Yes I have done a search and read as many of the threads as I can but here is another question I couldn't find an answer. My idle began to move higher and higher periodilcally until at times at a stop light it would idle at 2000. It became so anoying that I disconnected the IAC wire and drove it like that for a day. I re-torqued the intake and put in a new IAC tonight. I went through the adjustment steps according to Shoebox and it idled just perfect. I took it out for a drive and by the time I got back it was idling at about 1500 whenever you stop. I did a vacuum check and it is running about 14Hg at idle and seems to hold steady. Now here is the tickler. Sometimes it idles perfect at about 1,000 and the next time I stop and push in the clutch it will idle at about 1500 - 1800. I haven't done a spray for vacuum leak but will tomorrow. However with the solid readings I got from the gauge I can't see that there is much of a leak or could I be wrong??? Appreciate any help!!

 

You've got a wicked vacuum leak. Do you hear any whistling/whining as the car is idling? I just finished fixing my idle problems (I think - so far so good). I cleaned the TB and replaced the IAC hoping that would solve the problem (idle would climb when I put the clutch in only if the car was rolling). The new IAC didn't fix the problem though it did seem to improve it. The car pulled a consistent 19 Hg vacuum (thought it was pulling 21 at one point but was probably reading guage backwards - doh!). I believe 21 or so is a healthy vacuum count for these cars around sea level (I'm in FL). I sprayed TB cleaner around and faintly heard the engine respond.. turned out the midsection of a hose running to the intake manifold had just dissolved. It was over on the drivers side.. down low, toward the front of the car. There's a rubber plug with a metal fitting coming out of it.. a rubber hose connects that to another metal line that goes back into the intake manifold (I think) anyways it goes in under the TB (that end isn't even visible unless the TB is off). Well the visible end over on the drivers side.. that hose had a huge hole in it. I'd also suggest checking the little rubber elbow (looks like a 90* plug boot) it's part of the evap system.. it connects to a little metal fitting on the TB (pass side) directly below the pcv line (big fat rubber hose that runs back to the pass side valve cover). The other end of the elbow connects to a small hard plastic line. Both of these pieces were in bad shape - one badly cracked and possibly leaking and the other had a huge hole in it. Check these two spots.. I'm sure there are others but those are the two I ran into. Your intake manifold itself could also be leaking - I hear that's common on these cars but you say you retorqued it so I wouldn't worry about that.

I realize now that diagnosing a vacuum leak on these cars is difficult with a mechanical guage because you need to know what the IAC is doing. If the thing is fully closed, for example, and you're still pulling only 14 Hg of vaccum then you've definately got a leak. But if it's stuck wide open then.. well.. you've probably still got a leak since I don't think the thing could possibly let that much air in unless you had the little module/valve completly removed from the TB which no idiot would ever do. Since you've got a new unit I think it's a safe bet that the PCM has the thing fully closed trying to compensate for the tremendous vacuum leak you have.. obviously it can't compensate enough since you're still having idle problems.

I'd say you definately need to spray some TB cleaner around (be generous with it) and listen very closely for even a faint change in engine tone. You've probably got a hose that's off/damaged somewhere...

Do a search under my user name with key word idle or IAC and I think you'll find that I've had pretty much the same problem you're having now... I seem to have fixed it.

 

Whenever I clean my TB my idle will hang. Just dap some oil on the end of a QTip and lube the butterfly shaft where it meets the TB housing...problem solved.

 

Yeah sounds like you've just got something sticking. Definately check that TB and check your throttle cable, maybe something is binding there. I've never heard an LT1 run with an intake leak but I know when my 4.3L truck started idling higher and higher it was an intake leak. LT1's are somewhat known for that leak, so check that out too.

 

Why is it that so many of the problems we encounter turn out to be something so simple that we often overlook them??? I found the idle problem. Because it was intermitent it had me stumped. If it had been a vacuum it should have done it all the time. In any case I was going over it again this morning and found that the vacuum line that goes from the intake to the opti runs directly under the throttle cable connection on the TB. The arm would hang up on the line ever so slightly and then slowly slide over it. I used a small nylon tie wrap and moved over and it works great now!!!

 

Glad to hear you resolved the problem. I was wondering if it was something to do with the throttle cable, how you explained it kept sounding like it was just sticking on something. Finally something fixable on an LT1 that wasn't expensive.