87L98Vette

hello i have an 87 L98, the idling is everywhere, it goes really high to high haha, ranges from 1800-1500RPM. ive been checking for vacuum leaks, but cant seem to successfully find any. i did go through the readjustment process of the accelerator screw, because the previous owner did mess with it. also adjusted the TPS. any idea on what it could be, or what i can do to find out what the problem is?

antfarmer2

I would get a cheap cigar and blow smoke in the vacumme line that goes to the brake booster and find any leaks first.

87L98Vette

while the car is running?

antfarmer2

no

aklim

Hook it up to a scanner and see if it is really doing that RPM or whether you think it is doing it. See IAC counts. If it is 0 and you unscrew the adjustment screw, rev and it doesn't go up, you have a leak.

87L98Vette

When I did the adjustment, I brought it down to 800RPM, then I turned it off, and then went for a drive after and it was back up again

aklim

How did you adjust it? What did you use to determine RPM?

87L98Vette

When I readjusted the idle screw

crowz

My 87 did that a few months ago and the tps sensor had gone bad. Well it did that randomly for a few days then it wouldn't idle at all.

aklim

If you turn the screw randomly, you get what you got. There is a proper way

87L98Vette

I followed the online instructions, to do the readjustment

aklim

First off, that procedure is only good for a car that is 100% working. No air leaks, no bad wires, plugs, dirty injectors and most importantly, 100% accurate tach. Kinda why I usually dismiss the FSM method. If your tach is off, you adjust off. You don't really know what the ECM is seeing unless you have a scanner. If so, why not go the entire way and adjust via IAC counts which are going to be better and give you a good hint of an air leak?

87L98Vette

Plugs are good, wires are good, and injectors are new. And just by hearing the engine u can tell its waaaayyyyyy up. The car in drive without touching the gas idles at around 35mph.

aklim

I don't bother to hear what the idle is sounding like. I cannot hear what the ECM is commanding the idle to be nor can I hear what the ECM sees the ambient temp or engine temp is nor can I hear what the IAC is or what the actual RPM the engine is like other than "it sounds high".

I cannot understand why people are so adverse to putting a scanner on their cars and prefer to rely on guessing games to tell what is going on. Someone who tunes can tell you whether the MAF is getting a plausable value or what the ECM sees the engine temp is. With a timing light, we can see if the distributor has been moved or have an idea as to whether the Harmonic Balancer is suspect. Timing can affect idle as can air leaks. Engine temp can also affect idle. Fuel pressure should be checked. Ask someone in the scan and tune section what the plausible values for MAF is. If your IAC counts are 0, you either have an air leak or turned the screw in too much. That is a possibility worth mentioning.

crowz

Well one nice thing about the c4 its so old a volt meter is all you need to test most of its stuff.

Tps and mass can be diagnosed with a volt meter.

aklim

Why would you want to when it is so much simpler with a scanner especially when you can datalog?

crowz

Because most of the members here don't have a scan tool laying around and a volt meter was common and cheap to get if you don't have one.

I use a scan tool but again most don't have them. Especially for obd1. Obd2 is cheap and plentiful but obd1 is expensive and much harder to even find these days.

xrav22

The idle procedure is a pita. But it have all steps followed.
I unscrew the screw out before starting the procedure. I think you need to
Idle the car at 450 or so just so the computer can idle the car up, because it can't idle the car down. So thus it barely running during the procedure. I posted the procedure a few times here with tips and haven't had to adjust it over a yr now. Tps sensor must be reset anytime the throttle screw is moved because the computerneeds to know when your foot is off the gas. Pretty basic computing .

aklim

It's isn't that expensive when you consider that the scanner is useful for more than this car and not just today but also tomorrow and so on. Things, IMO are cheap and easy for a reason.

My scanner is an Auto X-ray for both and I also have a Snap On Brick and I also have Datamaster. Although I don't care for it much. Still like the Auto X-ray better than the Datamaster.

87L98Vette

Yes that's exactly what I did. Drop it down to 400, then when I restarted it was at 800, then drove it and it was back up to 1700 again