engine miss
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Pro
Joined: Oct 2005
Posts: 581
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From: Boynton Beach Florida
engine miss
When I remove the plug wire from #4 cylinder there is no rpm drop.I took a compression test and It only had 90 psi ... # 2 and#6 both have about 120 psi.. any ideas on what I should check first. the engine was replaced by chevy in 1983 and is a stock 205 hp
Le Mans Master
Joined: Oct 2006
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From: Gladstone MO
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Give that cylinder a shot or two of oil thru the spark plug hole, then check compression again. If it comes up, it's rings. Do a leak down check too. Turn the engine to #4 firing position, both valves closed, and put air pressure in the cylinder. You'll be able to hear air at the intake or the exhaust and know which valve is leaking.
Just had another thought here. Have you swapped plug wires? A known good wire and plug into that cylinder. It's possible that a bad wire has been there for a while and carboned up the valves. They're not seating properly, and now your compression is low. Maybe not, but it's worth a shot-- no cost thing.
Just had another thought here. Have you swapped plug wires? A known good wire and plug into that cylinder. It's possible that a bad wire has been there for a while and carboned up the valves. They're not seating properly, and now your compression is low. Maybe not, but it's worth a shot-- no cost thing.
Last edited by TimAT; Feb 4, 2007 at 12:02 PM.
Le Mans Master
Joined: Oct 2005
Posts: 9,429
Likes: 44
From: Macon, GA
Give that cylinder a shot or two of oil thru the spark plug hole, then check compression again. If it comes up, it's rings. Do a leak down check too. Turn the engine to #4 firing position, both valves closed, and put air pressure in the cylinder. You'll be able to hear air at the intake or the exhaust and know which valve is leaking.
Just had another thought here. Have you swapped plug wires? A known good wire and plug into that cylinder. It's possible that a bad wire has been there for a while and carboned up the valves. They're not seating properly, and now your compression is low. Maybe not, but it's worth a shot-- no cost thing.
Just had another thought here. Have you swapped plug wires? A known good wire and plug into that cylinder. It's possible that a bad wire has been there for a while and carboned up the valves. They're not seating properly, and now your compression is low. Maybe not, but it's worth a shot-- no cost thing.
90psi is still sufficient for combustion, but not ideal of course. Sounds to me like you may have a valve hanging open or worn rings like Tim says. You should still check for a spark problem.
Racer
Joined: Jan 2006
Posts: 382
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From: Uxbridge Ontario
I had the same problem with #5 cyclinder it was down to 50 lbs. Adding oil to the cyclinder did not improve the compression so it was felt it was the valves. Took the heads off, the rings and cyclinder walls looked almost new. Did a valve job, replaced all the valves, new springs and roller rockers. Runs perfect now. It seems all the years of running with unleaded gasoline really does a number on the valves of the older cars.
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Joined: Sep 2001
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From: Wellington, FL
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