Need help with backfiring-shutting off

[67SS509]

I'm having a problem with my 98 Camaro race car and hoping someone can help me with it. It has a LS7 427, GMPP carb style intake, running mafless OLSD making 650-700 hp at the crank. Transmission is a powerglide and the only accessorie on the engine is the alternator. I tuned it with EFIlive intergrated with a FJO wideband. It cranks and idles good, revs great and pulls strong. Problem is everytime I go WOT somewhere in the 4000 to 6500 rpm range it will backfire thru the intake or exhaust and shut off. My theory is the PCM drivers are turning the coils off. I have replaced all sensors, coilpacks and a different PCM. Fuel pressure never varies from 60psi. Wideband always shows <12.8 afr. I have done a full reflash, checked and double checked wiring and grounds and also crankshaft relearn. I also cut the VE table 25% and the IFR 25% to see if it was a torgue limit problem. Logs with my interpretation say the injectors keep firing hence the backfiring. Any help will be greatly appreciated because I have been battling this for 4 weeks now. My next move will be to change over to a 2002 pcm and wiring harness. I really hate to do that because of work and cost.

Attached is my current tune and a short log showing the pop. Actual pop is at frame 17.

[odd boy]

It is better if you get 99+wiring harness and then install new OS. I will look at your file when I get back home.

[wait4me]

YOU CAN'T turn off the mass air meter p0102 completely. What is happening is, the car for some reason is not flaging a failed mass air meter yet, so it is just doing a BLEND of VE vs MAF still. Since you arent using a Mass air meter, the system is just using full VE. BUT at 4000 rpms, the computer switches to high speed MAF only mode, so that is where you are failing.


So to fix your problem, you need to change p0101,p0102,p0103 codes to "C" non emmissions.

[wait4me]

Also using a 99 plus pcm isnt needed unless you are boosted..

[odd boy]

Also using a 99 plus pcm isnt needed unless you are boosted..

we face some problems with knock sensors if we use 98 harness

[wait4me]

Sensors are all tuneable. If you have an issue like that, just retune the sensor in question. I dont see how it would be beneficial to spend hundreds and hundreds of dollars and spend all that extra time to fix something simple like that. I have tuned so many cars of that year, and see all the people that think they have to swap to supposedly make more horsepower than the 1998 computer. An engine is an engine. Timing, fuel, ect... If they are the same, it should make the same power.

[odd boy]

I do agree with u, but a common problem with tuning 98 cammed cars, sometimes we get the cam DTC on even after replacing them with new ones

[67SS509]

YOU CAN'T turn off the mass air meter p0102 completely. What is happening is, the car for some reason is not flaging a failed mass air meter yet, so it is just doing a BLEND of VE vs MAF still. Since you arent using a Mass air meter, the system is just using full VE. BUT at 4000 rpms, the computer switches to high speed MAF only mode, so that is where you are failing.


So to fix your problem, you need to change p0101,p0102,p0103 codes to "C" non emmissions.

Would raising the (B0120) "rpm threshold for airflow calculation" to something like 12800rpm (limit)possibly help fix it also?

[wait4me]

Just try the above fix first.

[67SS509]

Set the MAF DTC's to non emissions today but it still shut down when about 50% throttle was reached. Then I tried raising the airflow calculation to it's limit of 12800. Still shut down. I can't even think of anything else to try or check.

[67SS509]

Hoping maybe I solved my problem today. I installed a 99-04 style stand alone wiring harness with a 2002 operating system and calibration. Then I copied most of the 98's tables to it. Cranked right up and idled good. After it warmed up some I revved it to 65-7000 rpm several times and no pop. So far so good but will know for sure next week when I start tuning the VE table and timing.