'92 GMC has fast idle, erratic shifting after warm up

[bwdearman]

I'm new to the forum so let me take the opportunity to say Thanks in advance.

I have a '92 GMC with a (not so recently) rebuilt 5.7 and an auto tranny. The truck runs like a scalded dog. In order to get it to pass inspection last year, I replaced the EGR, TPS, IAC, coil, plugs/wires/cap and rotor, ignition module, rebuilt the TBI, even replaced the catalytic converter. It finally passed and with flying colors.

Recently, it started acting really weird. In the morning, it starts fine and the idle starts a little high, but slows down to around 700 in a couple of minutes. I take off driving and it runs and shifts normally. After it warms up, things start to get weird. The idle runs up to around 1500 and pretty much stays there. The tranny starts shifting weird… in and out of OD for no apparent reason. It goes into OD at as low as 35 mph, but doesn't want to stay in OD at 65 mph. By the way, there's been no sign of a SES light.

I've checked for vacuum leaks from top to bottom and have found nothing, although I must confess I haven't actually measured the amount of vacuum with a gauge. I replaced the IAC again on the off chance the new one from last year failed. I reset the ECM and reset the new IAC as per normal procedure, but the problem remains.

The debate in my shop has come down to either a bad MAP (which ironically wasn't replaced last year with everything else), a bad TPS (which actually was replaced last year), or possibly the inside or outside speed sensors on the tray. The plan at this point is to measure the vacuum with the gauge and check the timing. My son has a scanner that we're thinking about hooking up but being that it's a '92, it probably won't tell us much. Worst case, I replace the MAP, then the TPS if necessary.

Sorry about the dissertation, but I wanted to be thorough. If anyone out there has anything to offer on this situation, please shoot me an email as soon as possible.

[ChevLoRay]

to the board!

There are a number of posts on this forum that have addressed your issues. My son-in-law has a '94 that also has the erratic idle issue and he says he's gonna have to replace the TPS. Without any reason to say so, it sounds like a possibility for you, too. Of the two, TPS and MAP sensor, I'd go the cheap route.

As far as scan tools go, Snap-On has a decent one (albeit expensive) that can get right down to the end-item(s). If you can locate someone who has one who will hook it up and scan if for you, that's good. Otherwise, the ALDL plug can be "encouraged" to flash your Check Light that will alert you to a component.

Try doing a search for idle problems and see what comes up. Any post that ChevyTech or bWood have commented on (among other knowledgeable folks) is reliable. I just mentioned those two because I'm blessed with a short memory issue.

But, glad to have you on board, even if you are from Texas....so's my wife and her family. Native Texans aren't all bad....

[bwdearman]

ChevLoRay,

Thanks for the good advice. Well, I started my truck this morning and it was idling just fine… until I put it in reverse to back out of the driveway. As soon as I did that, the RPM's jumped up and it kept acting flaky all the way to work (54 miles). Here's where it gets weird… It ran perfectly all the way home. A couple of times, it seemed like it was about to start getting flaky again, but it stayed together the whole way. It still seems like it shifts in and out of OD too easy, but I think that's a different problem. Picked up a vacuum gauge on my way home and it read 20 at idle. Bottom line… I'm springing for a TPS first because it's still in warranty. I'm going to try and find a used scanner or someone with a scanner and give it a "just in case" scan.

I'm glad to hear that you had the presence of mind to marry a Texan. When Rick Perry (the governor of Texas) finally gets a gut full of Washington D.C. (which shouldn't be much longer) and the State of Texas secedes, I feel certain that your in-laws will take you in. I say that with certainty because… well… they are Texans and that's just what we do.

[ChevLoRay]

Originally Posted by bwdearman

ChevLoRay,



I'm glad to hear that you had the presence of mind to marry a Texan. When Rick Perry (the governor of Texas) finally gets a gut full of Washington D.C. (which shouldn't be much longer) and the State of Texas secedes, I feel certain that your in-laws will take you in. I say that with certainty because… well… they are Texans and that's just what we do.

Yeah, they would. Got 'em scattered from Longview to Sugarland, if you don't count the ex-wife and in-laws in Paris and Greenville.

Seriously, when Texas secedes, there should actually be two of 'em...Texas and East Texas. If you live in east Texas, you know what I mean.

[bwdearman]

Well, the MAP didn't do the trick, so I'll be doing a TPS tomorrow night. I have a question about the sensors on the left side of the tranny. If I'm reading my manual right, there's an Input Speed Sensor and an Output Speed Sensor. What the heck do they do and are either of them the same as the Vehicle Speed Sensor? My reason for asking is that I noticed the connector for the Input Speed Sensor is a little flaky, so I'm wondering if it might be the source of my problem.

Now with regard to Texas, I think Gen. Sam Houston said it best. "Texas can do without the United States, but the United States can't do without Texas." (That ought to stir up the wasp's nest.)

[bwdearman]

Huzzah!!! The TPS did the trick! Thanks for your help. I'm sure I'll be back for more in the near future. I mean, it's a 92 with 173,000 miles on it, know what I mean?

bd

P.S. Long Live Texas