Hi All,

I have a query with anyone who has experience with 1990-1991 Corvettes with regards to the EGR.

My 1990 runs perfect until its warm then idles rough to a point it cut outs, surges when moving. If the temperature is cool it runs perfect and can run for 30-45 minutes. After changing out the coil, ignition control module, rotor button, verified old distributor cap is fine and replaced the TPS, the problem exists. Since there are no codes on the computer, I am beginning to think my EGR is sticking open causing the issue. However, I have a question?

Started doing this pretty much after I changed the rear section of the exhausts with Magnaflow mufflers. I've read that the 1990/1991 EGR can get issues when an aftermarket exhaust system is installed in that doesn't allow enough back pressure to allow normal operation (ie egr doesn't quite close).

Question is, could this be the tipping point (the straw that broke the camels back) on a 27 year old worn and carboned EGR sticking on throttle close and/or slow throttle speeds??

If so will replacing a new EGR and cleaning the exhaust tracts into the EGR solve the problem?

Thanks in advanced.

 

Why do you think it is the EGR if you aren't getting codes? I don't know much about EGRs because mine fell off but I honestly don't believe that the mufflers are going to be that critical. IF anything, it would be the exhaust manifolds if you replaced them with headers.

Why did you change the coil, ICM, rotor button and TPS? Please don't tell me you are throwing parts at it in hopes that something sticks and the car works. Lets start with the basics. I can also speculate that your injectors are Multec and probably aren't worth a damn but I don't know if you have ethanol in your gas at all. If something is wrong, I'd suspect them since they are probably crudded up some after all these years.

So lets start with the basics. Spark and Fuel. Do you have a steady and blue flame? Are your wires leaking? Did you verify fuel pressure at WOT and with the key on but engine off? Is the timing right? Do you have a scanner? If not, get one.

 

aklim,
Sorry should of confirmed a few more things.

Injectors have been changed to Bosch 3's and I am not running ethanol fuel. Spark wires are new as is the spark plugs (done a few thousand ago). Timing set (est wire disconnected) to the factory 6 btdc).

Yes I need to invest in a scanner, which one do you recommend?

 

Then I don't care. Ethanol can screw up Multecs and you changed them recently so they should be ok.

I have a Snap On MT2500 that can read my car. Some of the newer ones might. Just buy one which can read data streams off OBD1 and see if it works. If not, return.

 

CEL code 32 = EGR.

Cold engine disconnect the EGR @ the switch it is bolted to the thermostat housing.
You will probably get a CEL 32 but if the car runs properly after the engine is hot the EGR is probably the issue.

If no change the IAT &/or the CTS sensor is suspect.

 

Here in Virginia like most of the country we have our cars emissions tested every two years. Even though I am not required by law (car is over 25 years old) to have it tested I still take it in and have it done. I like to know that everything is in good working order.

My 1988 Coupe was running okay but when I had the emissions tested the NOx numbers were way out of range. It failed the emissions test so I went home bought myself the new EGR, EGR sensor and replacement parts. After changing it all out I still had minor issues. It turned out that the little vacuum solenoid that turns on and off the EGR had failed. I replaced that and now the emissions are back to normal.

If you are going to change your EGR valve be sure to start putting a good penetrating oil on all the hardware a week before you start taking things apart. This is a place where things like to break off.

Just be sure to check your vacuum hoses at every point in the system. Old dried out rubber lines do not seal very well. For what it is worth my car never gave me any codes indicating a problem.

 

Problem is that the ECM is very primitive and won't give much warning unless it is way out of spec. You are going to need to see what the sniffer says to get emissions and see if there is an air leak with the IAC.

 

I re-read your post this morning and I had another thought(s).....

It sounds like your car is starting okay in the "Open Loop" mode and running properly while it is in that mode. The trouble seems to start when the car switches or attempts to switch to the "Closed Loop" mode. In the "Open Loop" mode the car uses very few sensors until the O2 sensor warms up and then the transition to "Closed Loop" occurs. However if your ECM doesn't see the correct data it might be trying to correct for the erroneous condition that exists.

For the car to work properly in the "Closed Loop" the ECM needs to see the information provided by several sensors on the car.

One of the most frequent places we see problems like this is when the Temperature Sensors fail. Have you verified that your engines Temperature sensors are working properly? They are easy to test as they will have a specific resistance value when at a specific temperature.

What about your O2 sensor on the exhaust system? Is it giving the proper readings? A bad O2 sensor can drive you crazy if they are intermittent. You will need to be able to monitor the output voltage of the sensor if you don't have a good OBD1 system that has data streaming. It is possible to check the output voltage of the O2 sensor with a decent multimeter.

I had a intermittent failure of my O2 sensor on a Pontiac and I would be cruising along and bang the car would drop into "fail-safe" mode for a few seconds and then the sensor came back online and everything went back to normal. My car would buck, surge and ran like poop in the fail-safe mode. It would barely idle and was obvious that something was wrong.

I suspect that one of the prime sensors that the car uses for "closed loop" operation is what is plaguing your car. Do you have a Factory Service Manual for your car? I could copy the pages out of mine if you need them.

When you replaced the exhaust system the problems started, you said at one point. Did you replace the entire system or Cat back? If you did the entire system then it is possible that one of the wires might have been removed and not put back properly. The temperature sensors are not far from the manifolds.

Start at the temperature sensors and re-check the voltage at the TPS and verify that the IAC is working properly. Check the wiring connections and connectors as they dry out and break off which lets them get loose.

Someplace your car is getting bad data from an important sensor. On the 1988 model there are two temperature sensors, one for the gauge readout and the other tells the ECM what temperature the engine is. If your temperature read-out is correct/close on the dashboard then the other sensor could still be telling the engine that it is at some ridiculous temperature. This has haunted many a Corvette Owner and is responsible for countless starting issues.

When I removed and replaced my EGR system I found the passageways in the manifold full of carbon. I ended up taking off the entire intake system to clean all the carbon out. As expected the pintle in the EGR was plugged as well. Fortunately I was able to sand blast the manifold to get it clear of all the carbon and then I re-painted it and re-installed it.

Check out this article regarding the operation and associated problems with EGR's.
http://www.tomco-inc.com/Tech_Tips/ttt26.pdf

You might want to check out this website, they have some great information for "We the mechanical few" of this world.
http://www.tomco-inc.com/techinfo.html

Bad Alternator Diodes: A bad diode in the alternator will cause an AC signal. This "confuses" the ECM and affects operation. This came directly from the TOMCO TECHTIPS, just another possibility. Be sure to check out that website, it is great and full of useful information!

By the way, when you refer to it being "Cool" in your end of the world most Americans are not familiar with your definition. What is "Cool" in Australia? I lived in the southern hemisphere so I know that your Corvette season is just about to start down there while we are prepping our cars for a long sleep in the garage. Do you have seasons like we do or is it the "Dry season" and the "Rainy season"? My recollections of Vietnam are getting fuzzy but it did rain and it was HOT, frequently both at the same time.

Again, I hope to have helped in some small way for you to get out there and enjoy that beautiful Corvette of yours!

 

 

I would use an infrared thermometer and scan the sensor itself and get the data from the ECM to see what it sees. So if the sensor reports 200 degrees and the thermometer reports 195, it's good. OTOH, if the sensor reports 120 and the thermometer reports 210, well.....

I would say that if it is 5 years or older, replace it. It can also get "lazy".

Isn't there only 1 wire for a 1990? While we are on that subject, if he changes the O2 sensor, why not do a 2 or 4 wire one? What temperature sensor isn't far from the cat back system?

I'd agree with him chasing it with a scanner so he can see live data as he goes. TPS might have a dead spot. Also the Oil Temp sensor should be checked as the CTS and the IAT with the thermometer vs scanner.

You are a lot more patient than I. I'd have wiped out the code and closed off the EGR if they didn't do visual emissions checking.

 

Hi All,

A big thanks to aklim and ctmccloskey for posting to try and help.

I changed the EGR valve over the weekend (found a source of a small oil leak in the manifold and a coolant hose that was rubbing) but have not had suitable weather to test it properly. Test drove it and all is well since it was cool and I no longer have the slight pinging on initial throttle like i use to.

First of all, would this do the job for my 1990 scanning via a laptop? Anyone used this mob and/or software before? https://www.ebay.com/itm/GM-OBD1-Sca...cAAOSwiYFXGESA

ctmccloskey,
You make a very plausible theory with regards to close loop operation. What i need to do (since I do not have a scan tool and will take a while to arrive here in Australia) is test the CTS sensor when the car starts running rough after its warm. As a background a few years back when I got the car I changed out the CTS whilst draining the coolant. Aware that its probably irreverent information as this is already 5-6 years ago. I have a brand new 02 sensor at home and it has never been changed in its life.

Some questions I will answer below:
The exhaust I changed was the rear mufflers only. When I described cool, it was under 70 degree's Fahrenheit and in the dusk/evening. At night under 70 deg f ambient temp I can run the car for 40 minutes no worries and it will run like a champ.

The car would start to run rough after a drive, in 77 degree Fahrenheit and idling in traffic sitting in the sun. Upon take off it will sputter, surge and ultimately stall until it cools again.

Ultimately its time for me to get a scanner to know for sure whats going on with the sensors live when the car plays up. Whilst waiting, do i change out the 02 Sensor and CTS?

Cheers,
Gary.

 

I'd change the O2 but the CTS, I'd wait since I wouldn't want antifreeze on the floor but if not, I'd change the coolant if I was getting it on the floor and it was old. IIRC, MB recommends flushing brakes every 2 years and coolant every 3.

 

Personally I would change out O2 sensor lacking a scan tool. If you have the C68 climate control there are some things you might learn there though would expect little of use for your problem but just might offer a clue.

 

Hi All,

Finally got my 1990 going. Issue was the fuel pump. Lesson's learnt, I should of measured the fuel pressure and observed from the onset. In my defense, the symptoms were only present after a warm drive. Towards the end of the diagnosis, the stalling was happening from a cold start and almost immediately which helped the trouble shoot.

Ironic this Rockauto reproduction fuel pump was only 2 years old. Must admit, the new fuel pump has a more smoother and silent whirr upon key on compared to the one I changed out.

Lastly, I had an additional teething problem of a faulty brand new AC Delco TPS causing a very high idle (1750rpm) and I did an apprentice mistake of a kinked intake runner gasket. Put back my 28 year old TPS and all is well.

Performance is great and in the same driving conditions, I am getting better gas mileage most likely due to the new o2 sensor and new EGR valve.

Thanks All for Helping.

 

So I went to drive the Vette to breakfast Wednesday and I believe i now have a failed EGR system. The car will barely start, runs horrible and stalls. I have not had an issue before this beyond an occasional Check Engine Light
after longer drives that always leaves a H32 (EGR History) code. I jumped the correct pins to read the codes and I have 1 - C12, 4 - H32, 7 - Err, 10 - ---. I read through the Service Manual and the symptoms all point to the EGR
system, hoping to troubleshoot this weekend. Does this sound like the correct diagnoses based on these symptoms? Can I get the EGR valve off without removing the intake? Any other suggestions for diagnosing?

Thanks,
Chuck