Strange Idle fluctuation problem
 

I posted this on the other forum, but thought I'd try this one as well.

Well I've been trying to chase down this problem since I first got my car running just about a month ago. The idle won't stay constant. I've tried adjusting it at the BAC, and back on the throttle linkage... I can't remember what they call that screw. Anyway, I can adjust the idle and make it idle right at 750-800rpm where it should be, but it never stays constant when I start the car or drive the car. For example, when I start the car in the morning it may idle at 1500rpm. I think that's about normal since it's cold. However, once it warms up it may only go down to about 1200rpm. Or maybe when I start the car warm, it idles fine at 750rpm, but then when I stop at a stop light later, it will be idling at 1000prm. When I first got the car running after the rebuild, it didn't want to idle on it's own at all. Eventually it started idling, but it was really rough and was pretty inconsistant. I thought maybe it was just due to low compression. It's more smooth now, but it still isn't consistant. So I thought maybe a bad O2 sensor, but my gas mileage is pretty decent. About 17-18mpg on average. That's city/highway driving. I think it would be a lot worse if the O2 sensor was bad. So I thought maybe there was just part of the throttle linkage sticking somewhere, because the car had been sitting in a garage for several years before I got it. We squirted it down with WD40, but that didn't really help anything.

So after that I measured the resistance on the different solenoids/valves, throttle sensor, etc., and the Accelerated Warmup System valve (factory manual called this Air Bypass Solenoid valve I believe) was the only one that didn't measure out right according to the Hayne's manual. So I got a 10ohm resistor, which is within the tolerance for that solenoid and connected that in place of that connector, but that didn't really seem to change anything.

So does anyone have any idea what could be causing this problem? A vacuum leak was the only thing I could think of... I thought that would have a more constant effect though, but maybe not. I wouldn't know how to go about checking for that sort of thing either.

Ok too long for me to read as I am tired. But here are the rules. No idle= vac leak. Slow revving=vac leak fast reving=TPS is off

Please oh god search. There are a million threads on idle fluctuation.

Well I found a million on surging idle, but that's not really my problem. Thanks anyway.

yea, im having the same problem. sometimes it idles fine at 900, but sometimes it goes up to 1500, 2000. It occasionally starts to bounce a little, from 1200 - 1600 or so, but usually the idle is steady.

Yeah I forgot to mention that it does actually bounce a bit in the morning when it's cold. That's really the only time that happens though for me.

You may want to have a quick look at my page in sig under 'my own faq' and 'idle'

Hugues -

Thanks. I looked at your page a bit. Tomorrow I'll go out and go through all the possibilities that you've listed. Hopefully I'll come up with something. :)

Fully heat up the engine and let it idle. Install the initial set coupler. Did the idle change when the initial set coupler was installed?

Actually you need to start from square one. Set the timing with a timing light with the idle below 900 and the car fully heated up.

Then set the tps. I have no idea what series you have. IF it's a series four, set the tps so that it outputs one volt from the green/red wire on the tps connector with the car idling. Stick the meter lead up the back side of the green/red wire with the plug connected up.

Then set the idle using the bac screw.

IF the car won't idle below say 800 rpm with the initial set coupler installed and the bac screw adjusted the best you can.....then spray with starter fluid around the inlet duct and the bottom of the fuel injectors. Also around the back of the throttle body at idle.

IF it's a n/a, then check that the two small vac hose at the front of the dynamic chamber are not crossed up. There are three hose there. One big..two small. The two small are what might be crossed.

An outside chance after you do the above...is take the bac off and turn it over and fill it with some cleaner like fuel injector cleaner. Let it sit overnite then exercise it numerous times and reinstall it. Sometimes that piston in it will gunk up with something and cause it to not return to it's minimum position....causing a touch high idle.

It's hard to fix cars that one cannot see and hear/smell.