David94TA

This make be a crazy question, but is there a setting in LT1 Edit or Tuner Cats that if activated would prevent the PCM from going into PE mode?

This happened suddenly a few months ago and has continued. It had been running fine. Then one day, this.

When I get into the throttle, I get a lean back fire. It's not just a miss, but a loud back fire.

I'm just wondering if while looking around in my tune, I accidently turned something off or changed a setting. I do my tuning with a wideband and had my AFR set where I wanted and , like I said, it was running fine.

Car starts fine and runs fine as long as I'm not getting into the throttle. I know PCM's can go bad, but I would think it would exhibit other problems besides this.

I've tried richening the PE settings, a little at first and then more and more. Should be running extremely rich by now, but that hasn't solved the problem. Point is, it wouldn't have just done this on its own. NOTE: Just for kicks, I downloaded the free versions of Tuner Pro and EE Hack and was playing around with those. Just wondering if those might have changed something that I can't find. Seems that problems started after that. I don't think I changed anything, though.

I usually use LT1 Edit but I do have Tuner Cats also as a back up.

I'll keep looking at the tune to see if I can find anything, but wanted to check with you more experienced guys for anything specific to look at. Also ordered another PCM in case mine just went bad.

Lastly, my LT1 Edit data log showed nothing unusual--all parameters looked good and it looked like the PCM was commanding increased fuel at 100% throttle.

Injuneer

Somewhere in there is a table of RPM vs. throttle position that sets the parameters for entering PE mode. It isn’t entered only at WOT (100% TP%). Check your TPS to make sure it's producing accurate readings.

Do both the wide band and the stock narrow band sensors confirm that it's going lean? Does the logging software that comes with LT1_Edit (pcmcomm?) not show the PE mode flag?

David94TA

I have my settings to enter PE at 40% throttle position at 2000 and then 30% from 2400 up, so it should be entering PE pretty early.

As you might imagine, I don't have a lot of data logging because when it starts backfiring and acting up, it scares the heck out of me.

Here is what I got:

Rt inj Lf inj TP Rt O2 Lf O2
11.25 10.82 96% 888 861
11.41 11.76 100% 915 888
13.34 13.69 100% 932 906
12.31 13.69 100% 932 861
13.15 13.69 100% 36 71
11.37 12.02 100% 915 897
14.08 12.89 100% 906 728
12.01 12.68 100% 813 870
12.34 13.28 100% 138 80
12.10 11.51 26% 750 138

Looks like it reads 10 values per second. Starting RPM was 3200--let off at 4550.

Wide band showed spikes in AFR also. I don't see a PE mode flag on the data log.

What gets me is that it looks to me like the PCM is commanding fuel. So far I have changed out fuel injectors, fuel filter, coils, coil wires and even wiring harness (thought I might have a break or bad ground somewhere). Plugs are fairly new gapped to .028. I sent my LTCC module to Bob Bailey and he said it checked out ok. The only thing I haven't changed is the small wiring harness from the LTCC harness to the coils, so I have that ordered. Also ordered another PCM in case this one is on the fritz.

I read the PCM into Tuner Cats and noticed on the "Constants Table" one called "Cool Thresh For TPS For WOT (High Temp)". This was set at the max of 151Celcius, which seems way too high even though I'm not exactly sure what that is.

Injuneer

At 10 frames/sec, the above about 1 second. Appears it's an intermittent lean.... not continuous. Pulse widths don't fall off, only the O2’s. It seems to be in PE (recognizing the limits of the narrow band sensors).. Misfire - dumps air and fuel into the exhaust and it lights off ? ? ?, but on both banks simultaneously would be odd. Not what I was expecting from your description.

David94TA

See, I thought it might be a misfire. But immediately after new coils, plug wires and harness, the problem was still the same. Well, I'll make the changes mentioned above and see what happens. Thanks for looking at it and if you think of anything I should look at, let me know.

David94TA

What if the optical sensor in the opti is going bad? Could it work well enough to function under normal driving, but then be off slightly under sudden acceleration? Since the PCM uses that as a cam reference, any deviation would give it a bad read and cause a misfire or firing at the wrong time. What would be the probability of that happening? I noticed an occasional studder this evening when I was just cruising that I haven't noticed before now.

Injuneer

I thought about the Opti. But that would take a series of misfires on both banks, and I can't think of a failure, other than in intermittent loss of the low res signal. Then the question becomes, why didn't DTC 16 set. Or did it set.... you won't know it unless you scan for codes. DTC 16 does not turn on the SES light. And, DTC 16 won't set until the PCM detects two full revolutions of the crankshaft (based on the pulses from the high res signal) without detecting a low res pulse. What happens if you briefly lose both signals? Boggles the mind.

Time for a frequent scan of the PCM for codes, and a good (hard copy) data log.

David94TA

Problem solved--finally. It was an ignition problem causing a misfire.