Hello, All. I recently purchased a 1991 RS 5.0L TBI with 128k miles. It performs well for it's age but has one bad characteristic. After a cold start, the rpms go up to 2k and stay there for about 2 minuets before gradually coming down to normal. The whole process takes almost 5 minuets. This can not be good for the engine or the automatic trans if it is shifted into gear at this high idle. I have replaced the IAC, cleaned and tested the EGR valve, and checked the temperature sensors. All seems normal. This condition is the same whether I have the stock or JET eprom in the ECM. No codes set in the ECM. Help!

 

Hmmm...

 

2k isnt that bad, it could be worse.

 

The only thing I can say is.. it sounds normal lol...

I start at 2 k as well and in 5 mins I'm down to 750 idle.

 

I have the same problem with my z but i have done so much work that is could be anything. I have heard the an engine will run at a higher idle on cold start up for a bit. not sure why, maybe to get fluids moving and oil moving around. not sure mine idle at 2K all day. someday I will buy a new TB

 

Thanks for the input. I will add that I also own a 1995 Astro van with a 4.3L CPI and it never goes that hi on cold start. It also settles down to about 700 rpm in 30 seconds or so. It uses the same style ECM as the RS. A cold idle of 2k will definatly increase engine wear. It sounds like a programed parameter in the ECM. Thoughts?

 

if you are really worried about it then take it to a mechanic and have them adjust the choke.

 

Demonchild: Choke? What choke? There is no choke on a 91 TBI.

When I had the TBI on my '89, it would rev up to 2K occasionally. It bothered the heck out of me. I fixed it by swapping in a TPI

 

You know, I was thinking back to that and it occurred to me that when I pulled the TBI, I noticed that the gasket between the TBI and manifold was shot. It had a rather large vacuum leak, but ran pretty well. That could cause the idle to come up, especially cold, before the throttle body and intake expand. Couldn't hurt to look.

 

I have disconnected the Idle Air Control 'motor' and it stopped going to 2k on cold start. You have to give it a little throttle a first but it idles fine after that. It appears that the ECM is causing the hi idle but why?

 

Try this thread

https://www.thirdgen.org/techbb2/sho...hreadid=123706

look for my post

 

Maybe it is a fast idle to heat the car, because mine does it, but only at 1500. If I rev it once it will drop down though.

 

Thanks to Low C1500 for the pointer to the TBI group. As it happens, your post #154 is the best explanation for what is happening. Since it does not happen with the IAC disconnected I ruled out vacumm leaks. I also have the Camaro shop manual and have set the 'no IAC' idle to 700rpm. Now I have to determine if the injectors are running rich and the ECM is compensating with more than normal IAC or find a way to limit the IAC steps. Is this value re-programable?

 

You know if anything is changed on your motor from stock, (eg: timing a couple of degrees advanced) it will effect the start up rpm. This is because the ecm is throwing alot of gas in, along with lots of IAC counts, 4 deg advance could very well mean 500 rpm higher startup idle. May I suggest reseting your IAC, there is a good article on this site in the tech section. This very well could fix the problem. I don't know if you already did this (unplug IAC and put a paper clip from a to b on the aldl) or if you just unpluged the iac to set idle. But you need to use the paper clip.