99 GT won't start at idle


madlcd
07-11-2005, 07:17 PM
Noobie here

I have a 99 Grand Prix GT with a 3800 II engine and about 78000 Miles.
I have never done much more than change the oil and filters and the car has run just fine. We just had the remains of Hurricane Dennis blow through and the car sat out in the rain and humidity. It is still very humid and the car will not start at idle. It does start when I give it a little gas, but if I let the RPMs fall to 800-900 it will die, even when warmed up all the way. It does seem to be running on all six cylinders when I keep the RPMs up. Higher RPMs don't seem to be a problem. I tried disconnecting what I believe to be the MAF sensor but saw no difference.

Any ideas? Or, is there a way to bump up the idle to say 1200 so I can limp to a mechanic? I saw no obvious adjustment as on my 90 Toyota truck. (first generation EFI)

Thanks,
MAD

BNaylor
07-12-2005, 12:01 AM
Noobie here

I have a 99 Grand Prix GT with a 3800 II engine and about 78000 Miles.
I have never done much more than change the oil and filters and the car has run just fine. We just had the remains of Hurricane Dennis blow through and the car sat out in the rain and humidity. It is still very humid and the car will not start at idle. It does start when I give it a little gas, but if I let the RPMs fall to 800-900 it will die, even when warmed up all the way. It does seem to be running on all six cylinders when I keep the RPMs up. Higher RPMs don't seem to be a problem. I tried disconnecting what I believe to be the MAF sensor but saw no difference.

Any ideas? Or, is there a way to bump up the idle to say 1200 so I can limp to a mechanic? I saw no obvious adjustment as on my 90 Toyota truck. (first generation EFI)

Thanks,
MAD


How old are the spark plugs and wires? Depending on the age and mileage they could be susceptible to moisture. Also how about the fuel filter. You can check fuel system pressure at the rails (45 - 55 psi).

A defective Idle Air Control valve (IAC) can cause idle problems 1000 rpm and below. Its located on the throttle body.

Do you ever get a SES/CEL light?


http://i5.photobucket.com/albums/y186/lizzywiz/gtp.jpg
Thrasher CAI, DHP v1.0 PCM,
3.25 SC pulley, CAIT GMAF, Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 TB,
TB spacer, MSD 8.5mm wires, Autolite 104s,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials

madlcd
07-12-2005, 06:31 AM
How old are the spark plugs and wires? Depending on the age and mileage they could be susceptible to moisture. Also how about the fuel filter. You can check fuel system pressure at the rails (45 - 55 psi).

A defective Idle Air Control valve (IAC) can cause idle problems 1000 rpm and below. Its located on the throttle body.

Do you ever get a SES/CEL light?

Ah...
The IAC... That's what I was looking for to adjust the idle. On my old truck it's just a screw adjustable needle valve arangement that controls the flow of air that bypasses the butterfly valve. Now it's an electro mechanical thing that can go bad.

I did manage to bump the idle to about 1000 with the bottom end throttle adjustment screw. Crude, but at least it starts and my wife can drive the thing until I can figure this out or give up and take it into the shop. This also sounds consistent with a bad IAC. Higher rpms and driving around seem just fine.

I changed the fuel filter last spring, but the plugs and wires are the originals. The wires do seem to be in good shape and at least #3 gives a good strong spark with the 'ole screwdriver test'.

The dealer we bought the car from has 'free' oil changes for life. They usualy try to sell some service that you don't realy need if they are not too busy when you go in. A couple of oil changes ago the offered to clean the throttle body, but at $800 I figured it was a scam. Could this be a ligit service?

How would I diagnose a bad IAC? I have yet to get any SES/CEL indications.

Thanks,

MAD

richtazz
07-12-2005, 10:05 AM
I would bet the plug wires are bad. IF it only happened after they got wet, try this. Start the car at dark, and see if you can actually see spark jumping across the wires. If so, you found your problem. IF you can't see teh spark jump, due to the wires running under or behind something, buy a can of wire dryer at a Zone or Advance and spray the wires liberally, and see if it runs better. again, if it helps, your wires are bad and replace them, as wire dryer is a temporary fix.

BNaylor
07-12-2005, 10:35 AM
I agree with Richtazz. Wires to include spark plugs would be the best starting point especially with 78K miles and the moisture situation.

On the IAC, you can try removing it and use carb cleaner on the part where where it mates up to the throttle body. There are passages that carbon up and get dirty.

BTW - $800 for throttle body cleaning is a rip off. If you decide to clean the throttle body be very careful with the MAF sensor hot wire element. Carb cleaner will damage it. Use electrical contact cleaner only.

Let us know how it turns out. Good luck!



http://i5.photobucket.com/albums/y186/lizzywiz/gtp.jpg
Thrasher CAI, DHP v1.0 PCM,
3.25 SC pulley, CAIT GMAF, Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 TB,
TB spacer, MSD 8.5mm wires, Autolite 104s,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials

madlcd
07-18-2005, 06:49 AM
The IAC valve was the problem. It was pretty crudded up.
Curse GM for the tiny 4mm steel screws that held it in. They had welded themselves to the aluminum throttle body. I broke the heads off both of them and had to drill them out and retap. This made the half hour job take three hours, not including the trip to the parts store for some new screws.

I will move new plugs and wires up on my list of things to do as they probably were a contributing factor.

Thanks

MAD

BNaylor
07-18-2005, 10:21 AM
The IAC valve was the problem. It was pretty crudded up.
Curse GM for the tiny 4mm steel screws that held it in. They had welded themselves to the aluminum throttle body. I broke the heads off both of them and had to drill them out and retap. This made the half hour job take three hours, not including the trip to the parts store for some new screws.

I will move new plugs and wires up on my list of things to do as they probably were a contributing factor.

Thanks

MAD

Thanks for the feedback. Glad you got it resolved. Nothing like DIY and saving money.



http://i5.photobucket.com/albums/y186/lizzywiz/gtp.jpg
Thrasher CAI, DHP v1.0 PCM,
3.25 SC pulley, CAIT GMAF, Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 TB,
TB spacer, MSD 8.5mm wires, Autolite 104s,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials

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