1991 Oldsmobile Silhouette idles at about 600 after it has

PHAHN70

MEMBER
1991 OLDSMOBILE SILHOUETTE
6 CYL
FWD
AUTOMATIC
131,000 MILES

Hello,
I have a problem with my silhouette that is really baffling me. I do realize that some of the symptoms could be seperate issues, but the one I am really concerned about is:
Vehicle does start, but not as fast as when it is warm.
After the first start of the day, or after the van has cooled for at least 2-3hours. It idles at about 1100-1200 and then at about 30 seconds it starts to idle down to 750 but can take a good minute before it does. Then after I have driven the van for awhile and it is nice and hot (not overheating) I come to a stop light and the idle is at about 750 but then drops down slowly to about 600
I have replaced the o2 sensor, the IAC valve, map sensor, spark plugs, air cleaner, pcv valve and older emission tubing. It has had the fuel pump changed approx 1 year ago. The first time this happened the van died and I got a rich-o2 sensor and a bad map sensor fault code. I just changed them and figured if that wasn't the problem I'd at least know those were good. Codes have been cleared. A couple of times I have started the van after I have driven at least 20-30 minutes, gone inside and about 30-40 minutes later I go to start it and it starts, and the idles so low it died in about 3-5seconds- I start it after that and it starts and then idles up to 1100 and then comes down quicker than when its been started for the first time of the day. This has only happened 2x's in the past 2 months.
The van does not appear to be missing or running rough. No smoke coming out of the tail pipe. I do get the oil pressure guage-it also drops after the vehicle is warm-when the idle drops to 600-no light's come on-but its approx (under 40) it's a tad under the first mark.
I also have noticed that -at times- the voltage guage- I can see the marker slowly move back and forth.
I did not change the TPS because I would think that the computer would compensate and I would be feeling the difference. I had an escort one time where the tps was going bad and it would jerk while driving.

Could I have a bad thermastatic air control valve in the air cleaner?
Could it have something to do with the ignition or the distributor?
I don't want to be on the freeway stuck in traffic and have this die on me.
I'm totally lost here. Can you help?

Thanks for your consideration

7 Replies

KHLOW2008

MECHANIC
41,815 POSTS

Hi phahn70,

Thank you for the donation.

Problem seems to be an idling issue and vacumn leakages can cause the engine to stall.

Apart from those that you have replaced or tested, I would suggest cleaning the throttle plate and testing the injectors.

Test the fuel pressure to see if they are within specs.

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Tuesday, August 18th, 2009 AT 9:40 AM

PHAHN70

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4 POSTS

Stalling issues have been too few at this time. My concern and question-and I apologize for not being more clear- but it's the idling at 600 rpm's after it get warm.

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Tuesday, August 18th, 2009 AT 12:56 PM

KHLOW2008

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When the throttle plate is dirty. It would restrict the air flow and cause the idling to be lower than expected.

That is the reason I suggested cleaning the throttle plate.

Btw what is your engine type?

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Tuesday, August 18th, 2009 AT 1:17 PM

PHAHN70

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4 POSTS

Hello,
I appreciate it.-
I have a 6cyl 3.1L
It's been pretty good-knock on wood.
Anything in particular to clean the throttle plate with?
And areas to avoid?
You've been helpful.

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Tuesday, August 18th, 2009 AT 2:06 PM

KHLOW2008

MECHANIC
41,815 POSTS

The recommended procedure is to remove the throttle body to have it serviced.

Alternate way is to use carb cleaner.
Spray cleaner into the throttle body and let it soak for a while. ( Do not spray excessively each time to avoid extreme flooding.)
Start engine and let it run. ( Would be difficult to start and might stall or stumble due to the excessive carb cleaner in manifold, nothing to worry about)
Procedure might have to be repeated a few times as it is not easy to get it clean due to the construction.

Carb cleaner would sting when comes into contact with skin or eyes so extrra care has to be taken while spraying.

With low idling speed, most of the symptoms described are related so priorority is to get the engine running smooth before we start worring about others, which might have been resolved by then.