Tuning a '92 383 corvette with a 6-spd...

Idle is currently surging from ~500 to 1100rpm. MAP values are fluctuating between 50 and 70kpa. No codes, I've adjusted timing and A/F via the v/e tables so the mixture isn't the cause of my problem. IAC is good, no obvious vacuum or exhaust leaks. It is stuck in cell 18 at idle though, and I'm currently trying to learn more about that cell and what causes this condition. Not much info out there though.

Anybody want to take a crack at it?

Thanks!

Nick

 

Check the TPS sensor voltage, and make sure its less than 0.90V at idle.

 

Injuneer,

It's .63v at 0% throttle and 4.04v at WOT. Could the reading be faulty?

Thanks!

 

You should log the tps voltage while it sits there. Could be a faulty wire harness to the tps or corrosion in the tps. The problem is if theres little spikes in the tps signal it freaks the computer out.

Might also check out your speed sensor if its screwed up a little bit as its probably many years old now, and iirc could cause you to be stuck in cell 18 not moving.

Whats your timing doing while all this is going on? Fluctuating?

 

I'm seeing .63v at the TPS while logging with datamaster. It's not moving, I do have an extra TPS that I could swap out if needed..

The speed sensor or harness connection is bad and gives very erratic readings while at idle. Will that cause a bad idle problem? I just hadn't gotten around to fixing it yet.

Timing moves from 28-31 degrees, but that might be due to rpm variations (~500-1000rpm)? Closed tps spark advance vs. rpm is set to 26 at 400rpm, 29 at 800rpm, and 31 at 1200 rpm. Adjusting this table doesn't seem to help much.

Thanks!

 

400-800-1200 closed tps spark advance tables should be the same on any cammed car to prevent surging at idle. You want rock solid timing at idle to have a solid consistant idle. Try running it with 26 or 27 across the board there.

YES the vss screwing up will cause you to go into cell 18 why do you think I asked. Thats the definition of what cell 18 is, rolling with your foot off the gas at idle rpms. I'd fix that first before I changed any other parts. The pcm is just confused.

 

okay, I'll spend some time troubleshooting the vss. I must be mistaken in that the ecm's from the 92-93 cars didn't use the vss signal as opposed to the later pcm equipped cars that do. Have to recheck the reference materials on that.

Will also drop the closed tps timing as well.

Thanks!

 

Made some progress..

Swapped the VSS out for a new one and the erratic readings remained. I have an extra ecm that I swapped in, and the speedo stopped working altogether.

Good news is the car now idles in cell 16 with the ecm swap.

Some roughness is still present but I suspect that's tuning related since it's running pretty lean. I'm going to play with the mixture and timing this evening to get that part straightened out.

My current guess is that there is a short somewhere between the VSS and the ecm? Or I didn't get the VSS in there correctly. I don't think it's between the dash and ecm, as it wouldn't have been running so badly....

Any further input out there?? Thanks!

Nick

 

Time to inspect your harness between the vss and the ecm.

 

I'm trying to figure out how to check the VSS circuit for continuity... As far as I know the ecm won't provide a reference signal to the VSS since the sensor itself generates an AC pulse/signal and sends that to the ecm. Would like to confirm that the harness has a short somewhere before having to crawl under there for an extended period of time....

Do I have a choice?

 

Is it possible you have a high voltage wire too close to the VSS wires? Might be introducing interference into the circuit. A pulsed high voltage (e.g. - ignition) interference would create erratic VSS readings.

If you check the VSS wiring, the yellow wire (pin D12) is the ground, and should show 0V. You should get a variable voltage on the purple wire as you rotate the driveshaft. My diagram for the 92/93 Corvette shows pin "D1." for the purple wire, but that might be an error, with a second digit that didn't print fully.

 

I show D1 and D12 in my schematics as well.

I can't recall any high voltage wires in the vicinity, but it's possible.

I will raise the rear end, and disconnect the "D" connector from the ecm. While turning the rear wheels, I'll have an assistant monitor D1 and D12 terminals with a multimeter. Hopefully this will give me some clues?? Thanks!

Nick

 

I'm still not sure if the PCM provides a reference voltage. Some speedo systems run with the signal voltage referenced to ground. The output of the rotating magnet/sensor produces a variable voltage that swings from positive to negative. Other systems provide a bias voltage, allowing the output voltage to vary from 0 only on the plus side.