I am hoping for some suggestions from the group. I have a 1985 Vette with an extremely erratic idle. I have had 3 different mechanics look at it and nobody can seem to figure out what the problem is. We switched out the EGR and EGR temp sensor this weekend. We then put the diagnostic machine on it and there were no codes showing. It seemed to be running great on Sunday. Monday morning I got in it for work..when I started it the RPMS went to 780...the minute I put it in reverse, it dropped to 520 and started shaking and my dash lights were even dimming. I let it warm up a while longer and it then went to 1100 when I put it in drive and jumped to 1600 when I drove off. First stop light, it only dropped to 870 and the brake pedal started dropping, the brakes started groaning and the car was slowly inching forward into the intersection while I was continually pressing harder on the brake pedal. After that fiasco, I got to the parking lot of my work, and the car was going 14 MPH in the parking lot without touching the gas pedal..I had to brake to slow it down. Then when I parked at work, the RPMS jumped to 1200 when I put it in park. A "supposedly" top Phoenix shop had my car for over 2 months trying to figure it out and we finally gave up on them...every time they thought they had fixed it, it came back the same (after quite a few $$$). Hoping someone on the forum might have had the same issues and may be able to shed some light on this for me. Thank you in advance!!

 

I sure hope you have a extra 0 one those numbers look for a vacuum leak.
get a big cigar and blow smoke in the brake booster hose to the engine and see where it comes out and plug each leak as you find it. then plug that line and see if it runs better.
might evan be your brake booster itself.

 

What does the first part of your message mean about an extra 0 one? Thank you for the suggestion about blowing the smoke through the vacuum lines. We will have to try that, but I am guessing that the brake booster wouldn't have anything to do with the crazy up and down idle..

 

if you got to 16,000 rpm your car would have been a grenade
and if your booster has a hole in the diafram that is a vacuume leak pull the hose of it and plug it and see what it does.

 

LOL!! Did not notice I did that..car idles high, but not quite that high...we will try what you said..THANK YOU!!

 

Yes, antfarmer2 is correct! If you turned 16,000 RPM that engine would be all over the ground in pieces. The stock L98 for the 85 Corvette would max out about 5000 RPM.

Definitely sounds like a vacuum leak.

 

Have you checked the harmonic balancer? If it is out, your timing is out. What about your plugs, cap, rotor, wires? Everything there good?Fuel pressure regulator hose. Any gas smell? Fuel pressure. Take FPR hose off and what does it say? What is the bleed off time?

Have you tried setting the idle by IAC? What about your injectors. I;ll bet they are the stock junk Multecs.

You need to check fuel pressure but turning it on and not run. What pressure does it reach? How long does it hold?

Turn the throttle blades so they close. What is the IAC count with the engine only running in closed loop? It should be pretty high. As you screw in the screw, you open the valves and the IAC counts will drop.

 

My friend who installed the EGR, thinks it may be the throttle position sensor or the idle control valve...Any thoughts on these suggestions?

 

could be but I would start with the cheap cigar and putting your finger in the hose first

 

yes do the cigar and blow in the brake booster vac line. note, the car needs to be off!

blow alot of smoke in there!

 

If you keep throwing parts at it, sooner or later you will replace the broken part and the car will be fixed. What is a "diagnostic machine"? Is it a code reader or scanner? If it is a code reader, you can easily see what the TPS is doing. If your IAC counts are 0 and the blades are as closed as you can get, you have a leak somewhere. Pull out one hose at a time and close off the vacuum port. There aren't that many. IF you cap off a port, rev the motor and there are now idle counts when there were 0 before, you have a leak right there in that circuit. What is your command idle from the ECM? Have you checked timing? Does timing go up when you rev it? What is your fuel pressure like? What is it at simulated WOT when you remove the hose to the FPR? What pressure is it at WOT?

 

Don't think the 1985's have the Multecs.
When I researched changing injectors on my 85 everything I read said the originals were Bosch and they were worth cleaning and reusing. When I removed my injectors I found 6 Bosch that looked old enough I would say could be original and 2 new Accell.
I ended up buying the rebuilt Bosch III's from FIC and have been very happy with them.

 

I have heard conflicting opinions. Some have Multec Injectors as early as 85,some say 89,some end in 94 others go with the entire C4 line