I have a 91 Z28 thats having idling problems.....but only after closed loop. Here's a list of what Ive checked/replaced.

Timing(set at 10*)
ignition all new
new fuel filter
cleaned EGR valve
new 02 sensor

I checked the reference voltage to ALL sensors and was ok. I cleaned the IAC passages and the valve itself(didnt use TB cleaner on the valve). I checked all the sensors that I know how to check(MAP, IAT, TPS). My question is basically, what sensors if any, does the computer just use in closed loop??? I rechecked the 02 sensor(even put the old one in) and it was fine...ran the same way with either of them. The car starts on the first crank, runs good till it warms up. I have a 170 T-stat in it....but its been there for over a year, so I dont think thats what it is. I know this post may seem like its on the wrong board, but Im just curious what could cause it to run bad after its warmed up??? Could the coolant-temp sensor cause this? Also, I just replaced the exhaust from the headers back to where the catback hooks to.....BUT, the catback isnt on yet. SO basically my exhaust is from the headers to the "t" pipe where the catback would hook to. Would that cause a rough idle after warming up?????? The engine runs rich also.......I just wanted somebody to explain(if they can) how the sensors affect the idle on my TPI. Thanks in advance guys........

 

The difference in closed loop to open loop is that in closed loop the ecm uses feedback from the O2 sensor.

The ecm uses all the sensors all the time. Thou it might ignore the O2 at times.

During cold starts, immediately after a hot restart, and WOT the O2 is ignored.

 

Thanks...thats kinda what I figured but wanted to be certain. Based on what I said in the first post, what do you think it could be??? I double checked all ignition including the coil after the car had been driven.....no signs of it breaking down, along with the spark plug wires. Im really stumped on this one. It didnt do this till I put the new header pipes/cats on. I double checked for exhaust leaks also. Ive been told that the coolant-temp sensor can affect the entire fuel delivery system.....true? The only other thing I havent checked are the o-rings on the injectors.....maybe they are getting warm and leaking....but there are no signs of fuel anywhere. Also, could the charcoal canister "purge" valve affect the idle??? Please help....

 

kyl98,
What is your idle doing. Surging, barely running.
I was helping a bud fix his car. The minute his car would goto closed loop. his IAC would max out to 160 and car would barley idle at 5-600 rpm. I was stumped for a few days. Did all tyhe basic checking. I then programmed a chip for closed loop enable temp set to 260 degrees. Car ran great in open loop. Come to find out he installed hooker headers with the O2 in the #7 primary and that cyclinder was weak resulting ECM trying correct by adding fuel.
Adjusted valves. Installed O2 in the collector( better monitoring 4 cyclinders vs. 1) and car ran great. 90 corvette 7727 SD ECM.

This is just one scenerio. Can you post some datalog?
PS jusr reread your last post. Where is your O2? Far down stream with the headers. You may need a heated sensor???

Dennis

 

well...I went ahead and replaced the coolant-temp sensor....it was only $8 so I figured it couldnt hurt since the other was the original. The car idles fine in open loop, but after closed loop it starts idling rough. It seems to me like its "searching" for an idle speed, but it doesnt do it all the time. My 02 sensor is in the same place that it was in the manifold...actually may be a tad closer. Im starting to think its the IAC itself....but dont really know how to check it. I reset the ECM today after re-adjusting it for minimum air....and checked the TPS voltage and set it to .56 volts. I disconnected the neg batt cable and waited about 10-15min and reconnected it. Started car and idled great for about 5min....so I figured Id take it out on the open road and let the ECM "learn" again. All through the drive when I stopped it would idle "about" 700rpm in drive(at least on my factory tach). I got home and put it in park, and the idle went up to about 1100rpm in park. I also noticed when I was driving it that when taking off from a stop....SOMETIMES it would kinda hesitate for a second then go. I also noticed when it was running after I installed the new coolant sensor that when you revved it, it wouldnt go back to idle speed very fast....if you revved it to about 2000, it would fall back to about 1000 and then drop kinda slow......My dad seems to think its because the cat-back portion of the exhaust isnt on.....and is messing with the EGR valve. Is there a way to check the IAC motor???? I thought it would do it all the time if it was the IAC valve, unless it changes after closed loop. It doesnt seem to run as rich now that I changed the coolant-temp sensor. Ive also checked fuel pressure...it was ok.