Rough/hunting Idle when warm + auto-accel

[Chris]

Hi Guys,

I checked the forums but didn't find anything with both of my symptoms:

1) Rough idle / hunting during idle ( http://www.youtube.com/watch?v=AIlQosigIRI ). This was taken after a 20 minute drive on dry day.

2) The car accelerates up to about 25mph (very, very slow acceleration) with foot off the gas in Drive.

On my last STS, I replaced plugs & wires to get rid of the first problem, and had an auto-accelerate issue which turned out to be the Idle air control motor. However, last time that motor broke, the car idled close to 3000rpm and would idle close to 75mph.

With both of these symptoms occurring together, I thought I'd post here.

The only code that shows up (and the reason the SES light is on in the video) is the code showing fault with the Smog Pump - I have the fuse pulled due to an annoying whine.

Thanks for any assistance you can offer!

[voicepro]

The Northstar in my '98 deVille was idling rough -- hunting a bit -- but I replaced all four coils and now it runs like a scalded cat.

Can't speak to any other part of your problem. Good luck with it.

[KHE]

For the acceleration/high idle issue, check to see if anything is causing the accelerator to bind. It could be as simple as a floor mat that is partially on top of the accelerator pedal. Check the cable routing to make sure the cable is not kinked or damaged.

[Chris]

Thanks for the comments guys.

I've cleaned the TB and IAC, which seems to have stopped the "idle up to 25mph" thing. Still idles harshly....Because the harsh idle only happens at idle when fully warm, I figured I'd replace the IAC. That didn't have any effect.

voicepro - I thought bad coils would be more noticeable when load put on engine... I hadn't really considered it for an idle-only problem. Guess that's something to look into.

I've also read to check the Fuel Pressure Regulator (FPR).... but I don't have any problems on starting (which seems to be a common symptom of FPR). Where is it located on my car?

Thanks again!

[KevinW]

When did the hunting idle problem start? Are the oxygen sensors OEM? Did you check the torque on the intake manifold bolts?

[Chris]

When did the hunting idle problem start? Are the oxygen sensors OEM? Did you check the torque on the intake manifold bolts?

The problem started right around the beginning of the month... still dry and moderate temperatures. I think it got worse over a week or so and has stayed about the same since. I haven't been under the hood other than to check the oil since March when I replaced the air filter.

OEM O2 sensors.

Will check the intake manifold bolts on the weekend when I have access to a proper garage... thanks for the suggestion! I'm also going to check the intake tubing for cracks while I'm at it.

[Chris]

I wasn't able to look into this last year, but found a solution this year. Figured I should post here in case anyone finds this and has the same issue that I did.

Found a few threads on another forum suggesting that a tear in the intake plenum might be causing this. Most other users also report getting "lean" codes... seems to be somewhat common around 100k miles. I never got any lean codes but thought I'd investigate...

To test this, I removed the engine cover and idled the engine, sprayed lighter fluid (actually acetylene) around the plenum and found the engine would idle up.

Ordered new parts from RockAuto (they don't sell Delco intake plenum, so I went with the Dorman).

Replaced the plenum using step-by-step guide located at http://www.cadillacforums.com/forums/cadillac-tech-tips-how-fix/171319-2000-northstar-plenum-replacement-steps.html (Bruce, please remove if not appropriate, I didn't find anything similar here so thought it would be useful to post!).

I found that I had a friend under my intake manifold:

And here was my problem:

Note that the Dorman intake manifold gaskets do not fit properly. I just reused my existing gaskets and am loving the smooth idle!

[rockfangd]

lol. nice pictures. I completely agree with above. Just wonder if any of the debris under there got sucked in through that hole.

Last year I pulled the intake off my 96 and found a tiny mouse nest. It amazes me because there is not much room to squeeze in there but they still do.

[Cadillac Jim]

Excuse me, but are we to infer that a mouse or small squirrel with ready access to peanuts was living under your intake manifold?

Please tell me that this is a prank from a previous owner or a mechanic.

That intake isolator is really classic. Thanks for the pictures.

[KHE]

That's a mouse nest - the mouse evidently found a source of peanuts.

[Cadillac Jim]

Now, we have an unsolved mystery. Where are the peanuts??? If he has been stashing them where the throttle mechanism binds with them...

[Chris]

Excuse me, but are we to infer that a mouse or small squirrel with ready access to peanuts was living under your intake manifold?

Please tell me that this is a prank from a previous owner or a mechanic.

Definitely not a prank... here is the inhabitant:

lol. nice pictures. I completely agree with above. Just wonder if any of the debris under there got sucked in through that hole.

Last year I pulled the intake off my 96 and found a tiny mouse nest. It amazes me because there is not much room to squeeze in there but they still do.

Can't believe he not only got in there, but was even able to squeeze the peanuts through complete with shells!

[rockfangd]

lol. I have seen crazy things.I know of people that feed the squirrels peanuts and the mice take them and put them with their nest. Wherever it may be. In the valley is perfect because of the warmth and isolation. Until they get too big to where they cant squeeze out and die. I would definately not think of a prank when I see that.