oloten0

Just got my C4. Runs good but the idle is rough. Owner's manual says to run it @2000 rpm for 2 minutes to clear oxygen sensors. Yes? I just put in some fuel system cleaner. Any ideas?

pcolt94

Welcome
Ok your new to the Forum. Get your profile filled out, car year and name +.

When asking a question you need to state what year you car is to get an qualified answer. Different years have different ignition, fuel system and electronics and depending on what you have will affect the answers for you.

You will not get a lot of feedback if people do not know what you have. C4s vary a great deal from 84 - 96.

Cliff Harris

Rough idle is almost always a vacuum leak.

They can be very subtle and the vacuum setup changed quite a bit over the various years so as pcolt94 said, you really need to put the year and other pertinent information into your post.

oloten0

Thanks guys. It's a 94 bone stock as far as I know. Runs strong and all temps & pressures are okay.

DGXR

I've noticed my idle quality too (1995 LT1 6-speed), when fully warmed it shakes a little and spins 950rpm, not sure if this should be a concern since it runs great otherwise. Generally it's a little hard to believe that a high-compression pushrod V8 should idle "smooooooth like buttah" as some other CF users claim. Shouldn't there be at least a little rumble or lope? I still plan to check the intake manifold bolts, as well as theck the EGR, PCV, TPS, IAC, etc.

89dd

When I brought my new at the time 1990 in for its first annual inspestion, it ran so quiet and smooth, the mechanic had to asked if the car was running.
So yes they can run that well.

pcolt94

See if you can isolate it to a cylinder by pulling plug wires or fuel injector connectors and looking for no difference in running. Make sure all plug wires are on plugs all the way down. Check for any sparking in the dark from the wires.

Make sure the EGR is closed all the way and that it moves freely. You may find something doing the basics before going off into left field and complicated areas.

kimmer

Or instead of pulling wires use a timing light.

lt4obsesses

Well, depending on the maintainence history before you got it, it could be alot of things. I've experienced huge improvements in "new to me" used cars with a simple plug change.

oloten0

Thanks for all the ideas. So far a bottle of Techron has helped. My mechanic says run the thing, it's almost 20 years old and only has 80K it must have sat a lot.
I'm going to install new plugs as well.

adtbrown

My 93 idled rough until I changed the Opti. I'd bet the contacts in the cap are corroded.

oloten0

The Opti is distributor?

Kevin Woods

Yep.

KW

pcolt94

Yes.

It's for opti-spark. It has a normal distributor cap. But has an optical section that sends 2 sets of pulses for timing (to PCM) for the entire fuel and spark system. If the low resolution section develops a problem, engine might not run due to no injector pulses and no drive to coil (via ICM) to develop the spark.

It's a system that works great when it works properly with very accurate timing thru the RPM range. Unfortunately it lives under the water pump and is susceptible to failure if too much water (pump failure) pores on it. Keep it dry and don’t wash front of engine with a hose.

oloten0

Hey thanks for the information!

Bon Air

Try to de-carbon the combustion chambers; my '87 also idled rough after a lot of slow driving. I had it done professionally but you can do it yourself-they make a product sprayed
into the throttle body called Sea Foam. It works. In the old days you would just steam it
by trickling some water down the carb while revving it a bit. Anyway it worked for me.
BonAir