2twisty

First: I have searched, and have not foun my answers, or am using the wrong search terms. Now, with THAT out of the way...

I have a 93 Tahoe that the PO put a 92 2.8l in. Apparently there were some differences in these engines (namely the direction the cooling fan turns... )

Anyways, I have this odd problem that I can't seem to diagnose because it comes and goes.

I'll be driving along minding my own business, when suddenly the SES light will come on. At this point, the truck will try to idle at 2000+ RPM. Sometimes, jumping around as though I'm messing with the throttle.

It will correct itself as quickly as is comes on, and sometimes the truck will do this on and off for several days and then not at all for a week.

The PO told me that when it starts idling funny, just replace the IAC, and gave me a spare when I got the truck. Replacing the IAC did nothing.

I also replaced the TPS, thinking that the ECU thinks the throttle is open, and is adding fuel. No dice.

I do need to replace the O2 sensor (I did get a code for that a while back) but I don't get any meaningful codes regarding the idle problem, since when the problem goes away, the SES light goes out and the truck behaves properly.

One way of dealing with the problem when the engine is racing at idle is to shut off the engine and restart. That usually stops the problem for a while. I can only assume that the malfunctioning part resets when the computer is off and doesn't malfunction for a while. Sometimes, restarting does no good, but generally it is a fix that works.

I will be going out to put the O2 sensor in as soon as I send this message, and will come back in to check for replies when I am done.

Any ideas as to WTF is causing this erratic idle? My mind has turned toward the MAP sensor, but I don't want to replace it needlessly, due to its cost.

Ideas?

2twisty

OK - I went out and put the new O2 sensor in.

Also, I pulled the codes from the ECM.

I got:

22: TPS Voltage too low
33: Map Sensor
43: Electronic Spark Control

I then went out and replaced the MAP sensor. Replacing it did no good. I pulled the codes again:

22: TPS too low
33: Map Sensor
34: Vacuum sensor or Map Sensor
43: Electronic Spark Control

Do you have to do anything special to clear the codes from the ecu? Will it continue to limp until they are?

maxwedge

First of all what engine is in a Tahoe?, 2.8, what am I missing here, what vehicle is this? Get a full scan and look at map and tps voltage and see if they are in range, make sure you have proper vacuum to the map. A code reader will not do this. Disconnecting the battery will clear the codes.

2twisty

Sorry - 2.8l-- it was in the subject line, but I forgot to put it in the main message body.

Without paying through the nose, where can I get this diagnostic data you refer to?

I'll go pull the battery and see what happens after a full reset.

2twisty

OK -

I cleared the codes and drove it until the idle went screwy again. I then pulled the codes again and the only one is #22, TPS voltage low.

This TPS has been replaced, and perhaps it is not adjusted properly or there is a bad connection. The TPS was replaced due to this behavior, so I'm inclined to believe that the original unit was fine and we're dealing with a bad connection.. And the worst kind. Sporadic.

Any "nifty" ways to test the TPS to be sure it is behaving properly?

Is the procedure in the Haynes manual the right thing? (So many times in recent years I've found Haynes manuals to be "smoking crack" with the information they provide, yet some are really good (1976 Chevelle) and others are pure crap (1995 Miata). How does the S-10 Haynes rate?)

maxwedge

To check tps circuitry you need a circuit diagram, there is a 5v reference line, a ground and a signal line bak to the pcm, at idle there should be about .5v, but to check what the pcm sees and whether the tps voltage reacts smoothly to throttle opening, you will need a "real" scanner.

2twisty

Only time will tell now.

I checked the TPS by skewering the wires with straight pins and checked the voltages according to the instruction sheet on a new TPS I bought. For future reference for those searching:

Motor is a '91 2.8L TBI.

With the TPS plugged in and the key in the "ON" position, you should be able to get +5v on one line (reference) 0.00v on another (ground) and a small voltage on the third (signal)

verify that the IAC is fully retracted to make sure the throttle is fully closed.

Then, adjust the TPS so that with the throttle closed you get 0.5v-0.8v, then tighten the TPS. If you're working on a stock TPS, you'll have to remove the throttlebody from the engine and drill out 2 spot welds holding the bolts in place. In my case, I had to completely drill out the old bolts and retap for a slightly larger bolt and adjust the size of the slots in the TPS to accomodate. However, now that I've done that, I won't have to in the future...

Anyways, TPS with throttle FULLY closed should be 0.5-0.8v.

I adjusted mine to 0.55v.

I drove the truck for about an hour with no erratic idle and no SES light events.

Of course, since this thing has been known to "fix itself" for a few days at a time, I won't know for certain until some time has elapsed, but conditions are favorable, since my TPS was giving off 0.2v at fully closed throttle, and the ECM was giving code 22, "TPS Voltage to Low."

Cross your collective digital fingers, this thing might just be fixed.

Thanks!

maxwedge

Lesson learned here is there is more than just putting in parts, good job on checking the problem and backtracking what probably fixed your problem.