Chasing a misfire.

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DARTLARRY

DARTLARRY

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For a while now I have been running down why cylinder 8 is misfiring. I checked all ignition components, they are all fine. I did a compression check on the cylinder, and it was very low. If i loosen both rockers on that cylinder, compression comes right up. I am not an expert on this, but it seems that the valves are stuck open by the rockers. Is this an easy fix, Should I send the heads out to be repaired, or just buy new ones? This is a stock 2000 360 magnum for now, but eventually it will be getting a bigger cam. let me know what you guys would do.
 
DARTLARRY said:
For a while now I have been running down why cylinder 8 is misfiring. I checked all ignition components, they are all fine. I did a compression check on the cylinder, and it was very low. If i loosen both rockers on that cylinder, compression comes right up. I am not an expert on this, but it seems that the valves are stuck open by the rockers. Is this an easy fix, Should I send the heads out to be repaired, or just buy new ones? This is a stock 2000 360 magnum for now, but eventually it will be getting a bigger cam. let me know what you guys would do.
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Sounds like a lifter is siezed.
 
They still move up and down, just not enough apparently. I would much rather change a lifter than the head.
 
DARTLARRY said:
They still move up and down, just not enough apparently. I would much rather change a lifter than the head.
Click to expand...
Also check the valve stem heights compared to adjacent ones closed, maybe a seat has sunk, & the lifter ran out of accommodation.
 
new situation, i removed both rockers on cylinder #7, after I reinstalled them no compression on #7 now. It seems i am causing this problem. What am I doing wrong?
 
Start the engine and run it then retest compression to give lifter time to adjust
 
You can leave it sit over night and it will bleed down or pull coil wire and bump it a few times if you don't want it to start it will adjust its self
 
Did you drop something into the spark plug hole ? Did you run engine with compression tester hooked up? It can burn out the check valve and not hold compression
 
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Im not even checking with a tester, using my finger there is nothing. When i loosen the rockers, i have compression again. This just happened when I removed the rockers and put them back, I have done something wrong somewhere when reinstalling the rocker.
 
There are only a few possibilities. Here's all I can think of;
1) the lifter; is pumped up, or the seat is stuck up, or it is not on the base-circle
2) the pushrod; is too long, or is not seated properly, or has the wrong ends on it
3) the rocker arms are defective
4) there is a problem at the valve to seat interface , like a slightly bent valve
5) the cam is broken?

Of these, the most likely is #1, which @Killer6 already mentioned.
I would start comparing parts with another cylinder, with an eye to swapping them around.
You can back the rocker arms right off on the offending cylinder, and then do a LD test. The air pressure will push the piston to the bottom so wait for it to go. then read the leakage and determine where it is going. You don't even need a fancy tool for this, just inject compressed air and listen. Anything between 60psi and 120 is good but shut the compressor off so you can hear the leak.
 
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Back those rockers off a coupla turns, start the engine, and give each rocker a 1/4 turn while running.
At one point while tightening, it'll start to run ruff, stop tightening until engine smooths out, tighten a little more, it'll run ruff again, wait till it smooths, - repeat till rockers are tight, and engine smooth.
Replace cover .
 
I tried that, every time i tightened them even a little it would lose that cylinder. they never run off
 
I don't want to confuse everyone. I had compression, i removed the rockers and pushrods. when I reinstalled them, I lost compression.
 
last update: I went back out and did exactly what Inetia said. It took longer than thought it would initially, but it is running fine now,thanks for the help.
 
DARTLARRY said:
I don't want to confuse everyone. I had compression, i removed the rockers and pushrods. when I reinstalled them, I lost compression.
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Unless the pushrod(s) aren't seated in the rockers or lifters properly, it sounds like a lifter cup came up & stuck, why were they removed if I missed it?
 
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