Yes, yet another idle bounce thread...

[plharr]

I've researched past posts and can't quite get my plan of attack together on how to even out my idle. Hope you collectively can provide some ideas.

1st the facts: '94 Carrera Targa, 135K, engine pulls strong, consumes very little oil (< .5l over 1,200 miles). A relatively recent purchase and absolutely love the car.

The problem: car always starts right up, so no problem there. On startup, idle will go up to around 1,150, then settle down nicely to around 850. So far, so good. Then a couple minutes later, the idle will slow to around 650-700. I give it a little gas and it goes back up to 850, but will eventually drop back down after a few minutes. Happens when it is cold or warmed up. No hesitation when I take off and runs smoothly up the RPM scale.

Steps taken so far - thinking there's a leak somewhere, have (judiciously) sprayed some carb cleaner around the manifold intake gaskets. I get a little uptick (my imagination?) on #5, but am not positive I have a leak there. Don't want to unnecessarily replace the gaskets, unless I have a better sense that that is the problem.

All I've read about the O2 and ICV in other posts and they seem to point to idles bouncing up rather than down - can problems with those also have the idle dropping?

One final point worth mentioning although I don't think it is directly connected - right after I wash it, she won't start up until she dries outompletely - I plan to chk out coil, distributor cap and lines and will hunt it down.

Appreciate any thoughts on how to better narrow this down as I want to do whatever needed to get to a rock solid idle.

BTW, this is a terrific site. I ran across it about a month ago and have became addicted to reading this, much to my wife's chagrin. Already added the factory short shifter (via Pelican), which is terrific.

Again, thanks for suggestions on what this could be...

[dtxscott]

Idle mixture is an area that I would check first. It seems you may be a bit rich.

Given the timing where it "drops", it may be an O2 sensor as the DME hands control over to the O2 sensor circuit after the warmup period.

If you don't have a voltmeter to check the O2 sensor output or exhaust gas analyzer, then you may want to seek help from a shop. This should be fairly easy.

[plharr]

Thx. Would temporarily disconnecting the O2 help to isolate if it is the problem?

[dtxscott]

Yes, it would help eliminate it. It may run rough. You should still check the exhaust gas levels to be sure.

[plharr]

Unplugged the O2 sensor - no change. Still will run for a while at 850, then drop to around 600 or so. So I would think it is something other than the O2 sensor.

As I don't have an exhaust analyzer, any other suggestions on what to check? I keep thinking there is a leak somewhere that is causing this - will take another look tomorrow to see if I can flush anything out.

Help me understand something - it was mentioned that it may be running rich. I would have thought that if that was the case, it would cause the car to run at a higher RPM rather than a lower one or that I would smell gas in the exhaust, which I'm not picking up. Wouldn't running lean (eg if there's a leak, then it's sucking more air in which causes it to run lean) cause a lower RPM? Also, if I shoot a quick shot of carb cleaner into the air intake, it comes up to 850, which would seem to confirm it is running lean.

I really want to figure this out without sending to a shop as I want to begin working on the car more myself (if I can do it without tearing up the car or one of my body parts). Thx.

[dtxscott]

This post says it all..... idle bounce You may want to read the entire thread as it is very informative.

Rich will drop the idle while lean will raise it. My O2 voltage at idle is about 0.9 and the car is stumbling terribly. I will lean it out to 0.7 and expect a much smoother idle.

[plharr]

Thanks for the help, Scott - lots to absorb in the post you reference. Will read through it, play around with the mixture tonight and see what gives.