Help! Idling problems when warm

[kevknotch88]

I purchase the 4 pin wiring harness and a a9L computer for my 88 lx coupe. I installed them and an accufab mass air sensor and ran them for appx a week to be sure they were working properly. After installation no problems were noted. Also, fyi, the kit was the $30 item off ebay and the connections were sodered by a local shop.

Now for the fun. I installed the Trick Flow stage 1 cam (specs very similar to the FMS E cam) new lifters, push rods, valve springs, 1.6 fms roller rockers, gt-40 upper & lower, idle air bypass valve, and a 65mm tb. It runs like a scalded dog, however, After warm up, when approching a stop and the clutch is depressed, the engine will surge to 2500-3000 rpm and then slowly drop to 900-1000 idle.

I have adjusted the ilde air bypass for two weeks without helping this problem. I know you probably need more info, but if anyone has had similar problems, please help.

kevin

[PKRWUD]

It could be several things, but I'm curious as to what your TPS is set to. MAF ECM's have a narrower range than SD ECM's. Where it was okay to have your TPS adjusted to 1.0 vdc with an SD ECM, that same adjustment can cause problems with a MAF ECM. Also, the MAF ECM's tend to have their TPS voltage increase slightly when the engine warms up, which could push it over the idle limit, and fool your ECM into thinking you're cruising rather than idling. Just to be safe, you should set your TPS no higher than .95 vdc with the engine at operating temp. I don't recall if the one you've got was originally for an automatic or not, but if it was, they also have been well known for having a slow-to-return-to-idle-speed problem when installed in a vehicle with a standard transmission.

Take care,
-Chris

[kevknotch88]

Chris,
Thank you for your help. First of all, I do not know how to adjust the TPS. I have heard of people filing out the screw slots with a rat tail file to fool the computer, but I have no other knowlege basis to toally understand your help. The car is a manual and the tps is the same one used when it was speed density. Should I purchase a new tps for an 89-93 car? If not sounds like it needs to go to my local shop.
thanks again
kevin

[PKRWUD]

Do you own and know how to use a DMM (Digital MultiMeter)? If you do, I can teach you how to test it for flat spots and how to adjust it (two different tests). If a DMM is foreign to you, then you should have it looked at by someone who knows how to adjust the TPS properly. Replacing it may or may not help, because it needs to be adjusted after it's installed.

Take care,
-Chris

[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?

Any other ideas that could cause idling problems would be helpful.

[jimberg]

Besides adjusting the TPS properly which Chris will tell you how to do, another thing that people neglect to do after making major changes in the engine or its components is to reset the computer properly.

When your car is at operating temp, disconnect the negative battery cable from the battery for 15 minutes to make sure the computer is completely reset.

Reconnect the negative battery cable and start the car. Make sure that all of your accessories (AC, Heater, Radio, etc. ) are turned off. Let the car idle for 10 minutes.

Turn the car off, wait 5 minutes. Start the car again and turn all accessories on. Let car idle for 10 minutes. Take it for a drive. You computer should be properly reset at this point.

[kevknotch88]

Thanks to both, I will have my shop look at it. I do not know how to use this machine, but I will see if I can find some info to read.
Kevin

[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.