crazy misfire
Thread Starter
Staging Lane
Joined: Nov 2015
Posts: 82
Likes: 0
crazy misfire so i have a ls1 with PRC 2.5 head, 7.350 push rods, stock rocker arms, morel link bar lifter, and a 228r cam. everything from texas speed.
and a flashed ecu from texas speed as well.
with the motor started I'm getting a mis-fire from cylinder 2, 8, and 7.
i determined these are the cylinder were not firing because of there lack of heat in the header tubes.
I've checked to see if the spark plugs for these cylinders are producing a spark. and all of these cylinders are in fact producing a spark.
now for the check for fuel: the spark plugs are coated in gas, and smell of gas as well.
so im guessing that the missing portion for the combustion is air. is there a way to check to see if the valves are stuck open? or maybe there bent?
BY the way everything is brand new, just installed. Ive tried putting in 7.400, 7.300 push rods in the motor. the 7.400 push rods left the valves hanging open so the motor would start. I put the 7.300 push rods in and had a heavy metallic tapping sound, so purchased 7.350 which are in the head now.
and a flashed ecu from texas speed as well.
with the motor started I'm getting a mis-fire from cylinder 2, 8, and 7.
i determined these are the cylinder were not firing because of there lack of heat in the header tubes.
I've checked to see if the spark plugs for these cylinders are producing a spark. and all of these cylinders are in fact producing a spark.
now for the check for fuel: the spark plugs are coated in gas, and smell of gas as well.
so im guessing that the missing portion for the combustion is air. is there a way to check to see if the valves are stuck open? or maybe there bent?
BY the way everything is brand new, just installed. Ive tried putting in 7.400, 7.300 push rods in the motor. the 7.400 push rods left the valves hanging open so the motor would start. I put the 7.300 push rods in and had a heavy metallic tapping sound, so purchased 7.350 which are in the head now.
TECH Addict
Joined: Aug 2005
Posts: 2,305
Likes: 126
From: Monroe,NY
If those header primaries are staying cold those cylinders aren't firing. Make sure your wires are good. Swap a bad cylinder coil with a good one and see if the problem changes cylinders. I would guess especial 7 and 8 cylinder your wire isn't on all the way since those are hard to get to.
Yes you need to do a compression check. You also need to shoot oil in to test the rings (after initial test to look for increase)
Lots of possibilities here. Maybe the pushrods are short, but it should still get air. Possibly the overlong rods that were first used resulted in PV contact....
If compression is very low I would look for bent valves. If no bent valves, consider verifying cam centerline degree.
Lots of possibilities here. Maybe the pushrods are short, but it should still get air. Possibly the overlong rods that were first used resulted in PV contact....
If compression is very low I would look for bent valves. If no bent valves, consider verifying cam centerline degree.
Thread Starter
Staging Lane
Joined: Nov 2015
Posts: 82
Likes: 0
Zero compression on number 8 cylinder.
And 150psi on cylinder number 6. Weird thing is that all cylinders had 210psi before can swap and push rod swap
looks like i messed something up.
will there be visual evidence of the valves being bent, once i remove the heads?
And 150psi on cylinder number 6. Weird thing is that all cylinders had 210psi before can swap and push rod swap
looks like i messed something up.
will there be visual evidence of the valves being bent, once i remove the heads?
Last edited by atl1234; Mar 31, 2016 at 11:59 PM.
) Sometimes, you can look at the valves in the combustion chamber and see that they aren't fully closing. But that would have to be a pretty bad bend to see it that way.If you see any evidence at all, which I highly suspect you will in #8 with zero compression, You'll probably want to remove the valves and check them all. What I do is put the stem in a drill and bump the trigger on the drill. A good valve will not change with rotation. A bad valve the head will look like its wobbling as it spins. Check them all for peace of mind.
When I did this, it was a BMW 2.5L, and even valves I thought had not made contact were bent.
Before you disassemble anything it would be beneficial to do a leak-down test on the cylinders. By using compressed air to fill the cylinder you can determine if air is blowing into the intake (bent intake valve), into the exhaust (bent exhaust valve) or into the crankcase (blow by rings). But then again, if you have zero #8 compression it's got to come out anyway.
