The problem is with my 1990 camaro but the engine is almost exactly the same as the Vette engine and you people seem a lot more knowledgeable than others at camaro forums.

When at a stop light the car shakes (ruff idle) and when getting off the throttle it hesitates so bad it seems like its going to stall. The car seems to run pretty good other than that. Flooring it from a stop smokes the tires with no problem. So far I replaced the plugs, wires, and distributor. I also clean the iac.

Any ideas on what to look at next?

 

hi
I have an 85 vette and had problems with the idle. It does not sound like yours but there are some thing you can investigate.
You had cleaned the IAC, but have you cleaned the butterflys in the throttlebody as there will be a lot of carbon deposits behind them and that will disturb idle quite a lot. Then there is vacuum leaks. Check the vacuum lines and then spray some start gas or propane on the runner/intakemanifold (where the runners enters the intake manifold). If the engine increases the rpms there is a vacuum leak. I had that. Then you have the PCV valve. Check for proper operation. If you cannot hear the the bullit inside the valve is loose it isstuck and you have to get a new one.Then the most difficult thing. I had a bad EGR valve. It could not keep the vacuum because the membrane was bad.
You also have the vacuum solenoid, but I have not heard of anybody that had had failures on that one.
I would check the minimum air idle adjustment:
Put the ignition on with the A and B terminal (the upper two on far right)on the ALDL connector connected with a wire and wait for 30 seconds. Then take out the IAC connector. Then start the engine with the wire taken away. The car should have about 400 to 450 rpms. If it is slower adjust the butterfly position screw til 400-450 rpms.
Then restart the engine with the IAC connector on and check the TPS sensor for a voltage between .54Volt to .62Volt.

Good luck.

 

If after you have done all of the above and nothing changes check to see how much wear there is in your TB shaft.
Too much slop = erratic idle and hunting because too much air is leaking past the butterflys that don't seal and the IAC can not adjust enough.

I have just spent a week on the same problem , doing the above adjustments and now found out my TB has too much wear on the shaft despite the same TB in the same cond giving no problems on my stock motor.
Go figure.
Swapping in another old TB without any TPS or IAC adjustment got the idle back to 600 straight up.

 

Have you changed the fuel filter? I think basics is the first step. my .02

 

He has the same problem as me , motor runs fine but no idle.Is not a fuel supply problem
I ran 13.5 in full street trim through the mufflers two nights ago but won't idle below 1000rpm

 

Yeah I get what you are saying, and I agree. I was reading his comments and he just did not mention much, so I look at it as doing basics first, and if all is right there move on.
The above post by Jan erick is definitely a good start, you mentioned changing out TB excessive wear in shaft was cause, but in another set-up no probs.That would certainly make me go hmmmm.

 

Same problem on my 91. Talked to a friend that is a tech at the local Chevy dealship as I was thinking it was the IAC before I bought a scan tool. He told me to clean the throttlebody as it probably had lots of carbon in it. He was correct, I disassembled, and cleaned the throttlebody, then rebuilt it with a gasket kit from Advanced Auto, ($8.00). Also if you go this route, in the rebuild kit it will tell you to knock the plug out that Rochester Products puts in the unit when they set them up on the assembly line. This will give you more fine tuning to set your idle to your liking. Replace your PCV valve, (AC), as this circulates through throttlebody which is another source of the build up. This worked well for me, before I performed this procedure, the car would shake like I had a big cam in it, now it is nice and smooth.

 

ll above still doesn't fix it, try a coolant temp sensor ($7 at advance auto). The speed density engines are very dependant on this sensor for fueling.

 

Check your timing

 

wiring im good at but engine stuff is another story. i usually bring my cars to the mechanic because i am no good at troubleshooting and only with a helms or chilltons manual am i able to attempt anything. so please excuse any and all stupid questions that come up. the timing was checked when the plugs, wires and distributor were done about a month ago. i also cleaned out the tb when i cleaned the iac. tomorrows supposed to be a nice day so hopefully ill check out the rest of the stuff. i checked and i have no codes. if the egr valve, coolant temp sensor, or pcv valve is bad should that throw a code? thanks for all the help so far

 

Timing was checked? What is it set to? Was it checked with the EST wire disconnected?

EGR will throw a code (32 I believe, but I'm not sure off hand). PCV and temp sensors do not throw codes.

 

problem fixed. i adjusted the minimum idle air and it seems to run perfect now. thanks for everyones help.