I recently posted on here about starting trouble with my 86 IROC 305. You guys were right, it was the choke pull-off on the carb, and I appreciate your help. I recently had my Quadrajet rebuilt and they put on a new Choke Pulloff and a TPS, which I'm assuming means throttle position sensor. The car starts beautifully and warms up fine.

Unfortunately, another problem has emerged. The engine runs fine while warming up, but once it is warmed up, the idle "fluctuates" around 1200rpm, I mean the needle sort of bounces and the engine won't stay at a steady idle speed. The fluctuation seems to have gotten less the more I drive it, but if I let the car idle, the idle speed will slowly drop over the course of a minute or two until the engine sputters or stalls altogether.

Strangely enough, this never occurs above 2000rpm, and I can usually bring the engine back to life with a press of the throttle, but whenever it idles the RPMs slowly fall.

I don't know what could be causing it. I remember I did snap part of a small plastic hose that seems to go nowhere anyway, but I tried putting my finger over it and it didn't change the engine behavior, nor did I feel any suction or pressure. I tried spraying carb cleaner around some suspect vacuum lines, but no response. Actually I really can't tell which lines are vacuum lines because they have that ridiculous Air Injection system on the 86 H motor for pollution control. Besides, a vacuum leak would speed up the engine idle, wouldn't it???

I'm also thinking that maybe the previous owner jury-rigged the car to work with a broken carburetor. If he did, then maybe the rebuilt carb or the new TPS or CPO are sending signals the engine was J-rigged not to handle and this is causing a problem.

It may have also been a rebuild mistake. The shop I took the carb to is usually reliable, but I wouldn't know how to diagnose a carb problem/error. I can usually come pretty close on natural failures, I can't diagnose my or others's mistakes.

Does this symptom sound indicative of anything in particular? Any suggestions on what to try?

Any help would be greatly appreciated.

Thanks
Mark

Does your car use a MAF Sensor? I had a similar problem with my 88 Firebird v6.. When the car is in open loop (before it gets to temperature and the ECM kicks in) it uses pre-defined values to regulate the engine.. once it gets warmed up it goes into closed loop and starts reading the sensors. In my case, tapping the MAF with a screwdriver cause the engine to change the idle for a moment.. replacing the MAF fixed the problem. Although my car was fuel injection, I know that carborated engines also used a sensor to measure intake flow..

Now, if you are not using any sensors on the intake, then you might be right about it being j-rigged.. check the Chilton to see if something is missing.

Good Luck.. I hope this helps
ChadU

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ChadU 97 Camaro 3800II, PST, SLP, y87

Look around on the Qjet for a minimum idle speed screw. Set this up a little so the engine won't die out. Sounds like your idle speed positioner is not working correctly, even so it should not go slow enough to die.

If tom's suggestion doesn't work, it sounds like your idle circuit in your carb is screwed up. Maybe the crappy rebuild.

Is it easy to replace an MAF sensor yourself? Or do you need special equipment to do it? Do you think hooking it up to a computer would tell me for sure?

Hmm...is the minimum idle screw the one that stops the throttle linkage from closing any further? I adjusted that, but if I turn it up any higher, the engine will rev too high when it starts idling. The idle is high at first, then just slowly drops off. If the carb idle circuit was messed up, would the idle fall off slowly like that? Any thing in particular I could look for?

Thanks for all your help.
Mark

OK I called the auto shop where I got the carb rebuilt and talked to the guy who did it. He says it sounds like an idle mixture problem and told me that the idle mixtures weren't set when I got the carb.

Is that what you meant by an idle circuit problem? If I get the idle mixtures set do you think that'll make it run right?

Thanks again for everything.

Mark

If your idle mixture screws aren't capped, screw them in until they seat (gently) then backout 1.5-2 turns. Start it and it should run, let it warm until it will fall off fast idle. If it dies when you take it off fast idle, screw in the idle speed until it runs. If it won't, check the timing (check the timing anyway). If you have access to an exhaust gas analyzer, use it now. Since you don't, work between the idle speed and the mixture screws until you get a stable idle at an appropriate rpm (6-800). Never turn the mixture more than 1/4 turn at a time, always turn them both the same amount.

Thanks for the help.

Actually, I got the idle mix adjusted yesterday. The engine runs great now, very drivable. Thanks again for all the advice.

http://www.camaros.net/forum/smile.gif

Mark