No idle without acceleration

[BN2008]

Hello,

I recently drove my car about 1200 miles with a plugged catalytic converter. I had that replaced as well as 2 oxygen sensors and it was accelerating beautifully. Well, on my trip back prior to this replacement my car started shaking at idle where the RPM's would go up and down. Out of idle it was fine. After replacing the catalytic converter and the O2 sensors it was a little better, but not much. Then my husband found a crack in the upper intake manifold plenum and plugged it with glue, as well as replacing 2 leaky o-rings that we could hear sucking in air. We did pull a code that said "random misfire", so we replaced all the spark plugs and wires...it did not help either.

Now if I start my car it won't idle and will die unless I push on the accelerator for a few minutes or so. After it's warmed up I have no problems at idle. I've put in 92 octane gas and 2 bottles of fuel injector, but no luck. Any ideas on other things we should check out? THANK YOU!

[olopezm]

Welcome to the forum!

You should check the IAC valve, sometimes it gets stuck to the point it can't open or close freely and will cause the car to have problems at idle when cold. Take a look at it and give it a good clean with carburetor cleaner, just make sure you don't spray the electric contacts.

Also if the vacuum in the intake manifold is still leaking some vacuum you will get erratic idling problems since the air entering the engine is no the same amount measured by the MAF sensor.

Here's a link with a picture of the engine and a description of where to locate some common components.
http://www.automotiveforums.com/vbul...d.php?t=746374

[jakegday]

instead of fixing the intake plenum with glue, you should replace it.... when i bought my car i was having problems with a p0171 code.. it turned out there was a hole melted in my plenum right above the EGR port, which was caused by the previous owner driving to long with a bad catalytic converter. the bad cat caused too much back pressure, which caused the intake plenum to "break." im guessing the same thing happened to you

[olopezm]

As jakegday mentioned you should replace the intake plenum, it is the correct way to fix the problem, you might still try with the glue but the plenum might still not be properly sealed, if that's the case keep in mind that you will never get rid of the vacuum leak and your car will never idle properly. About the IAC here is a link with more information about cleaning it:

IAC cleaning

[BN2008]

Hi,
Well we cleaned the IAC and replaced the whole plenum, which was melted on the inside, which had created the crack on the outside....but, I still am not able to idle without acceleration and after it's warm I have no problem. It is a smidge bit better, but not really. Any other suggestions? Thanks everyone!

[biounlimited]

Well I am not up to speed on these pontiac engines yet, but what about temperature sensors?? If they are sending a signal to the computer that the engine is warm, it may be leaning out the fuel when cold, then run fine when warm. Not sure if these engines have multiple temp sensors or not. Maybe one of the more experienced pontiac people can chime in here.

[tblake]

Try unplugging the MAF sensor and see if it will idle.