laughatrice

iTrader: (12)

Any help would be appreciated. Whenever I do a hard WOT pull on the track/street it moves up the power band as expected but above 5800RPM or so it USUALLY backfires falls on it face and either shuts down or sputters hard always accompanied with at least one large backfire (it blew my y-pipe off the headers). I can ease into the power and avoid this usually but shifting starts the problem again. It's building about 16psi at 6000rpm.

Most of my exhaust valves are failing leakdown tests (10-20% leakdown depends on the cylinder) I am all set to pull the heads and fix that but first I need to diagnose this problem or write it off as associated with the exhaust valve issues. The car idles normal however under light power such as a burnout it sounds like the plugs are fouled again this may just be the valves. I intend to swap the plugs as well.

Here's what I have considered:

1.) Exhaust valves leaking at high power/boost
2.) Fouled plugs or spark blowing out
3.) Fuel (currently can not monitor fuel pressure at WOT)
4.) Electrical

Here's what I've done

I have checked all grounds and plug wires found to be normal. I will change the plugs but I really don't want to fix the valves only to have this problem occur again and possibly cause more damage to the new valves.

Thanks

EPP

iTrader: (22)

Have you checked the valvesprings? Bob

laughatrice

iTrader: (12)

Hey Bob didn't think about that I'll do that tomorrow thanks.

black98ws6ta

iTrader: (77)

I have never heard a backfire unless the car was lean.

laughatrice

iTrader: (12)

Turns out that was it. I was running two fuel pumps off one hotwire worked for awhile seems to have caught up to me though. Now running the second pump off dedicated hotwire/relay problem solved for now.

slow67

iTrader: (2)

Usually backfires out the intake mean lean, pops out the exhaust is rich.

black98ws6ta

iTrader: (77)

Yea......

slow67

iTrader: (2)

Sorry misread your original post

laughatrice

iTrader: (12)

well its still doing it... Keep in mind I really don't want to pay for dyno time to diagnose this when I'm about to pull the heads anyway.

-high rpm band 6000 plus WOT sputters pops looses all power/shutdowns fired right back up when started again
-I dont think it's running lean because it's only making 10psi of boost and 5800rpms or so it was turned for 16psi if anything it's rich the wideband showed 10.0:1 and 5800rpms and 10psi.

I'm almost convinced that it the exhaust valves causing this problem. I just realized the boost was so low and there's no other reason for that other than the valves not sealing up. Could they also be causing the shutdown.

Last check with the leakdown test showed up to 20% leakdown on some cylinders compression test showed the lowest cylinder at 145psi while good cylinders at 190psi.

Any other thoughts on this I would be really pissed if I tear it down fix the valves and still have this problem.

black98ws6ta

iTrader: (77)

Sounds like you have alot of problems to be attended to. Being rich or lean could cause this problem, as said before lean usually causes a backfire in the intake. Bring overly rich usually sounds like a 2 step/poping out the exhaust.

In my experience a miss fire will show lean on a wideband. How was the car tuned maf or sd? Why is it making 6 less psi now then before? Does it run perfectly fine up untill 5800-6000? Why do you think it is exhaust valves leaking down, can u hear it in the exhaust? If it was tuned with a maf it could be seeing less boost, having fuel for more boost, and is so rich it will not pull through. It could also be valvesprings, they are always known to cause higher rpm problems.

laughatrice

iTrader: (12)

Yea it seems like stuff is poping up everywhere. It drives excellent throughout the power band only under WOT throttle and usually starting from a dead stop it breaks up.

There's no reason the boost should be that low unless I'm getting some major belt slip.

I could put it on the dyno

MFIC

Check the ground for the coils. Had the ground coil come loose on a Procarged truck and it would pop only at wot and run fine when cruising someone forgot to tighten the coil ground it was only finger tight.

laughatrice

iTrader: (12)

I've double checked the three ground straps on the back of the driver side head are those the ones or is there another ground I should look for.

MFIC

Drivers side, front/middle of the block 2" or so above the oil pan is the coil ground well on the trucks anyways.....

laughatrice

iTrader: (12)

I think I found the problem....this is my fast 90


It explains the lower boost and it could be the cause of the burnt exhaust valves... and hopefully it's what is causing the back fire or it could be a result of the backfire. Anyway I'm pulling the heads

Also lots of oil in the intake ports

laughatrice

iTrader: (12)

Heads off exhaust valves show very lean condition white on both sides of the valve. Pictures to follow

FBracing1222

So what happened? I'm having most of the problems u r with out the boost. So just wondering if u found it??

laughatrice

iTrader: (12)

The final result is most of the valves seats (intake too) were bad I do not blame my tune I blame the heads I don't think that should have happened. I'm having a valve job and seats replaced at the moment.

I think the WOT problem was related directly to the intake crack. My air fuel ration was down around 10:1 or less when it was only making 9psi of boost with the cracked intake. I assume it got so rich it ignited fuel in the exhaust and or cutoff. Not really a backfire more the opposite.

I'm switching to a ported LS2 intake.

Compressor Surge

iTrader: (21)

Mine backfires when I stand on it with low fuel. My tank is not baffled, and i'll be midway thru a pull, and the pump pickup uncovers from the fuel sloshing.....BANG!!! lol. Time for a sumped tank...