ccollet

got most of the kinks out of my 1991, did the simple stuff like brakes myself.the last item is the idle, sometimes the car run fine with the idle around 700 rpm,

but sometimes it idles fast, around 1400,

and other times it turns over but wont hold an idle unless i put my foot on the gas. After running a minute or two with my foot on the gas it will run fine.

i first thought it was simply a vaccuum leak, but searched and searched couldn't find one. then i thought IAC, replaced that and still have issues. Next up, replaced the TPS, no luck. Then went and tried anothed IAC in case the first one i bought was bad, and still got the idle issues.

i am running out of ideas.....

chris

Joe C

my 2-cents, FWIW - sounds to me like the idle circuit within the throttle body is the issue. even though you replaced the IAC valve, the internal idle circuit most likely is dirty. the IAC valve seat within the TB, might be caked with crud. does the idle speed float all over the place? my suggestion, pull the throttle body and give it a good cleaning. a Band-Aid service cleaning on the car won't cut it. also, give a double check the voltage values from the throttle position sensor. not much adjustment on a 91, but give a check anyway.

ccollet

thanks Joe, i'll take the throttle body off and take it apart this evening and give it a good cleaning,

DRGVette

I have an 88 and seem to have a similar problem. It tries to die if I don't hit gas pedal when starting but then idles at 1200 rpm. If you put a load on it ( drive or reverse) it goes down to 600. It sometimes dies if very cold when it idles down. But even when it gets warn if it is in neutral or park it goes right back up to 1200. I cannot figure out why and I also assumed vacuum leak. Any help would be appreciated

yd328

I second Joes suggestion give the tb a good cleaning first. If you have access to a scanner that reads the computer info look at the iac counts, if memory serves me correctly 0 counts can be a vacuum leak.
Gary

DRGVette

I just had throttle body cleaned and I have a mac tools scanner that says I believe 140 counts might have to double check if that means something

yd328

There is a normal range for the iac to operate at. Someone here should know the range or you can google it.

ccollet

ok, cleaned the trottle body and IAC passages to a almost new condition. Still erratic idle. Replaced MAP sensor, but that wasnt it either.
I am gonna replace all the vacuum hoses in case there is a crack in one i am overlooking. Car only has about 63k miles

Cliff Harris

The ECM sets the IAC to around 140 when the engine is first started. That causes it to idle at about 1200 RPM (like the old "fast idle cam" on carburetors).

As the coolant warms up the ECM backs off on the idle speed. You should end up with an IAC count below 50 when the engine is fully warm. The exact count depends on how your mechanical idle is set.

Cliff Harris

A leaky or stuck EGR valve can cause idle problems.

DRGVette

So how do I change mechanical idle or should I replace IAC because when I put it in gear it sometimes stalls when cold so I don't want that idle to go down but when it is warm it still idles at 1200

yd328

If you already replaced the iac I wouldn't do it again, I threw the counts out there as a bit of info to use to diagnose it if you had access to a scan tool. Also, there is a procedure for manually adjusting the throttle plates. You can change the tps voltage setting by just turning the idle up or down using the throttle plate screw. As mentioned above the egr can be a source of an erratic idle. Are there any trouble codes in the ecm?

DRGVette

Don't believe there are any codes right now. I had a burn off diagnostic code but all of a sudden its gone. Also I did not replace IAC yet. I was asking if the throttle plates would make my idle lower when it has a load on engine because I don't want that

Cliff Harris

The mechanical idle is adjusted by the "Minimum Idle Adjustment", which also calibrates the TPS sensor:

http://forums.corvetteforum.com/1570563689-post1.html

ccollet

update,
searched all the hoses for a vaccuum leak again, and found none. I didnt really think that was it, because when starting it cant hold an idle without my foot on the gas and i don't think a leak would cause that.

So I bought a reman. ECM at advance auto, (found a 40% off coupon too) put that in the car and now it idles perfect. Problem solved.

guno89

I was just going to suggest this! My 89 had same problems I also replaced the IAC, ended up being the ECM, best $50 I ever spent on the car.