sometimes dies, but sometimes recovers and races back to 3500 rpm. This goes on for about 5 min, and then the car runs fine. The SES is NOT on. I tried to take the car to the local auto parts store to see if there were and stored codes, but they couldn't read it. The GTP that year is a OBD II system with the rectangular plug for the OBD I. They said only the dealer would have the adapter to read it, and they charge a lot. So, can anyone help with the cold start problem, and know how to a less expensive way to read the OBD II system?

richtazz · #2 12-09-2009, 11:48 AM

you either have a vacuum leak that goes away once everything expands a bit from heat or a bad IAC motor, both fairly common issues on that engine.

n758bb · #3 12-09-2009, 11:52 AM

I forgot to mention I removed and cleaned the IAC valve and checked the resistance according to the repair manual, and it checked good.

tblake · #4 12-09-2009, 05:58 PM

Common issue on the 3.4DOHC are vacuum leaks caused by bad intake gaskets.

n758bb · #5 12-12-2009, 01:33 PM

I got a chance to work on the car again. I checked for air leaks using a stethascope and either and found none. Some areas were hard to get to, but I did my best. I also pulled a small vacuum line and made my own intake leak to see the behavour of the engine with a leak and it was noticeably different. No stalling or fluctuating like when its cold.
Cleaned the MAF sensor, no change.
Cleaned the temp sensor, no change.
Unplugged the MAF, no change.
Unplugged the temp sender, no change.
Unplugged the IAC, car ran at 3000 rpm constant, no fluctuation. Problem gone.

Obviously I can't drive it around with it idling at 3000 rpm, so what would cause the computer to tell the IAC to go from one extreme to the other?
When its running very slow and almost dieing, the exhaust smells real rich.
How about the oxygen sensor?
Its a 3 wire type I think, so it should be heated. What if the heating element is broken?
Thats my best guess, what do you think?

Thanks
Mike

CrazyHorst · #6 12-12-2009, 01:45 PM

I still think you have a vacuum leak as noted above.

The IAC is simply responding to the difference between the target ERPM and the actual ERPM. It has a "rapid response" mode which is used to "stall save" but that activity creates an overshoot in the target/actual control loop. There just isn't enough damping to smooth the behavior back out.

When you unplug the IAC it likely defaults to wide open so the engine doesn't stall yet signals an astute driver there's something going wrong which may need repairs

And at 3krpm the unmetered air leakage isn't significant to be noticeable.

A technique...that I have read about when working on old tractors...and hesitate to repost...is to take an unlit propane torch and release a little whiff of propane near the suspected site of a vacuum leak. If the engine rpms improve or stabilize somewhat you've found the area and confirmed the problem. Of course this technique also has the potential to blow you a mile in the sky without any probability of survival. So proceed at your own risk.

n758bb · #7 12-12-2009, 02:56 PM

The propane test you mention is the same as the Either or starting fluid test I did. If the vacuum/intake leak sucks in starting fluid the rpm increases, and my test i didn't get any increase.

n758bb · #8 12-12-2009, 02:59 PM

Could it be a leak in the EGR valve? That would let unmetered air bypass throttle body.

CrazyHorst · #9 12-12-2009, 05:56 PM

Yes, if the EGR is stuck open this would be the same effect. The engine doesn't like *any* EGR at idle because it interferes with combustion stability.

I would think you'd also see some power loss due to that.

'94 being OBD-1 would not have any extended diagnostics for verifying function of the EGR...it would just be commanded in open loop and that's it.

n758bb · #10 12-12-2009, 08:27 PM

The 94' Gran Prix with the 3.4 liter IS OBD II. Only the 3.4 liters that year were. It was the beginning of the new electronics, but they used the plug from the OBD I system. Not many places other than the dealer can read it, because of that.
Tomorrow I will open the EGR valve and see if it is stuck open.

Thanks
Mike

tblake · #11 12-13-2009, 10:50 AM

Quote:

Originally Posted by n758bb

....When its running very slow and almost dieing, the exhaust smells real rich.
How about the oxygen sensor?
Its a 3 wire type I think, so it should be heated. What if the heating element is broken?
Thats my best guess, what do you think?

The o2 sensor does not determine fuel trim ratio's untill the car is warm and in closed loop.

I am not familiar with the 3.4DOHC but you could pull the vacuum line off the fuel pressure regulator, turn the key to on, and check for fuel dribbling out.

Have you checked fuel pressure on this one? If it pumps up and drops off, you have a leak somewhere. Probably in, which is why your car keeps idling down but can't.

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