WOT Backfire
WOT Backfire 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
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
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Verdad Gallardo 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.
-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.
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.
-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.
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.
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
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
Launching!
Joined: Aug 2007
Posts: 244
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From: Reno NV
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.
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
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
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.
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.
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...
