elliss12

iTrader: (1)

This is kind of an odd one so its going to be long and detailed, I apologize beforehand. 90k mile ls1 shortblock, good visual condition and clean, put 799 heads on and ls6 cam and 7.4 pushrods. I screwed up and had cam timing off, fixed timing and engine ran good. Later on developed a misfire once you drove the car and reved it up, found that cylinder 7 had 150psi compression cold while others were 210, compression would drop to 100psi once it started to misfire. I found bent pushrods so i pulled the heads, replaced all valves, put in a 228r cam, and got new pushrods. Engine started and ran normal, drove it and beat on it, same misfire. Now cylinder one has 150psi compression, 100psi once it misfires. Cylinder 7 has normal compression now and no misfire. Leakdown was the same on all cylinders and only audible leak was past rings. I bought an adjustable length pushrod checker, looked like i had a little too much preload so I went with the 7.35 pushrod which put me at .080" i believe preload. No fix there either. I pulled the engine and heads, found no cracks on the head, piston has no cracked ringland or abnormal wear, rings move free, cylinder has no scoring. I'm basically stumped at this point. If I had to guess, its rings or lifters pumping up or something stupid. Any help would be much appreciated. Thanks

elliss12

iTrader: (1)

I disassembled both lifters on the bad cylinder. Never torn one apart before but the piston that rides in the center moves freely, springs both have about the same compression. These are the same lifters that were in it when the valves hit the piston fyi. Anybody?

1FastBrick

iTrader: (16)

If the compression is low on 1 cylinder, look at the rings and the valve job on the cylinder

If it checks out mechanically, Then look into Fuel and spark.

What was the fuel pressure?

elliss12

iTrader: (1)

I dont see anything visually wrong with the rings/piston/cylinder. No air leaked past valves during leakdown so i'm ruling those out. Injector and plug were both firing when the miss occured, i checked those first. Im not sure on fuel pressure, all plugs look tan/dark. AFR's were all where they should be. Its definitely a mechanical issue im just not finding a single thing wrong. It also jumped from cylinder 7 to 1 after replacing valves and cam, thats why i was thinking lifters because they were likely swapped but they look normal to me.

1FastBrick

iTrader: (16)

Can you check crank endplay in the car off the balancer or something?

elliss12

iTrader: (1)

I could grab a friends indicator tomorrow and get a readout. What are you thinking?

1FastBrick

iTrader: (16)

Just want to verify it's not a thrust issue causing misfire.

Only other thing would be if the reluctor split or moved.

If everything checks out, Then You may need to de-sensitize the settings in the tune.

I am not a tuner, but I have heard this sometimes is required with an aftermarket cam.

I would verify everything checks out 100% before doing it though.

elliss12

iTrader: (1)

I'll verify tomorrow to see what the thrust is and compare to stock specs. The previous owner of the shortblock pinched the knock sensors so they arent pulling any timing or anything due to knock sensitivity. They arent working at all actually. When the miss happens, the only thing that changes is the compression on that one cylinder. Once it starts to miss, it will run on 7 cylinders until you let it cool down for awhile. Start it up later, runs on 8 until you get heavy on the throttle then dead misses for as long as you want to drive it.

joecar

Are you saying that once it misses you shutdown and do a hot compression test and find #7 low (and all others normal)...?

And then when it cools down you can start it and it runs ok until the misfire happens again...?

This does sound like lifters sticking, as you alluded to above.

What does your oil look like (does it have air bubbles in it, or anything else)...?

+1 check crank thrust/endplay.

I'm wondering if cam is walking (or trying to), check the retainer plate for wear.

RockinWs6

From here I'd look for 2 things.....cracked weak valve spring and tight valve stem holding valve open. Also very possible you have a loose valve seat.

elliss12

iTrader: (1)

Joecar- Yes, that cylinder always has less compression but when it is missing and I check it it drops to 100psi. The oil looked normal but to be honest I didn't pay too much attention. I'll check crank play then I could probably inspect the retainer.

Rockinws6- It did it before with yellow springs and it continued with the new PAC 1218s I put in. All valves seemed to go in smoothly with minimal drag. I didn't notice a loose seat but I can pull the valve and check.

RockinWs6

When you check it be sure to measure the stem clearance because cold it may move fine but once heated up it may be too tight. Also check the valves in that cylinder carefully for being even slightly bent. Here's another idea when this thing does this can you pull the valve cover quick before it cools off the loosen both rocker arms for that cylinder and check compression? It is possible for lifters to stick closed and hold a velve open but usually they stay that way. You have a odd problem, need to get creative.

I had one that would shut off in traffic when you come to a stop. Then it would restart and run perfect. ALL the lifters were bad, pumping up and holding the valves open. 3 other wrenches looked at it before I did and changed all kinds of things without fixing it. You can see its not always easy to diagnose a problem. You may need to seek out a very experienced wrench. Good Luck