I know it's not an f body, but this is the most helpful forum for LT1 cars. On a 92 LT1 vette, car runs good and idles smooth until I hit closed loop, then it starts running rich and popping badly through the exhaust at idle. Using scan software, the BLM's drop to 108 almost instantly. It has a new set of Bosch 02 sensors in it (i know, they aren't the best). Could it be a bad set of O2 sensors causing this problem, or something else? Also, once the car is warmed up, the scan tool says it is trying to maintain an idle speed of around 675 rpm. This is a six speed car, and I thought they were supposed to be at 800 rpm for idle when warm. Stock chip in it. Thanks in advance for the help!

 

What's the engine temperature sensor reading (using scanning sofware)? What voltage are you seeing at the O2's?

 

Engine temp sensor reads 190 or 195 while driving on the scan tool. If i unplug it while the car is running, the scan tool shows -47 or something like that, meaning it is working from what i understand. I was aware that a bad collant sensor could cause an overly rich condition.

O2 readings are swinging up and down from 0 to about 900 while driving. If i unplug an o2 sensor, the scan toll shows around 457 for a reading, which i believe is the default start up reading till they warm up. The car runs better at that point, because it is in open loop, untill i plug the O2 sensor back inn, then it goes back to running rough again.

 

Are both banks dropping to 108? I wouldn't think you'd get a bad SET of O2's. Does the car have any mods?

 

yes, both banks are dropping to 108. and while driving, you will see on or the other jump up to around 147, then quickly drop back down to 108 and stay there. it's like the computer is trying to get it back to 128, but can't.

only mods are cat back exhaust, 1.6rr's, and a bbk 52 mm throttle body.

 

450mV is the voltage the ECM supplies to the O2 sensor circuit. If you unplug the sensor, the ECM is reading its own voltage.

An open circuit on a thermistor gives you infinite resistance. Typically the ECM will indicate -40degF on an open circuit.

Your long term corrections (BLM's) are fluctuating as you drive because the ECM is is moving through a series of memory storage "cells", as a function of RPM and MAP. You need to get the long term correction that corresponds to each "Cell". Often, you can gain some insight into the cause of the problem by comparing the long term corrections in the cells as a function of engine load and rpm.

If you want more info about the Cells, long term (BLM) and short term (INT) fuel corrections, read the air/fuel management section of my online scanner guide:

http://members.aol.com/InjuneerZZ/ScanMast.htm

 

What are your MAP sensor, intake temperature, and TPS voltage readings like at idle and WOT?

You could also have an exhaust leak somewhere. Exhaust manifolds usually. That would cause the PCM to think it was lean and dump in more fuel.

 

No map sensor on a 92 vette, and the air intake temps are normal. They read outside air temp, 65 or so when i start it, then it runs around 90 or 100 maybe, depending on how long the car has been sitting idling or in traffic. I guess the exhaust leak could be possible. any good way to check that?

 

A 92 should have a MAP sensor, but no MAS.

 

Sorry, mis-read that, the MAP sensor is reading around 9.5 or 10 inches Hg at idle and varies up to i believe around 25 while driving.

 

Perhaps it is actually running rich due to a fuel or ignition problem.

Does the vehicle have headers? If so it's easy to check for non firing cylinders with an IR thermometer.

Have you checked for fuel pressure holding capability after the engine is shut down? Perhaps 1 or 2 is leaking a little in between firing.

I had the same symptom and my coil was shot. Only indication was a tell tale white mark on the side of the coil. Changed that and ran fine for a day and a half. Then same symptom back again but now the right O2 is not switching.

 

It does not have headers on it. I thought about ignition or fuel as well and i swapped injectors between left and right sides of the intake with no change. It is true dual though and the problem is occuring on both sides. I checked with a fuel pressure gauge and it is holding pressure for quite a while after shut off. Also checked the FPR for fuel in the vacuum line due to a leaking FPR, but it was fine. The coil is a possibility i guess. Any good way to check it?

 

But his system is reacting to a perceived "rich" condition, by reducing the fuel flow...... that's just the opposite of what you described.

Your MAP and IAT readings are normal.

 

Are your plugs showing signs of misfiring? You could possibly have a bad pcm or chip. Do you know anyone you could swap with just to see?

 

Plugs don't look too bad. I am running a step colder plugs due to the nitrous on the car though, could that be causing a problem? I haven't been using the nitrous though since it is running this way. I thought about a bad pcm or chip too. Especially since it was trying to idle at 675 rpm. that just seemed way too low. Unfortunately i don't have anyone to swap with to try a different one.