In my 88 GTU, I tested the voltage on my atmospheric pressure sensor, and got 2.5. I'm at a 410ft elevation. Does anyone find this to be a big problem?

 

That sounds out of whack. Try backprobing that item with the engine at idle and see if there's any change.

I believe if you disconnect the plug, the ECU defaults to sealevel. I'll check on that, using memory here right now. Also I'll give you my reading off one of my cars.....tomorrow.

I have noticed that if you disconnect it, the mixture gets richer across the board. See that with a wideband.

I've a RETK2.0 and it has the ability for you to put in your own altitude for the atmospheric pressure sensoe. I've put in 2000 ft before and seen the afr go leaner across the board doing that. Later. Oh, I'm at 700 something ft above sea level.

 

I tried it at idle-no difference. How can I reset the ECU to give an accurate reading?

 

It's not a ECU problem. It'd be a bad atmospheric pressue sensor if anything. They shoujld be pretty cheap at a junk yard, just hard to access imho. I'll give you a figure later when the sun comes up (on my cars at 720 ft altitude.

 

Pin 2H, green/yellow wire, altitude 720 ft. 3.90 vdc on one car and 3.92 on another.

Frankly the 2.5 your reading sounds like the air intake temp sensor located on the other end of the plug. Check your wire colors and make sure your on the right pin. The one with green/yellow colors, bottom row, forth one over from the right when looking into the ECU.