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

[GaryDoug]

The file is pretty much as I expected, corrupt with a lot of bad data. If I had to use that file, I would sort on engine run time and cut out anything outside a normal range for that. Even then, there is a lot more bad data. The 93 Y-car does have the CCM and that has caused me trouble getting something to work for those. I tried for months using a volunteer with a 95 car, but to no real avail. Without a working CCM or a complete car, I probably won't be able to do much more, and that module is pricey.

The connections and pcm are virtually the same as an F-car, but the CCM gets in the way. I could not get the CCM to disable communications during the scan, so it just "talks" over the scan cable signals and pcm all the time. I don't know if the engine will run ok if the CCM is unplugged. Perhaps he could try that but it looks to be difficult to access anyway.

I can't answer about the stuck O2 sensor. I don't think there is anything different in the code for that device on a Y-car. Perhaps it is really reading that true value?

[HellaciousA]

The driver's side cat is glowing bright red at just idle. I just checked all the grounds were clean and tight (including the one on the bellhousing), checked the chip in the ECM was seated properly (all wiring to the ecm was firmly connected as well), battery terminals tight. Up to 130-140 degrees it's smooth running, but immediately gets rough after that and sputtering.

[GaryDoug]

I am sure Fred will be along soon to help with this. He is the expert.
But I will play with analyzing it anyway....
Sounds like you are dumping raw, unburned fuel down the driver side exhaust. That might explain the very high O2 sensor reading. Could be (and this is where Fred would know) that either the ignition on that side is failing on one or more cylinders or that an injector is sticking open. Let's see what kind of grade I get on this when Fred comes back in ;-)

[AL 9C1]

So what do the plugs look like after running for a while? Glowing red cats, this should show up on plugs. I'm thinking a really lean bank to heat up a cat that hot. Make sure there's not a chunk of the old cat in the pipe restricting that side also. Just a few thoughts. Also I've seen before the O2 sensor wiring swapped from left to right and cause these issues. You said the cats were replaced, it's fairly simple on some of these harnesses to swap sides. I remember chasing one of these in the shop for a week after an engine swap.

[Injuneer]

Up to about 140°F the ECM operates in "open loop", ignoring the O2 sensors. At 140°F coolant, the O2sensors warm enough to operate, and a 206-Second timer times out, the ECM switches to "closed loop" and controls fuel based on O2 sensor feedback. What I need to see is whether I can sort through the good data to observe what happens when the ECM makes this transition.

You need to understand that the data is not "all over the place" because of the ECM, bad grounds, loose connectors, etc. It is most likely due to the fact (Gary, correct me or clarify if I am wrong) that the scan software is not fully compatible with the Corvette ECM data stream. The software was developed using the F-Body ECM. The F-Body does not have all those modules I mentioned in post #10. Your Corvette has them and they interface with the PCM. I would assume that they periodically insert some bit of data into the stream that throws off the column indexing the F-Body software is expecting.

Let me try and filter out the good data records from the bad data. If your driver's O2 is really stuck at 1.032 volts as it appears from my superficial look, it is going to throw the fueling way off, and how it is “off” will show up in the transition from open loop to closed loop. But I need a coherent set of data to trace the response of the O2 sensors, short term and long term fuel trims and injector pulse widths. May take a bit of time to sort the data.

[Injuneer]

Based on the somewhat suspect data, let's try and straighten out the driver's side O2 sensor. It hangs at 1.132 volts forever, driving the LTFT down to 108, then dropping the STFT to pull out even more fuel. Passenger side O2 seems relatively normal, cycling up and down in closed loop.

Eventually (945 seconds) when you start driving it, driver's O2 starts to very slowly drop, eventually reaching 350 mV. Never cycles, just very slowly drops. Once is drops below 450 mV the ECM starts ratcheting up the LTFT, richening the mixture, and pulls the LTFT up to 149 in idle cell (16).

Driver's side O2 sensor never drops below 350 mV, not even when you are in decel mode and the DCFO has shut the fuel injectors off (pulse width = 0.00). Passenger side O2 correctly drops to 0 mV.

Swap the two O2 sensors side-to-side, start/drive, rescan to see if the problem follows the sensor to the passenger side (sensor problem) or stays on the driver's side (wiring or ECM problem).

Questionable data frequently shows the ECM has one code. But when you pull the codes you indicate there are none. How are you pulling the codes? 94/95 PCM in Scan9495 shows # of codes and lists them. 93 does not.

When you recently replaced the spark plugs, what did the old ones look like, for each bank?

Is your exhaust totally stock? Are the exhaust tips coated with black soot on both sides?

Is the car in California? Has it been emissions tested at any time in the last 8 years?

[AL 9C1]

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.

[popo8]

Almost positive this may be part of your issue. MSD has a horrible reputation with Opti's. New and even rebuilt units are garbage.

Also, please change your font color.

This... my msd was my problem. Only Delco for any of my LT1s

Sent from my SM-G955U using Tapatalk

[HellaciousA]

So what do the plugs look like after running for a while? Glowing red cats, this should show up on plugs. I'm thinking a really lean bank to heat up a cat that hot. Make sure there's not a chunk of the old cat in the pipe restricting that side also. Just a few thoughts. Also I've seen before the O2 sensor wiring swapped from left to right and cause these issues. You said the cats were replaced, it's fairly simple on some of these harnesses to swap sides. I remember chasing one of these in the shop for a week after an engine swap.

The plugs I removed last week were actually in pretty good shape. Electrodes were not worn bad, but the driver's bank looked a bit brownish, but still plenty of white. The plugs do not look fouled on the driver bank after I pulled one out most recently.

Based on the somewhat suspect data, let's try and straighten out the driver's side O2 sensor. It hangs at 1.132 volts forever, driving the LTFT down to 108, then dropping the STFT to pull out even more fuel. Passenger side O2 seems relatively normal, cycling up and down in closed loop.


Eventually (945 seconds) when you start driving it, driver's O2 starts to very slowly drop, eventually reaching 350 mV. Never cycles, just very slowly drops. Once is drops below 450 mV the ECM starts ratcheting up the LTFT, richening the mixture, and pulls the LTFT up to 149 in idle cell (16).


Driver's side O2 sensor never drops below 350 mV, not even when you are in decel mode and the DCFO has shut the fuel injectors off (pulse width = 0.00). Passenger side O2 correctly drops to 0 mV.


Swap the two O2 sensors side-to-side, start/drive, rescan to see if the problem follows the sensor to the passenger side (sensor problem) or stays on the driver's side (wiring or ECM problem).


Questionable data frequently shows the ECM has one code. But when you pull the codes you indicate there are none. How are you pulling the codes? 94/95 PCM in Scan9495 shows # of codes and lists them. 93 does not.


When you recently replaced the spark plugs, what did the old ones look like, for each bank?


Is your exhaust totally stock? Are the exhaust tips coated with black soot on both sides?


Is the car in California? Has it been emissions tested at any time in the last 8 years?

scan94/95 showed only a single code for knock--that was the only code I saw in the program. I have not pulled any other codes as I don't have a separate OBDI scanner. Exhaust is totally stock. Exhaust tips are clean and no soot on rear bumper cover. Car is in CA, and it passed smog up until this year it failed because it was idling too rough. O2 sensors have been replaced twice (once by a shop, once by me). I do not know if the shop swapped harnesses, but I know all I did was replace the sensors.


I will swap sensors this weekend and re-scan to see if it moves to the other bank.

[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?