Backfire at low rpm

**5.02GO** · 11-04-2003, 06:40 PM

I have been having some backfires at low rpm. Usually in 3rd, 4th and 5th gears. Generally under 2000rpm.

I am running a Novi 1000 with about 4-5psi of boost. Fuel pressure is about 43psi. I am using Motorcraft AGSP 32c plugs. I have Edelbrock Performer 5.0 heads machined with the "O" ring groove.

I had this problem when i was running my GSS (ball drive) blower. Its like i let off of the throttle a little then get back in it, it has a backfire or two. I am running an FMU and a bypass valve. They are both irrelavent cause it did this befor.

Any suggestions?

Thanks...........:(

**regattacoupe** · 11-04-2003, 06:52 PM

what is your fmu calibrated at? when you stab the throttle you may be getting a fuel spike. Do you see any boost this low?

**UNTCHBL** · 11-04-2003, 07:48 PM

Sounds like it's way rich, and then igniting some of that unburnt fuel when it's still in the header.

**Chuck** · 11-04-2003, 07:53 PM

Does it backfire through the exhaust or the intake?

I had a problem with lean poping through the intake and I fixed it with a couple more pounds of fuel pressure.

**UNTCHBL** · 11-04-2003, 07:57 PM

I'll bet good money there is an e-cam involved too. :)

what kind of timing, and have you tried backing the fp off some?

**5.02GO** · 11-04-2003, 08:42 PM

Okay, the FMU is calibrated for 30lbhr injectors. Or atleast supposed to be. I had the backfiring befor i started using it though.

I cant tell if the backfiring is through the intake or exhaust. How can i tell. I kinda think its thru the intake cause its kinda quiet.

Yes, i am running an E-cam. I am currently running 8* timing. I have also ran 10* too. No change.

I havent reduced fuel pressure cause i really dont think i have enough fuel as it is. But i can try.

Anything else?

Thank You

**UNTCHBL** · 11-04-2003, 08:45 PM

Take it down 1# at a time. If you hear it ping, step it back up. Check your 02's. They could be fouled. Check to make sure there are no major exhuast leaks at the headers to head gasket. And check for unmetered air.

**5.02GO** · 11-04-2003, 09:08 PM

Originally posted by UNTCHBL
Take it down 1# at a time. If you hear it ping, step it back up. Check your 02's. They could be fouled. Check to make sure there are no major exhuast leaks at the headers to head gasket. And check for unmetered air.

okay...I am gonna go take it for a drive. Will report back in a few

**UNTCHBL** · 11-04-2003, 09:14 PM

cool.

**Chuck** · 11-04-2003, 09:16 PM

If it back fires through the intake its usually lean.

**UNTCHBL** · 11-04-2003, 09:29 PM

if it's through the exhaust it's usually rich :)

**green93notch** · 11-04-2003, 09:42 PM

if it's through the exhaust it's usually rich

ive had this happen, sounds like a gunshot, had the kids running around by my house screaming for help. lol. same with the lean one thru the intake, not a pleasent sound.

**5.02GO** · 11-04-2003, 09:54 PM

Okay gentlemen. Back from the test run. I think you'll be surprised with the results.

I backed the fuel pressure down to 40psi. It still backfired. I then took it down to 33psi. It STILL backfired.

I just read some new replies to this post and maybe i didnt describe the sound very good. Its not a LOUD backfire. More like a cough or something.

Anyway, i was going down the road with regular or normal driving. I was in 5th gear doing about 65. My tach was just under 2000. I decided to hit wide open throttle real quick. It coughed or backfired quite a bit. When i hit the throttle, i pushed .5 to 1 psi of boost.

Explain that? Why would the mothertrucker cough & stuff like that?

I dont get it:(