Lost Compression in the #4 cylinder...
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Joined: Aug 2001
Posts: 75
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From: Pueblo Co
Lost Compression in the #4 cylinder...
I lost compression in the #4 cylinder and started pulling my hair out. I rebuilt my 85 three years ago, and have about 5000 mile on the rebuild. During the rebuild, I add AFR Heads, TPIS cam, Hooker Comp headers, I ported the runners & plenum, 24lbs injectors, 58mm Throtle body and a new chip.
I had 60 lbs of pressure in the #4 and 140lbs in #2. When I pulled the valve cover the intake rocker was real loose. I lashed it back down and the compression went back to 140lbs. I then started checking the rest of the rockers and #5 was a little loose too.
What would cause this? Could it be just the rocker nuts, or something else? I bought the locking nuts today for about $35 and am going to put them on tonight, but I would like to find out if I have a bigger problem. Any input would be greatful.
I had 60 lbs of pressure in the #4 and 140lbs in #2. When I pulled the valve cover the intake rocker was real loose. I lashed it back down and the compression went back to 140lbs. I then started checking the rest of the rockers and #5 was a little loose too.
What would cause this? Could it be just the rocker nuts, or something else? I bought the locking nuts today for about $35 and am going to put them on tonight, but I would like to find out if I have a bigger problem. Any input would be greatful.
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Joined: Feb 2004
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Originally Posted by racer69
Once I relashed the valve, the compression came back up.
Racer
Joined: Nov 2005
Posts: 456
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From: Sumner Texas
There are two ways to determine if compression is from valves or piston/cylinder. Obnoxus mentioned a leak down tester which is the method I prefer.
With a leak down tester, apply pressure to the weak cylinder and listen for where the air is hissing. If you here it from the intake manifold, the problem is an intake valve, if from the exhaust pipe the problem is an exhaust valve and if you here it hissing at the oil filler cap it is a piston/cylinder/ring problem.
The other way is to take your compression reading and make note of the value, then squirt a little oil in the cylinder through the spark plug hole and then take the compression reading again. If the compression comes up significantly, the problem is piston/cylinder/ring related, if not it is a valve or something else.
Good luck,
With a leak down tester, apply pressure to the weak cylinder and listen for where the air is hissing. If you here it from the intake manifold, the problem is an intake valve, if from the exhaust pipe the problem is an exhaust valve and if you here it hissing at the oil filler cap it is a piston/cylinder/ring problem.
The other way is to take your compression reading and make note of the value, then squirt a little oil in the cylinder through the spark plug hole and then take the compression reading again. If the compression comes up significantly, the problem is piston/cylinder/ring related, if not it is a valve or something else.
Good luck,
Team Owner
Joined: Oct 2004
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From: altered state
St. Jude Donor '05
Do you have pressed in or screw in studs. The press ins have been known to back out over time if you are using a higher lift cam..That would defintely create a problem
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Joined: Jun 1999
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From: Merritt Ils Fl
Originally Posted by cuisinartvette
Do you have pressed in or screw in studs. The press ins have been known to back out over time if you are using a higher lift cam..That would defintely create a problem
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Thread Starter
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Joined: Aug 2001
Posts: 75
Likes: 0
From: Pueblo Co
It turned out to be a couple of bad lifters. I don't know how this could happen with so few miles but it did. I need to pull the intake this weekend anyway, so I guess I will get some new lifters.
