Help! Idling problems when warm

[PKRWUD]

Originally posted by Mustang_289
Chris:

I've got an idling problem with our 88GT, replaced plugs, wires, dist cap & rotor, fuel filter, and trying to get to the PCV valve.

Can you submit a post on how to adjust the TPS voltage on an SP stang- I'm interested and very familar with a DMM?

Sure.

First off, never set it above .95 vdc at idle. Doing so brings it dangerously close to the 1.1 vdc (for SD only!! MAF is 1.0vdc) point at which the ECM determines you are driving rather than idling, and a rough, hunting idle often results. Besides, there isn't a human alive with reflexes quick enough to tell the difference between .95 and .98 vdc, so don't buy into the hot rod hype, and adjust it too high (.98 vdc is too high).

As far as actually testing your TPS, this is best done with a DMM that has a bar graph, but any MM will work. Disconnect the TPS from the harness, and attach your meters pos. lead to the Green wire pin, and the neg. lead to the Black wire pin, at the TPS connector. Set your DMM to ohms (4k scale), and rotate the throttle slowly by hand. Observe the DMM. It should increase and decrease smoothly with the throttle opening and closing. If it jumps around at all, replace the TPS. If it passes, before you hook it back up, turn the key "on" and measure for voltage (20dcv scale) between the orange wire and the black wire at the harness connector. It should be 5.0vdc (+/- .4vdc). Turn key "off".

Bring engine to operating temp., and shut off. To adjust the TPS after it's in place, you will need to access the green wire while the TPS is connected to the harness. You can try to backprobe it with a DMM lead or pierce the wire with a very small safety pin. Attach the DMM pos lead to the pin (or backprobe), and the DMM neg lead to good engine ground. Turn the key "on". Set your DMM (to 20vdc scale). With the throttle closed, at idle, the DMM should read no higher than .95 vdc and no lower than .75vdc. If it does not fall within this range, loosen the two TPS mounting screws and rotate the TPS as best you can until you reach that range. Sometimes the plastic openings on the TPS need to be widened to allow more movement. If and when the TPS falls within that range, slowly rotate the throttle, while watching the DMM, to WOT. It should rise smoothly, and read over 4.5 vdc at WOT.

Originally posted by Mustang_289
Any other ideas that could cause idling problems would be helpful.

Other problems? make sure your spark plug wires aren't touching each other after the distributor cap, make sure the IAB is clean, check for vac leaks. Have you tried to pull any codes? Describe in detail it's rough idle.


Hope that helps.

Take care,
-Chris

P.S. As a footnote to Jim's method of resetting the ECM, if 15 minutes is too much time, you can get by with having the neg cable disconnected for a minute if you turn on the headlight switch while the cable is off the battery. This discharges any capacitors that were retaining memory for the ECM.