Had to adjust my idle - why?

[89 Cobra LX]

This weekend I finally grew tired of my car idling at 1200 RPM so I disconnected the TPS sensor and lowered the idle to around 850 RPM. I shut the car off, connected the TPS sensor and when I fired it back up everything was fine.... nice and lumpy.

Why would my idle jump up to around 1200 RPM? Before this winter it idled at 850 RPM. Would just sitting in the garage for a couple of months over the winter cause this?

Do I need to re-set my TPS voltage after adjusting the idle? I'm pretty sure I won't need to since I disconnected the sensor before adjusting....

Thanks

[PKRWUD]

Your idle could have been due to the IAC getting "sticky" while sitting for a few months, who knows. As far as the TPS is concerned, you need to check and reset it, and then clear the ECM and allow it to relearn the new settings. You may find that the problem was the TPS all along, and that by adjusting the idle, you set it into a range where it should be.

Take care,
-Chris

[89 Cobra LX]

Thanks - I'm noticing a bit of a surge until it really warms up. This is probably somewhat normal, but I think it could be fixed. How do you reset the TPS?

[PKRWUD]

I just wrote this in another thread:

At the TPS:

Either by back probing the connector, or by piercing the wires with a small safety pin, attach a DMM pos lead to the green wire, and the neg lead to the black wire. Turn key on, but do not start the engine. At closed throttle, the DMM (set to VDC 10 scale) should read below 1 vdc. Rotate the throttle to WOT, slowly, and see that the voltage increases smoothly. At WOT, it must be more than 4.5vdc. To adjust the setting, loosen the two bolts that secure the TPS, and try moving it. The optimum setting at idle is .95 vdc. If your reading is way off, and you cant move the TPS enough to get within range, remove the TPS and try elongating the holes with a rat-tail file.

After resetting the TPS, disconnect the negative battery cable for ten minutes. While the cable is disconnected, turn on the headlight switch to drain any capacitors. Turn the switch off just before reconnecting the battery cable. Start the engine, and let it idle for 1 minute. Then, raise it to 2000 rpms for 1 minute. Shut off for ten seconds. Restart, and turn on every load you can (a/c with the fan set to high, high beam headlights, etc.), and let it idle for 1 minute. Then, with the loads still on, raise it to 2000 rpms for 1 minute. Shut off all loads, and shut off engine. Wait ten seconds. Restart, and go for a 10 minute test drive. Drive fairly aggressively. Shut off engine. Restart after 10 seconds, and check the idle.

Take care,
-Chris

[89 Cobra LX]

Thanks again and great instructions. Shouldn't you be sleeping? It's like 3am in CA!

[PKRWUD]

I got up at 5:30pm yesterday. It feels like dinner time to me right now. Between working on the Sprint car and trying to make a living, I work whenever I can get it. I've been working nights the past few days servicing a fleet of Ford vans and small trucks for a local moving company. They like to have their vehicles available during the day, so servicing them at night works best for me. It usually means some unusual hours, but I'm used to it. I have a few fleets that I service this way, so I usually spend the weekends we aren't racing working on them. I have no family out here, so sleeping all day on Easter and yesterday was no big deal. Today will be the toughest. I will try and sleep for a few hours later this morning, and then head out to Moorpark for the afternoon to work on the Sprint car. Tonight I expect to sleep like a baby.

Take care,
-Chris