Ok so I have a horrible backfire and I'm trying to start my new 396 for the first time.

At first it happened I thought it was the OPTI since it was used and had stored codes... I just replaced it with a new GM unit, and tried starting it again, same exact miss.

Here what it does, you start it, starts right up, idles real low then starts backfiring like its loading with fuel....my scanmaster shows the 02 readings at 800+ constantly (no cycling). My ecu was tuned by PCMFORLESS for this exact setup. I'm coming to the conclusion it is the culprit unless you guys can help; btw they're closed for the weekend, I already tried calling.

I already replaced the opti, coil, ICM, Opti Harness, and Coil Wire. All spark plug wires are Taylor 10.4 mm high quality units.

It just seems like it's getting way way too much fuel. Any ideas what to do???


Setup:
forged 396
1994
non-vented opti
no FMU
Vortech V-1 (breaking it in with an A-trim)
Crane Cams Hi-6
Hotcam Kit
4l60E

 

Bryan's tunes for my engines have always been spot-on and I've never had a problem with the tuning. I'm sure your gave him the injector size.

Have you checked the fuel pressure and the regulator?

Also, is the 0-Lash adjustment correct?

WD

 

I reset lash twice now...it's spot on.

I don't have a fuel pressure gauge so I can't check that until I borrow one.

Here's an update that may or may not help: When I unplug the MAF it'll fire right up, won't backfire and will run nice 'n sweet for about 3-5 seconds then just turn right off.

I replaced the MAF with a new one....still backfires.

 

I would put money on it not being the tune. Sounds like you have some plug wires on the wrong cylinders. Ya it sounds stupid, but I had a buddy have the same problem. He thought it was the opti. Come to find out, he had two wires wrong.

 

I have been humbled.

Plug wire #5 and #7 were switched. It's now almost 3 in the morning, and everything seems fine. I can't believe I missed such a simple step.

My new concern could this possibly have done any damage? My assumption is cylinder 5 would be firing late (on the downswing of the power stroke) since it had cylinder 7 spark timing, and cylinder 7 would be firing early (on the upswing of the compression stroke). The engine ran for nearly 7minutes the first time I started it (without backfiring) which is what threw me for such a loop through this whole ordeal. Could cylinder 7 be damaged from 7 minutes of what I'd assume to be garunteed detonation at idle/light throttle?

The car has never left the jack stands, and when it was first started the highest it was revved was 3k.

Today has been insane. I am however incredibly relieved the issue has been resolved. Thanks guys for all the help.

 

The timing is off. Or a plug wire is crossed (which amounts to the same thing).

Rich

ps: I guess you already figured that out, I suppose I shoould read the whole thread before replying.

 

I would say you are alright for the amount of time you ran it. My buddy ran his a lot more than that and his has been fine.

 

First, I'm glad to hear you found the problem and I know you were dealing with some serious stress! I've also had those occasions with my LT engine projects. I don't think the 5-7 plug reversal will damage your engine, or cause any long-range problems.

Once you get your 396 broke-in and have some tract times, post the info and let everyone know it runs.

Here’s some advance advice; if you don’t already have a 6-point roll-bar and 5-point harness, you’re going to need it! Because I’m sure you’ll be running times below 11.50 (11.49 and below NHRA requires a roll-bar)! My 396 went 11.52 first time down the track with a very easy launch.

WD