3500 RPM Hesitation
 

Im helping a friend with his 88 NA, factory car except for the air pump being routed straight to the cat (which is another question I have). His car was hesitating at 3500 RPM and idling low and rough. It drives ok untill then, then it's like the car just falls on its face. I broused around looking for obvious signs of a problem and found one problem, the vac line going from the pressure sensor (passenger side shock tower) to the LIM was disconnected. I pluged it back up and we went for a test drive. The car idled better but the hesitation was still present. We stopped by Advance to get directions to do a diagnostic check but the Haynes manual was for 89 and on, so I disconnected the battery, waited a min, and went back to the house. Nothing was reset and the problem was still there. Later this evening while I was at work he called and let me know that the hesitation doesnt occur when the engine is cold. You guys have any ideas? Anyone had a similar problem? And about the air pump, is it bad to have it connected straight to the cat constantly. I keep hearing both sides of the story. I dont see any problem with it but want some more oppinions.

The hesitation is usually due to bad grounding. It's odd though because it was fixed (at least it was when the car rolled off the line) in 87+ FCs. I would start with cleaning up all the factory grounds.

There is a how to thread on how to directly ground the ecu on the evil forum.

TPS isn't within specs? Try and test it to see if it helps with the hesitation.

http://rotarycarclub.com/showthread.php?t=1509
http://rotarycarclub.com/showthread.php?t=1475

Yeah I think its the TPS too, Im gonna borrow Charles' tester he made since im to lazy at the moment to make one.

Im gonna use a multimeter to test the TPS on my friend car, and in the link that Phoenix7 posted I noticed that intercooler was mentioned. The car I'll be working on is an NA, Does this matter? Are the TPS similar?

TPS are the same for TII and N/A within a series. S4 and S5 don't interchange (without work, not worth it).

Were making progress. Yesterday morning when I got off work I checked and adjusted the TPS, checked for ECU trouble codes (there weren't any), and repaired a missing ground from the firewall to the tranny. Unfortunately the problem is still present. Im going to change the TPS and if the problem still persists I'm going to swap out the mass air sensor. While I was workin on his car he told me that recently he had manually opened the flap while the engine was running, not sure why but I assume this may be the culprit of all the problems. I apprecriate all of you guys input and if you have any more ideas please feel free to comment.