I'm just looking for some ideas. My car is in the sig, a 1991 Z28 LB9 5-spd with 85,000 miles and I'm the original owner. About 2 months ago it developed a stalling problem - when cold it started fine and idled fine, but after the car warmed up, especially after highway driving (65 MPH +), the car would stall at the first stop/light. It would start back up without a problem and then the RPM's would drop and stall. The next morning the car would start & idle fine until it warmed up. So I searched the boards and found the tech articles on IAC & TPS. Since I know this was never done on my car, I removed the throttle body and give it, the IAC, and all the passages a real good cleaning using Valvoline SynPower TH/Carb cleaner. The IAC gasket broke apart in my hand, so I bought the gasket kit from AC-Delco and replaced it and the throttle body gasket. While setting the min air, I could not get the car to idle any lower than 600 RPM - I jumped the A & B ALDL, I unplugged the IAC & EST and the car was warmed up when I tried. I adjusted the min air screw all the way out until it was no longer making contact with the throttle linkage/stopper (I think that's what its called). I plugged the IAC in and the car idled @700 RPM. I also replaced the TPS and the lowest I could get my voltage was .599 VDC. I cleared the computer and test drove it and it idled @750 RPM, but the good news is that it doesn't stall anymore.

Now what's happening is that on start up, when cold, the car idles @1100 - 1250 for about a minute and then comes down to @750- 800, which I think is ok, but it does this after its been driven and is warm too. For example stop to get gas after drving for 30+ minutes. When I restart it, it again idles/surges to around 1100 - 1250 RPM then comes back down to@750 - 800 RPM. One other issue is after driving at highway speeds for a while, when I come to a stop the car seems to idle high @1000 RPM and stay there for a few seconds and then comes down to again @750 - 800. Any ideas would be great and sorry for the long post.

Other info - other than the idle the car seems to start & drive fine. Other items replaced recently: fuel filter, new alternator/battery is new & was recharged, plugs/wires/cap/rotor have less than 16K on them. Fuel injectors were chemically cleaned while still in the car by a Chevy dealer about 3 years ago, soon as I get my fuel pressure gauge I plan on checking for a leaking injector.

I plan on checking the TPS again & pulling a couple spark plugs out to see if its running rich. Any other ideas?

Thanks in advance.

 

Sounds to me like one of 2 things:

1. Your throttles aren't closing all the way. Dirt & crud is stuck in them somewhere, or their shafts bind, or something.

2. You have a vacuum leak. A broken vacuum line, failed gasket, loose hardware at a runner, etc. etc. etc.

I'd be more inclined to suspect #1 since your TPS voltage is being held too high; which tends to indicate that the throttle position itself is being held too far open.

 

I thought I cleaned the throttle body pretty good, but I could have messed something up. Question: I have seen the voltage for the TPS listed as .54 vdc +/- .07 - I fall within that range, do you think that it's still to high? I remember reading on the site that several folks are running the TPS with voltage of .60 or more until they get a SES, to improve throttle response. I plan on checking it again, thanks.

Other info I just thought about:

O2 sensor replaced @3 years ago, I have one bad cat and my speedometer has gone nuts. It will work for a while, then will just bounce around or just not work. I don't think that it's my idle issue, but its just another issue with several articles on the site devoted to it. I love this site.

 

Your TPS voltage sounds fine. The throttle hanging open a little bit for a few seconds on accelerator letoff could be a dirty TB, but I doubt it. My old 305TPI did the same thing. I believe it's just the way the computer handles complete let-off deceleration and probably can be changed with prom tuning, but maybe not else wise. I never considered it a problem. I've got the opposite condition going on with my 383 SuperRam, especially with the A/C on, but that's a different thread.

I'm not sure you really have a problem. But your TPS voltage is fine. If you have a problem, that's not it. It possible the TB blades are open a bit far. They can be adjusted closed a little more to maybe help your problem.

Have a Great Day!!!


- Vern

 

The TPS voltage you have is not so much of a problem, in and of itself; the problem with it is, it's at the extreme edge of the acceptable range, and it won't adjust into the adjustment range like it's supposed to. In other words, it's trying to tell you that the throttle is slightly propped open. That's what the "minumu air" setting is all about; it's supposed to set the throttles to a realtively closed position, and then the IAC allows additional air in, to control the idle speed. If the throttles are hanging open, then the IAC can shut itself down all the way, and the idle will still be too high.

 

Thanks for the info, I'll check the TB again as well as the TPS.