Have you checked the EGR Valve for leakage? if it is leaking the valve will get very hot at idle right away after a cold start, also the intake manifold gaskets leak on these, it can leak under the manifold and on the top do a propane enrichment to see if engine smooths out at idle. I will bet the EGR or the intake gaskets are the problem, u also never mentioned the all important electronic engine control fault code test, go to the link for instructions on how to do this test without a scanner.
http://www.extreme-check-engine-light-codes.com/GM%20OBD1%20Decoder.htm
off idle it would run fine, have had 2 C4's now and pretty well know the problems with them, please do the checks i suggested.
there is nothing mysterious about propane enrich procedure, spay propane all around the top of the engine, looks for signs of change in the idle speed, also spray into the air intake, if the engine smooths out you have a posss under the intake leak. another thing have you cleaned the throttle valve and the Idle speed control air bypass valve? if they get clogged with gunk it causes idle issues
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If you have a no start condition or if the L-98 starts and then dies,
check the above items in sequence to see if all the events are
occurring as required.
A Scan Tool makes this job much easier and is a highly recommended troubleshooting aid for these sorts of problems.
Most of the C4 Corvettes used a MAF (Mass Air Flow) sensor to
determine how much air is being pulled into the intake manifold. The
exceptions are the 1984 Corvette that used a speed density system--a
sort of predictive method of measurement---and the 1990 through 1993 C4
models which were also speed density based. In 1994, Corvette went back
to the MAF based system but used the speed density approach as a back
up. (1989 Bosch MAF installation shown at right).
A Mass Air Flow sensor has an extremely fine wire inside its bore.
The 1985 through 1989 C4 engines used a Bosch MAF sensor that heated the
wire to 100 C. The 1994 and later C4 models used a AC/Delco MAF that
heated the wire to 200 C. The amount of current required to reach the
temperature is measured in each case. (Note: the LT-5 engine used in the
ZR-1 used a speed density system and continued to use that system in
1994 and 1995 since the engines had already been made prior to the last
two years of production. The ZR-1 therefore has no MAF even after
Corvette went back to the MAF based system).
Faulty MAF sensors will normally light the check engine light on the
drivers information center if the problem is constant and store a
trouble code. If intermittent, a trouble code will still be stored as
long as the battery is not disconnected.
Normally, the problem is a poor connection at the sensor and wiggling
the wires, unplugging and reinserting the connector will often cure the
problem.
A faulty MAF will normally cause a no start or difficult start
condition and although you can eventually get the car into the
"limp-home" mode in most cases, you need to attend to the problem ASAP.
This flow chart might help: Corvette86/FuelSystemDiagnosis.pdf
Hope this helps with your wrong; keep us updated.
Description: Logic module did not see a change in air/fuel ratio when EGR circuit was activated
Power loss light: Off
Limp-in mode: None
Possible Causes
EGR valve stuck open or closed - If the vehicle fails emmisions, then the valve may be shut. If the enigne runs poorly at low rpm or at idle, the valve may be stuck open. It may be the valve or the transducer valve. Test the transducer valve vacuum output. At idle, the transducer valve vacuum should be mostly bled, and there should be very little, if any vacuum at the output. When the engine is revved, the vacuum should increase. If the transducer is functioning properly, replace the EGR valve.
Bad EGR solenoid or Canister purge/EGR solenoid - The solenoid is located in the right, front fender next to one to three other solenoids. See the Understanding Solenoids page for information about how to test and repair solenoids.
Bad connection or wiring - Check the wiring and connections between the solenoid and the logic module (or SMEC). Clean and regrease any corroded connectors with dielectric grease and repair any shorts.
Check this TSB; I'm not positive but isn't there a diagnostic switch in this EGR system? A little single wire switch or sensor that is in the EGR port somewhere? Maybe it's bad. I'm trying to picture this setup in my head and can see this switch on it. I could be wrong but sounds like you've covered everything else. It's a fairly simple system until you get a problem that doesn't go away.
Also thinking that maybe the scanner is not compatible with this car. I know the GM scanner needed a special adapter to hook on Tech 1 to access a Corvette ALDL. One other thought, maybe the EGR "system" is okay and there is no exhaust gases flowing through the valve due to clogged ports?
I would go to a Chevy dealer and gat a copy of this TSB. It may help you troubleshoot the problem.
9216chevy10
Number: 91-327-6E
Section: 6E
Date: MAY 1991
Corporate Bulletin No.: 116515
Subject:
ENHANCED EGR DIAGNOSTICS
Model and Year:
1990-91 CAMARO AND CORVETTE WITH 5.0L AND 5.7L ENGINES
SERVICE UPDATE:
This bulletin serves to enhance EGR diagnostics (Code 32) in the 1990 5.7L (VIN 8) "Y" Carline and 1990-91 5.0L (VIN F) and 5.7L (VIN 8) "F" Carline Service Manuals, Section 6E3 "Driveability And Emissions."
Keep us updated.
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The ECM gives a 32 trouble code.....EGR system, but is it not more specific than that. would the vehicle drive well on the road with good acceleration, etc., if the intake manifold gaskets were leaking?
Thanks
I tried to Google "Propane Enrichment Test" but didn't find anything very specific ore useful. Do you have a preferred method that you could share with me?
Thanks aagain.
I've done both tests. Nothing discernible with the propane. It was running roughly all during the test, I sprayed propane all over the intake manifold and the idle never improved. When the engine was cold (overnight) I started it up and ran it for five minutes. The coolant temperature reached 143 degrees, the runners from the plenum were at 67 degrees, and the EGR valve never reached over 89 degrees. I didn't drive it all all from Friday evening to Monday morning. It started immediately and idled nicely for about five minutes, then the same old thing.
What's next?
Thanks
Rob
For a 1989 Corvette L98, basically stock. Hard to start, particularly when hot. Usually will die on first attempt after catching momentarily, second time will start but will have a very rough idle. Always have a strong smell of gasoline after starting, exhaust is black and sooty but not oily. Runs fine off idle, good acceleration, power, etc. Will backfire when coasting (letting off of the accelerator) especially from a higher speed. Have already replaced all eight fuel injectors. Replaced the EGR temperature sensor with a used one in as much as a new one is impossible to locate. I have a 32 trouble code in the system. Supposedly the fuel pressure is good, voltage is correct as well. I did a propane enrichment test the other day and found no discernible change in the rough idle. The EGR valve after idling from a cold start is only 89 degrees after 5 minutes of running, the runners are 68 and the coolant temp is 143 degrees. What other info do I need to provide to help you locate my problem?
Thanks
Thanks for your input. I had a failed MAF sensor years ago. The problem I am having presently is not the same. My trouble code is a 32 and the code for the MAF is 33. When it was bad, it stayed in limp mode both on and off idle. My car runs fine when on the highway, it just idles really poorly. It starts quickly when cold......just not hot.
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