2005 lm7 5.3l
chopacabra cam
With springs
Tuned on e85
straight pipe with headers
Fuel pressure is at 58psi
When idling or crusing it runs perfectly fine but i try to give it alot Throttle it back fires thro intake. It randomly started after fueling up. Need some help with this

 

Backfiring through intake is intake valve related MOST of the time. I’d start by pulling the valvecovers and looking for a broken spring/retainer/lock or possibly youve bent an intake valve via piston contact….maybe an overrev….and it can’t fully close now. Thats where I’d start anyway. A leak-down test will confirm valve sealing. If all is good there, I’d go to data-logging while driving and look at that side of the issue.

 

running lean?

 

I wasn't ever reving at the time I just got some e85, I took off, then it started backfiring

Could bad e85 cause backfire? My truck is only tuned for 87 and e85

 

You do have a flex fuel sensor since you mentioned it is tuned for E85?
Or do you have to flash in a different tune for E85?

 

yes i do, it was tuned by calispeed in Bakersfield, ca on dyno. Couple of mechanics saying it probably has too water in the blend.

 

Since it happened right after fueling up then that would be the first thing I would check

 

Or it has so much extra timing in the FF and multiplier tables, it's lighting the plug off before the intake valve is closed.

I do not see how water in the fuel would make it backfire out of the intake.

The intake valve has to be open for that to happen.

 

what would cause extra timing? Bad camshaft sensor? Or some how an injector Decided to become leaky when I put fuel in it at the same time

update i smell fuel in oil, im guessing bad injector now

 

I would assume its a lean condition causing the "POP" or misfire condition.
The timing would have to be how far advanced to actually ignite before the intake valve closes????? That sounds absurd if it was running fine and this suddenly happened right after refueling

 

Was driving it all day hauling **** was driving perfectly normal. Until I fueled up, soon I left the gas station it started popping

 

Run through the tank and fill up with 87 and see what happens. If it drives normal, there you go. Always go back to the last things you did. Liljay is right, though. Check your timing tables on E.
Chevelle is also right, backfire on accel is intake valve related. If problem persists after swapping fuel, pull the heads.

 

Was planing to siphon fuel out and fill up on 87 and see if my flex fuel sensor is working.

 

It was the fuel, I just put 15gals of 87. Now its running good, imma start bringing tester now

 

The flex fuel sensor. It detects the amount of ethonal in the fuel, it adjusts the stoich value and can add additional timing.

Another thing is make sure the FF is actually working and reading properly.

If it's not, that will cause a very lean condition, but you would know because it would run like *** all the time.

Do you have a way to data log it?

 

No I dont have anything to data log but I can look at the values for everything but I cant log it. But the alcohol number was at 84% after fueling 87 it dropped down to 30% so I think it's working

 

Yeah if it dropped that much after filling up with 87 I'd say it's working.

 

Stared happening again more constantly now, runs like **** low rpms, imma pull heads soon

 

You pull the heads, do a leak down and compression test.

 

Maybe you missed the part where I mentioned the FF spark adder tables or don't understand how they work.