Weird EGR Problem

My supercharged (Procharger) 87 has a weird EGR problem. When I went to have my emissions test done it came back with high NOX, well the L-98 seems to have some problems with the EGR passageways in the lower intake getting clogged and the SES light had come on a few time with a EGR code.
I thought, no biggie, just pop the plenum and runners off of it, clean it out, blow every thing out, new EGR valve (GM) and it would be fine.
I put it back together, put my new cat on that I keep for emissions testing and it still failed miserably, hmmm.
I put vacuum on the EGR valve and the diaphragm moves up and down but it does not change the sound of the motor at idle a bit.
So at this point I am thinking that something HAS to be clogged up.
I pull the lower intake, runner and plenum and had them hot tanked and ensure that EVERY passageway was free and clear.
With the intake on the bench and vacuum on the EGR valve you can blow into the port where the exhaust goes into the manifold and it goes past the EGR and into the lower manifold and plenum like it is made to. The EGR tube leading to the exhaust is clear also, you can blow past it with no restriction, and can feel and hear air coming out of the rear pipes.

This car has passed the test for the last 2 years with the blower on it and the anti-surge valve is working.
The only thing I can think of is that there is more pressure for some reason in front of the throttle butterflies (upper deck pressure) than there should be at part throttle, so when the EGR valve opens up the blower is pushing air backward into the EGR system.

I can not understand what may have changed or why the EGR system is not working correctly, IE letting exhaust gas flow into the manifold.

Any ideas?

 

I have the same problem with my '87. I replaced the EGR valve with a new GM unit (not aftermarket) and verified the passages are not plugged. I tee'd in a vacuum gauge at the EGR valve and drove the car with the gauge tied to the drivers wiper arm. I am getting vacuum at cruise rpm and no vacuum when I stop at a light. The EGR solenoid appears to be working correctly. I checked the plug wires for external damage and also checked them with an ohmmeter. All 8 plugs are the same color. I installed a new 3 way main cat (no precats). It still fails the Nox test. HC and CO are well within the pass range. The injectors all ohm out with good readings. I had them cleaned and flow checked last year at Cruzin Performance. It has a 180 thermostat and usually runs at about 185 to 190. I am looking for answers also.

 

This is a shot in the dark. Are you sure you have the correct EGR. I read somewhere, (here?) there is a normaly open and a normaly closed EGR for L98s depending what year car. If you have the wrong one it could be opening when it should be closed.

 

I researched this before I replaced the valve. The '87 uses a "negative backpressure" valve and has an N stamped on the top of the valve after the part number. I replaced it with an identical valve.
blown 87 ---- didn't mean to hijack your thread. I'm just as frustrated as you probably are with this problem.

 

Any help is welcome.

The part I do not understand is that when I put vacuum to the EGR and the EGR moves at idle it should "kill"the engine, it is not doing that.

It acts like there is no flow, like it is stopped up, but I can attest to the fact that it is open.

I got it to pass by running some water into the throttle body during the test, but it still is not right and I am at a loss on this.

The bad part about this is, we are the shop in our area that gets all the "wont pass" cars to fix and have fixed them all, but one, mine.

I am missing something here, something.

 

The test procedure for a "negative backpressure" EGR valve is different from what your used to. The valve will not open with vacuum applied when the engine is running at idle speed. The procedure (from my '87 Helms manual) is as follows:
Ign. "OFF"
Connect a vacuum pump to EGR valve.
Using a mirror, observe EGR diaphragm while applying vacuum.
Diaphragm should move freely and hold vacuum for at least 20 seconds.
If OK, go to next test.
Apply vacuum to EGR valve.
Start engine and immediately observe vacuum.
Valve is good if valve moves to seated position (valve closed) and vacuum dropped while starting engine.
If OK, check park/neutral switch (if auto trans.)

Do a Google search on "negative backpressure" EGR valves. I found some good web sites that explain in detail how this valve actually functions.

 

Not sure about the 89, but an 87 has an EGR switch, the EGR code on my 87 was intermittent, and with a little help here I discovered (according to Forum members) 95% of the time it's the switch...replaced mine and it solved the problem.

 

Sorry, No EGR experience here(threw it away), but I do have some info if your without the FSM.
http://members.shaw.ca/corvette86/EGR%20System.pdf

 

I guess I should have read the freaking manual

I just looked in my shop key and it says about the same test.

Got to a bit more checking, but having a valve that will not open at idle when vacuum is on it is what I have.

Thanks guys

 

My '87 has never set an EGR fault code. I have the Helms FSM and have done every test listed. Everything checks good but it still fails the Nox test. Sooner or later I'll figure this out, but I'm at a loss right now. I know Nox is caused by high combustion temps, but the car does not run hot. I've had the injectors checked, backed off the timing a few degrees, performed a leakdown check, and drove the car with my scanner taking data. All the numbers look good -- BLM's, Integrator, O2, EGR duty cycle, ect.

 

This may not be your problem, but we have fixed a bunch of cars with high NOX when every thing else is good by replacing the cat.
Even when they light up, the three way cats can lose the ability to control NOX.
That is why I keep a new cat just to put on to run the test on mine.
If all else fails you can rig up a plastic drink bottle with a needle valve and run some water into the intake in small amounts to cool the combustion chamber while taking the test.
Good luck.

 

I may have to resort to the plastic bottle set up. The main cat is new. I installed it after I failed the test the first time.