I have a 1984 Pontiac Trans Am LG4 Auto 4 Barrel Carb. All Stock. I think I have an EGR Issue because I am having some symptoms related to a flaky EGR.

My current EGR Valve (Which I believe to be original as the car only has 90K miles) has the OEM number on it then an "N" at the end of the OEM number. I have read this means "Negative Backpressure". I also understand (if I read correctly) I should be able to apply vacuum with my vacuum pump and it should hold vacuum and also move the diaphram up and down. It does not do that when I apply vacuum (neither hold vacuum nor move the diaphram) with my vacuum pump. Now I can manually move the valve from underneath with my hands so I know it is not "seized" but I cannot confirm if the diaphram is torn. I also read that if it was "positive backpressure" EGR it would not hold a vacuum nor move the diaphram with a vacuum pump, only with engine vacuum.

So I am a bit confused. To make matters worse, every parts store I go to (napa, autozone, etc.) sell the EGR but it says "positive backpressure" on it and nobody seems to have a "negative backpressure" valve. I am guessing since mine has a "N" I need a negative valve and a positive backpressure EGR would not work. If I am incorrect there please let me know.

So my questions are:

1) Does my "N" at the end of the OEM part number on my EGR valve mean "negative backpressure"

2) Is it true that a Negative Backpressure EGR Valve if vacuum is applied with a vacuum pump it should hold vacuum and move the valve? and Positive back pressure EGR valve would not? If that is the case (and a negative backpressure should hold vacuum and move the diaphram) I feel I have found my issue.

Thanks for any advice/help .....

 

I also have an 1984 Pontiac Trans Am LG4 Auto 4 Barrel Carb.
Positive or negative backpressure I can't say.
But I can tell you that my EGR valve could be tested with a vacuum pump and the diaphragm would move open with vacuum applied.
It sounds like your diaphragm has a hole in it.

 

You should be able to operate the valve by sucking on it. It sounds like yours doesn't work. This was the part number on mine:



I replaced it with a Standard Motor Products EGV393 from RockAuto because I couldn't find an oem one, it seems to be working. According to my datalogger the engine has requested egr and I didn't notice anything when it did, anyway.

 

So I resolved this issue. Figured would update for others. If you have an erratic idle, vacuum leak and you cannot find it anywhere, be sure to look at the EGR Condition. Mine 100% did have a failed diaphram which was causing a vacuum leak. Also there is a definite distinction between Negative and Positive backpressure EGR's and cannot be interchanged (well they can but it just won't work correctly). I ended up replacing my OEM Negative backpressure EGR with a Wells which I bought from Autozone number EGR141. It was a bit amazing nobody I talked too at parts stores could distinguish between negative and positive backpressure so i ended up calling WELLS who told me the correct part number and who sold them.

Figured may help someone else who is chasing these issues......Thanks for everyone's input.

 

Did you speak with Brian Garrow at Wells? He's pretty astute, and could be a wealth of information. He may have retired by now, but if not and you got him, you very likely got the right/best answer.

 

Not sure who I spoke with initially. They did not know answer at first (negative or positive) but researched and came back with the answer about 4 hours later. Very thorough and was impressed with them.