I usually post on F-body.com but that site seems to either be gone or constantly under maintenence. Here's an overview of the car:
1994 T/A LT1 A4 355 build with AI 200 cc heads, mid 220s cam, 113 LSA, stock intake, SLP shorty headers, Magnaflow cat, 3200 stall, 850 RPM hot idle in part, 800 in drive. It passed smog 2 years ago.

I've been having a problem getting my car smogged this year. The first test I had was pretty bad...HC in the mid 60s, NOx in the 600s to 700s. I then changed the plugs, fixed a possible bad plug wire, went to a tune where I passed 2 years ago, and cleaned the EGR. The HCs went down to high 40s and NO went to low 600s for 15 mph, high 400s for 25 MPH.

So I fixed a possible exhaust leak, put in a new Magnaflow cat, made sure the tune was 14.7, cleaned the MAF and air filter. The new readings were much better but still failed:
15 mph: 13 HC, 0.01 CO%, 577 NO, 14.45 CO2%, 0.62 O2%
25 mph: 11 HC, 0.01 CO%, 528 NO, 14.50 CO2%, 0.56 O2%

So since they changed the smog limits on my car, it failed with these numbers. The NO is too high (max is high 400s). I can only figure the EGR is having issues for the high NOx. I also noticed my CO% is way down and O2% looks pretty low.

So I did Shoebox's test. At idle, I depressed the EGR diaphram and noticed a very slight change in RPM, but not much. I continued depressing again and again, then nothing. No change in RPM. So either the EGR is not working, or the cam is preventing it from stalling due to already having overlap, or maybe a high idle? I have no idea on that, but I was wondering, does this test work for cammed cars? Any time I depress the diaphram, it's supposed to change RPM and/or stall right?

Thanks in advance.

 

If you open the EGR valve at idle, the exhaust gas flows into the intake manifold, displaces air, and throws the A/F ratio out of whack. Doesn't matter whether its cammed or not.

f-body.com is alive and well. Delete your cookies and try again. You aren't the only one who has reported this problem though.

 

Thanks Fred, I will try to delete my cookies or try to access from home. I'm glad it's not down.

And ok, so it sounds like my EGR is bad, or maybe the passages are plugged. I tried to clean it after I failed the first time, but I don't know how deep the passages go through? Maybe I'll try to find a diagram of how the hole looks like. Either that or maybe the EGR has a hole in the diaphram. But the car runs pretty good and it doesn't show signs of a vacuum leak like that.

 

How is the corrugated tube that runs to the intake manifold? That may have a crack in it. There's in internal passage that connect the flange where the corrugated tube connects, and runs to the EGR valve. The pintle on the EGR valve controls the opening and closing of that passage to the passage that runs under/inside the bottom of the intake manifold. I can't remember if the EGR combines with the idle air, or dumps in somewhere else. I blew compressed air the passage, but that was more than 10 years ago, and I can't remember where it came out.

 

The tube that runs to the intake had a small split on one end. It was very small, but maybe enough to create a small leak. I tried to weld it, but it's so thin that the welder melts the material completely. But I did get it to where I think there's no leak and inspected every single passage and the EGR valve itself. There is no blockage - very minimal dirt and blew through a tube out of every passage. I blew threw the corrugated tube hole in the manifold and air came out of the hole on the intake where the EGR valve goes. The header to corrugated tube flows well too. I blew through the EGR when pushing the diaphram and it blows well. I also tried to start it with the EGR off, proving that if it did start, then the EGR to intake air passage was blocked. But it didn't start at all - so I assume the square EGR passage was fine. I also put 12 v on the solenoid and could hear it "click".

So I put everything back in place, started it and pushed the diaphram again. The idle characteristic did slightly change. It ran a tiny bit rougher, but idle RPM didn't change at all. And not even close to dying. I even stuffed rags in the exhaust, hoping to create backpressure, then pushed the diaphram, and still hardly any change after bottoming out the EGR. Just a tiny bit rougher.

I'm not sure what to do next. Maybe the 850-900 park idle is too high for it to die? Or maybe something else is causing the EGR effect to be so weak. I just can't figure it out.

Thanks as always for your help, Fred.

 

So one thing real quick that I also noticed....the LT1 edit files show very strange things for my EGR parameters. Here they are:


For EGR On, I'm not sure if the MAP and RPM is an AND or OR statement, but if it's an AND, then the programmer disabled my EGR due to the RPM requirement? Is this this case, or is it an OR statement where it turns on with MAP > 41.9? A stock file appears to be the same MAP but 900-2100 RPM. Also, I noticed that DTC 27 (EGR solenoid) under the hardware verification tab is set to "No". DTC 32 (EGR system failure) is also set to "No". A stock file I downloaded has them both at "Yes"

But I do see that partially through the smog test, Datamaster shows EGR DC at 12.5 pulsating. Not all the time, but some of the time. Can this show voltage, yet LT1 edit still disabled it?

 

So just to give an update, I went ahead and put the stock EGR On settings from a stock LT1, which was:
MAP 41.9
RPM > 900 and < 2100

I ran a scan on my way to work and found that the EGR DC is now 100% during those conditions. Prior it was flashing 12.5 continually on, then off, then on, then off. Now, it's 100 and holding steady. I'm a little confused though. Fred's scanmaster help page said Scanmaster readss the pulse modulation, and the solenoid is pulse modulated. That's why I thought the prior reading of 12.5 flashing on and off was correct. But now, with the stock settings, it reads 100% and doesn't turn off until the EGR Off parameters are met, or if the EGR On parameters are met.

So my question is, since the solenoid is pulse modulated, am I supposed to see it flashing on and off for every record in Datamaster? Or is a constant 100 reading under those conditions correct?

 

100 means the PWM is 100%. 12.5 means the PWM is 12.5%. From the settings above, is appears it would only turn the EGR valve on when RPM was greater than 6375 RPM (max binary value that can be entered = 255 * 25 scalar = 6375). Your system appears to have been disabled by the programmer, including the diagnostic. Note that I do not tune the LT1 PCM, because I don't run my engine with an LT1 PCM, and haven't since 1999. So there may be another interpretation, but this is my take on it.

I'm still puzzled there is no stumble when you move the valve. Did you verify at any point that when you press on the valve, the pintle actually moves? Feeling air flow would seem to indicate it is.

 

I am puzzled by that too. To test it, I pulled the EGR completely off and started blowing into the ciruclar tube where the exhaust gas goes in. I then depressed the diaphragm when blowing and felt a ton of air go across the square hole, which is supposed to be where it let's air into the intake.

Sorry, I'll describe it a little more...I did feel a slight stumble, like a change in the smoothness for the idle, but it wasn't much and the RPMs didn't change.

And thanks for the interpretation on the program, I calculated it the same way too. It looks like the programmer did remove EGR. When I put it back in with the stock EGR programming, and see the constant 100% under the given conditions, it still worries me that I'm not seeing flucuation...just 100% until I let off or go away from the RPM range.

I also re-enabled DTC-27 checking, and I am now getting DTC-27, EGR solenoid fault. I hope that the constant 100% DC didn't fry it. I'm just not sure what to see in a stock LT1 for the EGR DC...if it's constant or flucuates. I'll test the solenoid when I get home using Shoebox's method. The only question I have with Shoebox's method is he writes:
For testing purposes, grounding the DLC output/field service enable terminal (1994-up), with the key ON and the engine not running, will operate the solenoid and allow vacuum to pass to the EGR valve.

Yet, in the DLC output chart for pre-1995, I don't see a output/field service enable terminal. So I'm not sure how to do this part?

 

Stock EGR reading is not always 100%. It varies. Generally appears at higher MAP and lower RPM. With your cam already producing a high MAP, you may need to bump the MAP minimum enable up a bit.... but I've already warned you.... I'm not an LT1 PCM programmer. Just using logic.

 

Thanks Fred. Your advice is always appreciated. I'll look into seeing if I should bump the value, or just maybe leave that program for smog only.

As far as the EGR solenoid, to get DTC 27, that means the PCM detected a circuit fault in the solenoid? I wonder if that means that the solenoid isn't working at all, or if it could throw the code for another reason.

 

As far as I know, all it does is check the voltage on the circuit, and see if it matches the commanded solenoid state - 12V when the solenoid is supposed to be closed, and low volts when it is supposed to be open.

Check the resistance of the solenoid. I think mine was about 15 ohms. Make sure you have 12V on the brown wire in the harness connector. The PCM provides the ground via the gray wire.

 

Thanks Fred, I'll check that tonight. If it checks out fine, I'll revert back to programming DTC-27 out so it won't display the SES light for it anymore.