KYL98

Im having some idle problems with my 91 Z28 L98/700R4. It idles fine until it goes into closed loop then it idles really rough...almost like a dead miss sort of. It sometimes stumbles from a dead stop also(only after closed loop). Here is a list of everything I've changed....

02 sensor(Bosch)
coolant temp sensor
spark plugs, wires, cap, coil
checked timing(8*BTDC)


This started after I replaced the 02 sensor and adj for min air on the IAC valve. I did do something stupid though...and started the engine with the jumper between A & B terminals on the ALDL. I have checked the TPS and MAP sensors...both were fine. On my laptop, it shows everything to be working ok....the 02 voltage jumps around alot, but everything shows ok. My fellow co-worker checked it on the Snap-ON scanner and everything appeared to be working ok....no codes either. Im wondering if I may have gotten some cleaner in the IAC or something..??? Anything else I may be overlooking? Did I mess up the ESC module by starting the car with the jumper in?? Any suggestions would be appreciated.......

BTW, does the computer only reference the IAC valve after closed loop?? Is there anyway to check the IAC to make sure it works ok? ...... Help......

KYL98

Anybody?? I need to get this fixed before this weekend... A friend of mine said my injectors could cause this if they are leaky...or if the windings are shorting out.....???

IROCThe5.7L

Maybe your MAF?

KYL98

Dont have a MAF sensor....its Speed Density....uses the MAP sensor

Air_Adam

Idle speed set too low? Maybe shot knock sensor? I think (not sure) that if the knock sensor goes on permanent vacation, the computer will just run it as rich as it can, because it doesnt know whats going on and doesn't want the engine to grenade unless it tells it to.

KYL98

hmm didnt think about the knock sensor...but wouldnt that do it all the time, not just in closed loop? Ive heard that the Bosch 02 sensors arent as accurate as the ac delco, so maybe thats it.....IM about ready to pay some serious money and have the car hooked to an engine scope to see what it is.....any other suggestions?>

IROCThe5.7L

Whoops, didn't look at the year.

KYL98

OK, did some datalogging today....Looks like its something fuel related.....BLM's are right around 118-120 on a cold start up....and the INT is at 128...BUT right before closed loop and afterwards, the BLM's go to 108 and its stuck in cell 4. The INT goes down to 118 and bounces around 108, etc. Sound like the Bosch 02 sensor is junk???? Its only a couple months old.....but I heard it can't read fast enough?? Im gonna try replacing it with an OEM ac/delco and see what happens......

87kevroc

Based on your description of the problem I would think either o2 sensor or ECM. I'm running a Bosch sensor no problems, and thought it was OEM.

KYL98

Well, I replaced my 02 sensor....seemed to run good, at first. Then after a few minutes of driving I was sitting at the light, and it started idling rough again. It still sometimes hesitates for a second from a stop....like there's a dead spot in the throttle. I have checked and rechecked the voltage on the TPS and checked for and flat spots in the voltage....both of which checked out ok. ITs weird, because other than the ****ty idle, it runs ok, except for an occasional slight "surge" when in 3rd gear going about 45 or so....nothing severe though. It has all kinds of power, and gets decent mileage still. I cleaned my EGR passages last fall thinking that was the case....but no luck. The EGR valve looked clean, I blew it out a little but used no cleaner inside the valve's body. That made no change though. Would a Knock sensor cause this to happen only in closed loop? Im not showing a bunch of knock counts on the laptop. The only thing left would be my fuel injectors....which when I checked the resistance only 2 of 8 were in spec. Would that cause my idle problems? I just have a hard time believing JUST the injectors would cause that, being that the problem seems to happen "about" the same time, right before it goes into closed loop. Im running out of options....and dont really want to start throwing parts on it without knowing the cause. Would the program on my laptop(ECM852) show a faulty ECM? the program works ok when hooked up to the ALDL....it shows all data ok. Should I check the EGR to see if thats it? Or, give it to somebody and let them mess with it?????

tom3

What is your scanner showing for the IAC counts at hot idle? If it is below 15 or so I'd guess a vacuum leak somewhere. Might also see a higher than normal idle when cold on first startup.

KYL98

its only showing the IAC counts between 3-10 at idle in operating temp....so that means a vacuum leak?? Or is the TB blades open too far?? Theres only about two miles of vacuum hoses...so it should be easy to find..lol

dimented24x7

iTrader: (2)

IIRC, at idle its supposed to be stuck in cell #4. What is the engines vacuum doind while in open and then in closed loop? How does the exaust smell at idle? it certanly sounds like its a possibly egr valve problem. Id be tempted to say the IAC as well but that would show up as soon as the car started to idle down. Could check it, though, by setting the min air to allow the car to idle at its stock speed, extending the IAC, and then pulling the plug and see how it does. This might point to a funky IAC, but is not an absolute indicator that its bad.

As for starting it with the paper clip in, that wont harm the computer, itll jsut be in field service mode.

KYL98

I was beginning to wonder about the EGR myself. When you first start the car, the exhaust smells ok..kinda rich, but its usually kinda cool outside, so that's pretty normal. After the car warms up though, the smell sometimes gets stronger. I did notice last night that while driving in OD under light throttle, the car made a "bucking" motion.....not sure if it was just the TQ converter unlocking or the engine hesitating. The idle Im talking about kinda feels like a dead miss, like its not running on all 8, but I have verified that. Could it be a stuck valve in the motor or anything?? I kinda ruled that out also, because it wouldnt do it RIGHT before closed loop everytime. Another friend said it could be the Canister Purge valve stuck open or closed, BUT I took a vacuum cap and blocked off the hole in the TB where it goes, and no change in idle quality. What are some characteristics of a bad EGR?? Im thinking if too much air from the EGR gets back into the combustion chamber, it'll make the mixture too weak or lean......would that explain my rich condition showing VIA 02 sensor??? IS there a way to disconnect the EGR just to see if it's the culprit?? My MAP sensor shows about 40-45 KPA @idle and usually around 1.6volts. The IAC counts are around 5-10, which is ok....they can be anywhere from 0-30 depending on how things are set up. Man this is annoying... Im going to try looking for vacuum leaks anyway today....Should I just use some soapy water in a spray bottle???

dimented24x7

iTrader: (2)

If its getting a strong odor at idle that really points to an egr or fuel problem. Try unhooking the vacuum from the egr so its disabled and see waht happens. Since your car is SD a vacuum leak would be like jsut opening up the throttle and giving it more air unless it was right at the base of the intake where it meets the heads.

KYL98

Im going to recheck the fuel pressure tomorrow...and try disconnecting the EGR and see if that changes anything. Also, since my fuel injectors are original and they are basically shot(6 of 8 anyway ) Im gonna replace them with the SLP 25lb er's....maybe one of the seals on the fuel injectors are leaking......I just kinda ruled out the vacuum leak because the MAP was reading like it should....but at this point...Im NOT ruling anything out....lol

JulieGTA

I had the exact same problem with my car. (91 L98) Out of the blue one day, it was going dead rich as soon as it kicked into closed loop. After a long and thorough search, sensors, leaks, etc, everything checked out ok, except when we checked the resistance of the fuel injectors, there were 2 dead on either side. In other words they were stuck open. Fixing them solved the problem.

KYL98

sounds about like mine....what were the readings for your "dead" ones?? all but two of mine are NOT in spec. I tried playing with the IAC today. I let the car idle WITHOUT the IAC, same results. I tried a brand new TPS(just for kicks) and no results. I also capped off where the canister purge goes into the TB, no change either. I even pulled the vacuum line off the EGR(line from the solenoid) and no change......The injectors are the only thing left. Just for kicks though Im gonna do a compression check and check the fuel pressure(again). Im also gonna double check all vacuum lines, etc while Im waiting for the injectors to get here..lol

87kevroc

When you installed your o2 sensor, did you use a torque wrench? The spec is 30 ft/lbs. Maybe not tight enough, and an easy thing to check.

87kevroc

double post

KYL98

Nope...didnt use a torque wrench...but didnt get it too tight or too loose. The car idled the same with the O2 sensor unplugged....and Ive changed it numerous times with no problems....Im in the process of changing the injectors. Most of the seals for the injectors on the Intake side were dry rotted, so hopefully thats the culprit.

85Rock

I am having similar problems. did you ever get this fixed ?
and if so what wasit? thank you for the reply

Kris