random idle
 

i have a 1988 SE n/a and it idles randomly between 800 and 1200 rpms....what is causing this? is it my injectors plugged up??



crossbar

I doubt it's your injectors, but I'm not saying that it couldn't be. My FC has been idling weird for over 3 years ever since I bought it. I just ordered a new (used) TPS, because I'm almost positive that IT is at least contributing to my idle problems. I also have a couple of vacuum leaks due to some broken solenoids. I'd guess that yours is probably due to similar problems. Check for vacuum leaks, check the TPS, check the BAC, etc. There are quite a few things that could cause these symptoms, so it's really hard to tell.

Good Luck,
Derek

Let's see if I've got your problem right: you start the car, and it idles @ 2k RPM 'cause it's cold. It never quite drops from there. You come out of the store, and now it's idling at 800 RPM. As you drive down the road back to your house, you notice at a light that it's now idling at 1400 RPM. Once you get home, it's dropped down to 1000 RPM, but won't go lower no matter how long you sit. Correct?

I have the same problem, and it's the thermowax in your throttle body. Time to do the throttle body mod. Muhahahaha!

Brandon

check my sig i have done the TB mod


crossbar and once i drive the car idle is fine again after about 30 minutes worth of driving i can stop and she idles fine @900 rpms, this is only when it is running and hasnt been driven yet, and it will do it for awhile or until i drive it, for example i let it run for 25 minutes and went out and it was still doing it, i went out shut it off, then 2 hours later went out started it drove off and went crusing for 30 minutes stopped and it was fine....confuding for me i dont knwo what it could be


crossbar

No, what mine does is when it is in gear (with the clutch in obviously) it will go up and down and do a "hunt" or whatever you want to call it. That's why I think it's my TPS.

It's more likely your BAC or ABSV. Your TPS only tells the computer how far open the throttle is. Your idle can't increase unless something is adding more air into the intake (like opening the throttle). The TPS is just a sensor, so it can't make your idle hunt.

As for the original query, I wouldn't worry about it if it settles down eventually. It could just be the TB itself heating up. More interesting is your 900 RPM idle. Try adjusting the throttle stop screw to bring it down to 750 RPM, where it should be. Then see what happens :)

Brandon

random idle is pissing me off onhe of my coolant lines near the BAC as a small leak could that be it, it drips once every 10 minuites or so


crossbar

what can i check????

No, a coolant line wouldn't do it.

Here's what I think: there's one component in your intake stream that opens when the engine's cold, just like it should, but doesn't close fully until your intake tract is nice and warm. My first guess? Thermowax. You've done the TB mod, so my next guess is the BAC. If you've cleaned that sucker (try brake parts cleaner), how about the ABSV? Have you cleaned that? If yes, try checking for intake leaks.

That's about all the help I can give ya. FWIW, my car does the same damn thing. Until I get my TB nice and toasty, my car idles (progressively lower than) 2100 RPM. I'm not too worried about it :)

Brandon

Just a thought,


If you think it's the BAC, check the plug on it. Over time, they can get brittle from the heat, to the point that the wires in the plug short. When that happens, it will burn a transistor in the ECU. I thought my BAC was bad, but it was actually a shorted out plug. There is a cheap fix for this particular problem, using a $1.50 Radio Shack part. I did this a couple years ago, I'll see if I can find the link for the fix.

Or, you could take the friggin' thing off!




Gregg

I think you need to go to square one. Get the car hot. Jumper the initial set connector. Set the idle and the variable resistor while your there. After setting the idle to 750 rpm, pull the initial set coupler. If you could not get 750, stop and write back or fix the idle so it is close to 750. If you don't then the next step will be compromised if the idle is too high. Now set the timing. Now pull the plug off the bac. Did the idle drop?? Let us know what happened. I think you have a idle setting problem that is causing confusion with the bac. Bac is trying to correct an idle problem. Oh! Try reading this http://www.teamfc3s.org/info/articles/idle.html I know its for a turbo, but the bottom part about the operation of the bac helps.