1983LU5TA

Should the EGR valve on an 83 LU5 305 Trans Am be a positive or a negative style? My GM FSM doesn’t specify. I have been battling a light throttle misfire and have tracked it down to the EGR valve. The one on the car is an aftermarket positive style. Vacuum is applied a few seconds off idle. With the EGR disconnected and the line capped, its smooth as can be. I have owned 15 thirdgen Trans Ams and have never seen a positive style EGR. I blocked it off and took it for a drive, and it runs beautifully, however an occasional ping can be heard.

Schurkey

Find the OEM part number for your application. The Genuine GM valve should be labeled "P" or "N" if it's a Positive- or Negative-backpressure valve.

I dislike backpressure-sensing valves on general principle; any changes to the exhaust system has the potential to throw off the EGR calibration. But we don't get to choose how GM engineers the vehicle.

Huh? There's a timer in the EGR vacuum plumbing???

1983LU5TA

Correct. There is a timer. After roughly 2 seconds of part throttle/off idle operation, the switch clicks and allows vacuum to the valve. As far as finding the number to the correct valve, that’s what I’ve been trying to do. If I had the number, it wouldn’t be a big deal determining what valve is correct. As I said, I have owned 15 thirdgen Trans Ams, and I have always seen negative EGRs. I don’t remember what my 82 LU5 had. It’s been a few years since I owned it.