So, I built my 6.0-370 almost 2 years ago. Been fighting with a cylinder 6 misfire situation. It used to be that the engine would run on all 8 cylinders for maybe 30 minutes, then start misfiring on #6 intermittently, then if you kept going after 30 more minutes-#6 would be dead. Lately it will only run on all 8 for the first 5 minutes, then #6 starts misfiring. It's funny because it only misfires between idle and 2500RPM. Once the engine is over 2500 I think it's running on all 8.. But maybe it's just the higher engine speeds are masking the misfire. But, the cylinder drops out after 5 minutes and will not come back for the remainder of that trip. Due to the misfire, the computer was dumping too much fuel on bank 2 and it was just killing all the engines power..

I changed out the following items trying to fix the #6 misfire;
Coil pack (multiple times, even swapped one coil stack from one side of engine to the other)
Fuel injector for #6
Wiring harness (entire engine harness)
Computer (flashed the new computer with HP tuners with old tune)
O2 sensor for that bank (thinking the sensor was giving a false signal and causing an excessively rich condition)
Spark plug (multiple times)
Spark plug wire (multiple times)

Compression test shows 165PSI on #6, and on 3 other cylinders that I tested.

Today I began thinking that maybe the lifters are bleeding down, or bad cam lobe. I removed the valve cover on bank 2, and started the engine. I wanted to see if a valve spring was broken, and if the valves for that cylinder were opening like the rest. The valves for #6 seem to be moving just as much as the others on either side. I did see something quite strange though.. While the other cylinders were dribbling oil at the rockers, BOTH pushrods for #6 were not... Then for some unexplainable reason, the intake valve on #6 squirted about a half ounce of oil out-all at once.. Then, while I was watching, about a minute after the intake valve, the exhaust valve for #6 squirted about a half ounce of oil out-all at once.. NONE of the other valves did this during the entire duration of the test.. What does this mean???? If anything..

I pulled the head, and can't see anything obvious. All pistons on bank 2 are completely carboned up. Because of the #6 misfire, the computer had been dumping excess fuel because it thought bank 2 was lean (unburned air in the exhaust from #6). Just a few days ago, I switched over to OL w/MAF sensor still active. I figured it would at least let cylinders 2,4, & 8 get the correct amount of fuel once I turned off the O2 sensors...

With the head off, I filled the intake chamber with water to see if the intake valve was leaking.. #6 was NOT leaking.. #2 was definitely leaking. Also, all 8 valves have damaged tips where the rockers push on them. The stem tips on all 8 valves mushroomed. You can squeeze the tops of the valves above the keepers and feel the rolled over edges. The face of the rockers, where they touch the valve stems have galled depressions in them. I pulled the lifters for #6 and there is no damage. The cam looks good. Maybe one of the 2 lifters is not pumping up, or is bleeding off, while the engine is running? I'm grasping at straws here. If anyone has ANY other ideas as to what I should be checking for-I desperately need your help.

I'm replacing the following items;
All lifters on bank 2
Head gasket
Sending head to machine shop to have all 8 valves replaced with brand new valves
5 Angle valve job
New rockers (will move my trunion bushings over to the new rockers)

Push rods were new when I built the engine and they show NO signs of wear on any of the ends. Engine has always had excellent oil pressure. Cold idle is 70-75 PSI, hot idle is 45-50. At 65 MPH, engine is 2100RPM and oil pressure is about 75PSI. No metal in oil. Engine is extremely clean. The rockers in the engine now were brand new, with the trunion bushings, when the engine was built. Cam is the Elgin P-1839. Nothing crazy...

I pray that fixing these things will magically cure the #6 misfire.. But I have my doubts.

The one other thing that is odd is that the pistons (flat tops) have raised carbon ridges on the tops matching the diameter of the intake valves. Almost like the pistons were hitting the intake valves-BUT, the opposite.. Instead of indentations in the piston top, there are raised carbon ridges on top of the piston that perfectly match the intake valve diameter... It's almost like the carbon build up is so bad on the piston tops that the intake valves have left witness marks in the carbon... So... Anyone that has read all of this.. Please tell me what I'm over-looking. What would you do differently? Should I just start with another engine? I feel like this has been the LEAST reliable engine I've had the misfortune of being involved with. I've built several other LS engines after this one and they are all still running perfectly fine. The longest this engine has run without issues has been just a few weeks.. Seriously.. For 2 years, it has not run consistently for more than a few weeks at a time... Maybe the car gods want me to suffer, or they are trying to tell me to just scrap this engine and start over. For sure, if the fixes I'm doing now do not make it more reliable, I will build a 408 stroker and literally sledge hammer this engine into bits just to feel better..

 

Man that **** is rough. I'm so sorry.

The sudden oil squirt out the pushrods could indicate the lifters were stuck. One thing that will help protect your valve tips is lash caps. If they get messed up it's a $40 fix vs cylinder head work.

Another thing that might be worth trying is closing the gap up to about .035 to decrease resistance and have a better chance of getting a spark.

Double, triple, and quadruple check your grounds on the backs of the heads.

If all else fails I have never seen an engine I could not melt in under 30 minutes....

 

My THEORY is the head on that side has a guide to valve clearance set too tight and the rockers are having to force the valves down, which would indicate the torn up rocker face and the tip of valves being mushroomed. Cyl 6 is a tad tighter than the other 3, and it’s causing one of those valves...maybe both...to hang open, just a smidge....(which should cause backfiring through the intake or exhaust but it’s not) when engine gets to temp....and causing the misfire. Man that’s a thin theory, but it’s early in the day and I haven’t thought about this one at all...
Id do exactly what your doing with it, and throw new lifters at it and get the head re-done by a good machinist. Might want to look at other side also before you take him the one head. Might need both re-done. Sounds like new rockers also if they are gouged up. If that doesn’t fix it, have Jake over and y’all melt that sucker into a puddle. Film it so we can watch!

 

The funky valve tips, galled rockers, and funky oil squirt thing make me think you have an upper end oiling issue. Mushrooming of the valve tips could be ptv contact related, hard to say but you did say you have some going on.

I haven't looked closely at the oiling setup in a while but on the later motors they have that screen underneath the oil sending unit, that is called a filter and it's for the lifters. Make sure it isn't clogged if you have one. Look at the lifters closely and the bores, any scoring or galling there?

How long are your pushrods? It's odd you're having ptv contact with a smaller cam, something is up there too. The ridges are close enough to call it ptv contact since they're apparently lightly touching.

May want to confirm your cam alignment is correct and you're dot to do while you have it torn down. I would also hook up to the front oil galley plug on the side of the block and blow some oil through there while the lifters are out to watch for good flow and to blow out any possible contaminates or restrictions in the upper end just in case you have something blocking the oil going to the top side.

 

Thanks for the support, and ideas. I dropped off the heads-both of them-today. I also ordered all new lifters (all 16), and all new rockers. I am also going with thicker, or standard thickness, head gaskets. Currently running the Cometic .041" gaskets. Going back with standard thickness to gain a little additional valve clearance. Not that I think that is the problem... The only contact the valves are having is with the carbon build up on top of the pistons on bank 2. When I pulled the bank 1 head-there is very little carbon build up on any of the pistons, and there is no intake valve impressions on that side of the motor. I am praying that this fixes the issue.. If not, the good news is, I have another engine that can be pulled and installed.

 

Offer to melt it stands

 

Darth... I honestly just want to be able to find the actual culprit of all this stress. It kills me that something has caused me to just want to walk away from this truck forever. If I can find whatever it is that is the cause, maybe I can mentally reconcile keeping the truck. I also would like to know what it is so that I can avoid experiencing the same issue in the future.

 

Ten four. Was trying to be funny and it was unhelpful.

After retreading your OP, I agree with top end oiling issue. But I am not convinced new lifters by themselves will be the fix. The #6 pushrod thing sounds very debris-like. As in something got through and plugged the pushrod and then suddenly let go, resulting in that shot of oil. Or same debris was not letting the lifter pump down and then broke loose.

If you do trace it down to lifters I would also replace pushrods.

 

On the pushrods.. I specifically pulled #6 intake & exhaust push rod and blew through them with my mouth to clear the oil. Neither one was plugged. Then I held them up to the sky and I could see light through them. I then checked all the other push rods for bank 2 by blowing compressed air through them. I have the push rods punched through a piece of cardboard so that I could keep everything organized and separate. None of the pushrods were blocked.

Should I disassemble the lifters for cylinder 6? I'm not re-using them anyways and maybe I could find something inside that would explain this. I think it's just a matter of removing the clip that holds it together. I'm gonna go ahead and do that just to see 1. Can i get them apart. 2. Is there anything obviously wrong with them inside if I can get them apart. 3. Check for debris inside and check the feed/bleed holes for obstructions.

Can anyone reccommend a good set of roller tipped rockers that are DROP IN and do not require studs, or guide plates??? I have another post going but not having any luck finding out of the box, drop in style lifters. Someone on the other post mentioned Comp Cams but I can't seem to find them.

 

On the oiling issue... It could very well be an oiling issue. The machine shop pressed new cam bearings into the block. I did the assembly here. Cam was brand new, lifters new, rockers (GM Style) new, trunion bushings new, push rods new, timing chain set new, oil pump new..... Could they have pressed the cam bushings in not clocked correctly and it's starving the top end for oil? Some GM engines have a groove cut all the way around behind the bushings so that no matter how it's clocked the oil will have a channel to go around until it gets to the feed hole. I don't remember on the LS if that is how it is or not.

 

I'm also running the after-market, aluminum, dumb bell piece in the back of the engine block.. In case that has any bearing on upper end oiling. This block is a 2002. Not sure if it has an oil filter under the oil pressure sending unit like someone mentioned above. Are there any other things I can check oiling wise without tearing the motor down further? I've got both heads off, but the valley cover is still on.

 

I know this is a old thread but from the sounds of it my 05 5.3 Silverado is doing this as we’ll heads coming off tomorrow and all the the symptoms are the same 2500 rpm misfire and at idle cylinder 3 ,motor just has a baby’s cam and supporting mods have done all the above coils, plugs wires, injector flow testing, electrical diag just was wondering thanks hope to not dig up a old thread 👍🏻 Not long ago the cam was installed but just recently it did this 02s are running normal in spec only thing that is weird is when you unplug the maf it seems to run normal have already swapped mafs and it didn’t correct the problem any info would help thanks 🙏🏽