93 corvette--poor idle/running rich/surging/bucking running out of ideas

[Injuneer]

The 93 F-Body ECM allows you to pull the codes by shorting the DLC pins and flashing the codes on the SES light. Is that not possible with the Corvette ECM? Or did they use a system that displayed the codes on the digital speedometer readout?

[Injuneer]

Injuneer you seem to be the guru here. I can't help but remember an incident when the 02 harness was accidentally swapped from side to side on a similar vehicle. Acted a whole lot like this one. What's the easiest test to determine it's lined up to the proper bank. I'm sure the trans and exhaust has been dropped in this car at least once for a clutch or something. The harness is almost identical to either side. Don't know the easy way to test without live OBD2 data.

I was going to give you the easy answer.... the harness wire colors are different left and right. That's the case with the F-Body, all years. But before I made an ass of myself I figured I better check the 93 Corvette ECM diagram. Sure enough, both left and right wires on the Corvette are the same color - purple. So much for that idea.

I've seen a couple of cases of swapped harnesses, and it screwed up both banks. In one extreme it drove one bank to a 160 LTFT and the other to a 108. And they just stayed there. In this case the right bank appears to be acting normally. The left bank bottomed out at 108, but as the car was driven, reversed to a 149. And when DFCO turned off the injectors, right bank went right to 0 mV but the left bank didn’t. Leaky injector.... maybe one that loosens up with heat and stops leaking? ? ?

At this point I'm doubting myself. Things that I have never seen before, after looking at 100's of data logs. Unfortunately compounded by the fact that the log is peppered with faulty data. I've seen logs with sporadic data errors, and those can usually be blamed on interference on the data wire. But this case is beyond that. Maybe it’s interference from the other modules that communicate with the ECM as mentioned above, or maybe it IS a faulty ECM.

Never looked at data from a Corvette using Scan9495, and Gary notes he didn't think it would work at all. There is a version of TTS Datamaster that supports the 93 Corvette and F-Body. No longer supported by TTS, no longer available for download. But if someone has a file to download, that might be an option here. I can open the logs. I think Gary has the 94/95 Datamaster file available. Not sure about 1993.

[AL 9C1]

I would love to do some tinkering on it. Love a gremlin hunt. It's not gonna be anything very complex. Vacuum, wiring, maybe even mechanical. It's sounding like the control system is reacting to what it is reading.

[GaryDoug]

The CCM should show any engine trouble codes on the car's display. The Y-car ECM may not be able to do that by the shorting method because of the CCM capability making it unnecessary.

To answer Fred's comment, I only have an F-car version of the old and new Datamaster programs. Y-car would be different as far as I know, again because of the CCM conflict problem. Same goes for TunerProRT. Not sure if anyone has a good working file to use with TunerProRT.

In case I am not clear with this scanning issue. Here is what is happening. The F-car's have an ECM, SIR, and ABS controller on the signal line together. The ECM is the master and the other are slaves, meaning that they don't talk until talked to by the ECM. So we programmers use a command to request the ECM to stay quiet and wait for requests for data. The Y-car is similar except that the CCM is the master shared with the ECM so we have to issue requests for both of those to stay quiet, but nobody (except GM) seems to know how to do that for the CCM.

For what it is worth, my suggestion to unplug the CCM won't work. I forgot that without the CCM connected, the Y-car ECM won't send out any data.

For the long term, if I owned a Y-car, I would buy one of the Tech II clones from China for about $250. That way, you would at least get good info. I have two and they work fine. Unfortunately, they don't do much useful datalogging.

[AL 9C1]

Just gotta borrow an old Tech1 from a dealer. If they can even find it anymore.

[GaryDoug]

Popo, good to see you back "on the job" here. I have a feeling that you will do just fine with this health issue, just like my step-son.

[GaryDoug]

Just gotta borrow an old Tech1 from a dealer. If they can even find it anymore.

My local Chevy dealer does some of the repair work on my TA. I got to talk with the older mechanics there. They have no Tech I's and only one Tech II, which is assigned to a guy about my age who looks a lot like a ZZ Top guy with long beard ;-) I dread it when he is assigned to my car.

[AL 9C1]

There's a dark age of vehicle manufacturing. It's somewhere between carburetors and OBD2.

[HellaciousA]

I just swapped o2 sensors from right to left and the problem didn't move. Driver's side cat is still getting extremely hot while the passenger bank stays fine.

[Injuneer]

@GaryDoug

I opened my copy of the 1993 version of TTS Datamaster. In the "setup" screen there are options for:

- Y-Body Manual Trans

- Y-Body Auto Trans

- F-Body Manual Trans

- F-Body Auto Trans

If it works, I think it would be worthwhile for him to download your 1993 Datamaster File. That way when we can look at data we can have more confidence in.

What do you think of the fact that after the O2 sensors were switched side-to-side, the problem stayed on the driver's side? With a single wire O2 sensor, and the problem not being the sensor, what in the circuit would cause the voltage to hang at what may be the upper limit (1,132 mV) for 15 minutes of idle, then when the car was driven, slowly drift down to 350 mV and stay there?


@HellaciousA

When you say the problem remained on the driver's side, did you scan it and if so, did your driver's side O2 voltage show the same pattern described immediately above?