Gurus, looking for some enlightenment regarding the EGR system.

Background: '92 coupe, 73k miles, stock LT1 save for a 52mm Performance Products throttle body, Magnaflow high flow cats, and a Corsa cat-back installed just this Spring. Plugs, wires and 02 sensors are all less than 3 years / 10k miles old. Stock tune. The car runs just about flawlessly.

On the way home from work last week, my SES light came on. Car continued to run fine, no issues. When I got home, I scanned it and found a code 32 - EGR failure.

It had thrown this code once before, a couple of years ago. This was right about the time I was chasing down an on-again/off-again high idle situation that turned out to be a flaky ECM. At the time, I wasn't sure where the code 32 would have come from, so I went ahead and pulled the EGR valve just to see what kind of condition it was in. It was obviously original and was really caked up with gunk in the passages, so I figured screw it, and I replaced it with a Borg Warner OE equivalent (part #EGR1098). Until just last week, the car hadn't thrown a code since.

So now, my curiosity is getting the best of me, and I'd like to understand this more. I dig out my FSM and start reading through the section for troubleshooting a code 32. From what I can tell the ECM will run a code 32 test, for the most part, when the car is under cruising conditions (speed greater than 50 mph, TPS within a low percentage range, MAP readings within a certain range, and EGR in an active duty cycle). It appears the test consists of the ECM stopping the EGR duty cycle for a short period of time, and if it doesn't sense a certain change at the 02 sensors in response, it will light up a code 32. Ok, got that. But one of the things I read is that the car is equipped with a "negative backpressure" type of EGR valve. So I start digging around on the Web and find all sorts of info on how EGR systems work and the difference between negative and positive backpressure EGR valves, and for the life of me, I can't seem to wrap my head around this negative/positive backpressure thing. I also find evidence that a high-flow exhaust system can play games with EGR functionality, but supposedly negative backpressure systems are somewhat less susceptible to this.

So basically, I'm trying to get clear on the difference between a positive and negative backpressure EGR valve, and also to see if anyone else has installed an aftermarket exhaust and seen EGR issues afterward. Any thoughts?

Ron

negative is vacuum.

positive is not vacuum.

Quote:

That's helpful :-)

Quote:

Actually, with Andy's comments and these two links that I found, I get it...

http://www.tomco-inc.com/Tech_Tips/ttt26.pdf

http://www.tomco-inc.com/Tech_Tips/ttt27.pdf

After reading those two links, it hit me that an EGR valve in operation is actually opening and closing rapidly. The negative backpressure valve uses manifold vacuum underneath the diaphragm to more or less "suck" the valve closed, while vacuum, when applied by the EGR solenoid, from the vacuum port above the diaphragm pulls it upward. The counteracting forces combined with a sealing/unsealing of a port in the diaphragm causes the valve to open/close rapidly. What they refer to as "dithering" in the links. I was under the impression that when the solenoid provides vacuum to the port on EGR valve, it held it open constantly. Nope. Vacuum applied at the vacuum port allows the valve to flap. With no vacuum applied to the port, "negative" manifold vacuum below the diaphragm, plus spring pressure, holds it constantly closed.

At least I think that's how it works. ;)

I'm thinking that in my situation, it's not necessarily the EGR valve or solenoid that caused the code 32. It was just that the ECM saw something non-optimal in the O2 reading at the time it ran the code 32 test, so it "blamed" it on the EGR.

It's been a while since I ran a Datamaster data log. Time to go for a ride.

BTW, I hope that other readers here aren't too put off by my lengthy/wordy posts. I know that I do have a tendency to get carried away, but sometimes when I'm troubleshooting or trying to work through a technical concept, it helps to write it all out, and bounce it off of others who know their stuff. My posts sorta' become notes to myself that I can go back to. Hopefully, others find them useful too. I work in IT, and do a fair amount of technical writing, so I guess it's just second nature.

Not in the least... It's posts like this that separate the men from the boys, so to speak.

I am glad my terse little comment triggered it for you. I kinda thought it might.

I have to agree, I thought the valve opened and that was that.

When mine failed, it held no vacuum, this meant that when it tried to open, it couldn't... and at the same time, it was so loose that I think it was just flopping in the breeze, so to speak. Meaning, it was being forced open by exhaust gases, and not staying closed as it should.

This leads to a combination exhaust and intake leak. Wanna say "It ran like crap?"

Posts only get too long when you begin repeating yourself or the pertinent facts. Your opening post was informative and seemed complete.

BTW, when you had the EGR off, did you happen to look into the passages the EGR uses. How clean were they. It could be all gummed up like the EGR valve itself was.

Bob - It's funny, but the ports on the intake, tho black and gnarly, are usually ok... it's the bends around the valve that get coked up.

Counterpoint, tho... Honda engines back in the 90s had issue with EGR. They used a rather high-tech solution for EGR injection; these little 1/8" diameter holes that ran above the intake runner. They would get coked up and clog. There were two fixes... one worked on the I-4's, and that was to drill out the casting riser, and drill the tubes clean. The other option, and the only fix for the V6, was to remove the intake and hot tank it.

Quote:

I had the EGR go out, it did happen when I switched to a 3 in. chambered cat back but that was just coincidence since the new valve's been on 5 years with no codes.

Quote:

I do recall looking into those ports and I do recall them being a bit sooty and kinda' nasty looking. I thought about what I might be able to do to clean them a bit, but pretty much realized that short of pulling the intake off, there's really not a lot one can do about those. I didn't think it was a good idea to try to clean them with the intake still on for fear of bits of gunk falling back into the ports.