Blacked Out

Hey guys I have had a backfiring problem in my car for a while now. It only happens under a low rpm high load scenario. It happens like when I am on the highway going up hills at about 73mph. I will have to down shift to 5th to grab some rpm and it goes right away. It also does it occasionally on take off where it feels like it is stumbling then is backfires and goes. It also does it simply if I am driving at about 1700rpm or so and I apply decent throttle, it stumbles and backfires until more rpms are attained. My mods are a MS3 cam package from Texas Speed, under drive pulley, intake, ls6 intake mani and 32lb injectors. I checked the plug wires last night but did not check the plugs due to it getting real late. We plan on checking the plugs soon when we do a compression test too. We are confused because the car runs real strong in the mid to upper rpm situations, so if it was a bad plug it would suck the entire time right? We also checked for vacuum leaks but we could not find any. The MAF was cleaned not to long ago as well. We are tuning with EFI live and I have the last batch files if anyone wants to take a look at them. Any help or ideas would be appreciated guys thanks.

Ryne @ CMS

iTrader: (10)

sounds to me you dont have the car tuned properly down low, your fuel trims are properly way positive, and its going way lean that is way it backfiring, you should log the fuel trims and try to simulate the problem then look at fuel trims.

Blacked Out

Ok here is the deal with that. Last night we were logging using wide band only. It was some deal where we turned off the 02's so we were in open loop only (I think its open not closed loop when the 02's arent used) as well as turned off the MAF. We did a few runs like this with the first being lean. So we increased fuel according to the amount the wide band said like 20%. The next log it was still pretty lean so we increased fuel by the amount according to the wide band about 20% again. Then we logged again and it was still lean which makes no sense. It would back fire the same way and times it does normally when the MAF and 02's are in play. The guy who was helping just did this to his vette the other night and it came out perfect in to logs. The wide band it telling you how lean rich you are then you change it accordingly, so it should be straight in just a few flashes. However mine never really made significant improvements away from lean. He thinks there may be an underlying issue affecting the tunability if you will. While doing this my normal after cat AFR was 15-17 not at WOT. He is unsure if it is truely a lean pop that we are hearing.

Blacked Out

Bump for today. No one else has any other ideas? I am going to clean my MAF tonight and see if that helps cause in my log the MAF line was pretty jagged and not smooth at all.

Billdog350

iTrader: (3)

Ryne, I agree that it sounded like a lean pop (due to bad tune or defective 02's causing Positive LTFT's). However, the LTFT's weren't part of the equation since we were doing an Auto VE tune (LT learning is turned off). His VE didn't respond as quickly as it did when I tuned my C5 (after cut and paste my tune was damn close but Patricks was still off by quite a bit even after multiple tunes). If we clear the LTFT's, change the maf/map tables, etc...the problem seems to be the same...which points my guess toward a spark issue (timing is set about the same as my PRC 243, 228/232, LS6, Vararam, LG Street setup on my C5) so its not like timing is over advanced.

To reiterate: the backfiring occurs with fuel trims on or off, cold ECT or warm ECT, open loop, closed loop.

The strange part is that its only under low rpm and heavy load (lugging uphill in low gear, or punching the throttle at low rpm), which seems to align with a lean tune.

3.4camaro

iTrader: (1)

Low rpm + high load + sputtering/backfiring = textbook ignition problem.

Look at some new plugs and wires.

Billdog350

iTrader: (3)

The wires and plugs only have 10k mi on them, we just removed and ohm'd each wire out and got almost exactly 670 ohms for each one so they seem to be ok. I agree pulling plugs will tell alot. Compression test will also show if there is a burnt or sticking valve. Sounds like that's the next direction. Thx!

Blacked Out

Yeah I should try for a compression test/spark plug check for next week assuming that its not snowing. If compression is ok and plugs are fine/replaced and it still sux I will chime back in. Thanks guys.