So I did a search, and this is what I came up with...

First, the problem: So I get up to go to school, car starts fine, drives fine, nice easy cruise, not too much throttle...anyways, just as I am about to shut it off, the idle starts dropping to ~500rpm and back to 900~ repeatedly. It almost wants to stall out and then bounces right back. The car was in neutral with the clutch depressed and no electrical running except for an aftermarket deck.

Now the weird part...was changing the oil yesterday and I noticed that one of the vac lines was disconnected (hadn't seen a difference in driving with the vac line disconnected). I put the vac line back on, and nothing changed, so I assumed that everything was alright.

Now the fix: When I get home from school today, going to look at TPS, Idle Adjustment screw on throttle body, plugs, wires, and all vac lines again. Is there anything else that I should be checking? In some other threads it has also been mentionned that water temp sensors and O2 sensors can also cause this problem...is that correct???

BTW my car is an '87 GXL with streetport and custom exhaust.

Thanks for the help,
Brad

 

Don't touch the idle adjustment screw. It doesn't randomly un-adjust itself, and since you have some other problem, messing with it will make things worse.

You need to go over the entire engine and make sure there are no vacuum leaks. What you describe sounds like a leak, and leaks are the cause of 90% of idle problems. Check especially around the plastic intake duct.

Next, check your BAC valve. Verify it is not sticking or clogged, and test it according to the procedures in the Haynes/factory manual.

After that, check your TPS. Make sure it is within range, and there are no dead sponds in it's travel. A scope is best for this, but an analog ohmmeter can be used as well. Connect up the meter (instructions in manual) and slowly sweep the TPS through it's range. If at any time the needle of the meter bounces around, the TPS is dead.

After that, you have to look for more exotic causes. The O2 sensor doesn't effect idle at all. The water temp sensor certainly can by making the car run very rich. Testing procedures are in the manual.

Obviously, all this assumes that your plugs and wires are fine as well. But you will most likely see that the problem is the BAC valve.

 

cool, thanks for the help...

on a side note...can I use a digital multimeter instead of an analog ohm meter??

 

It's much harder to tell that the TPS is rough with a digital since by nature, the digitals like to "hop around". A scope or analog meter is best.

 

cool...I'll go pick up an analog ohm meter on the way home