84 low compression
Thread Starter
Heel & Toe
Joined: Apr 2004
Posts: 21
Likes: 0
From: Humboldt tn
84 low compression
I think I'm in big trouble. I bought my sons '84 after keeping it in my garage for 5 years. It was never started. After spending a ton of cash getting throttle body gaskets, MAP sensor and throttle position sensor replaced at a dealer. I know I got took and don't need to be reminded. Anyway, after getting it back it ran like crap and would backfire and die. I checked the compression on all the cylinders and #4 is low only 20 PSI. Is this an indication of a valve problem or something bigger? I really need help on this as I know nothing about engines. If its the valve is there a rebuilt unit I can get or will I get took again? :beatdeadhorse: :mad
Race Director
Joined: Feb 2004
Posts: 12,517
Likes: 19
Re: 84 low compression (tenntomt)
I would have a leakdown test done. You can tell alot with this test. The air will go somewhere. Out tailpipe , intake,radiator, oil pan etc. This will narrow the problem down and let you know what you are in for without disassembling engine.
Drifting
Joined: Mar 1999
Posts: 1,931
Likes: 1
From: Norcross GA
Re: 84 low compression (tenntomt)
One step at a time. Lets find out what is wrong, before trying to fix it.
I'd pull the valve cover off that side and have a look at the #4 valve train. Are the push rods not bent, and properly seated in the rockers? Are the springs broken? When you turn over the engine,... are both #4 rockers moving by the same amount? Are the rockers properly adjusted?
If all looks good,.. I'd pull off both rockers from #4, then do a compression test on that cyl again. Did it improve?
What does #4 plug look like. Black? Wet w/ fuel or oil?
I'd pull the valve cover off that side and have a look at the #4 valve train. Are the push rods not bent, and properly seated in the rockers? Are the springs broken? When you turn over the engine,... are both #4 rockers moving by the same amount? Are the rockers properly adjusted?
If all looks good,.. I'd pull off both rockers from #4, then do a compression test on that cyl again. Did it improve?
What does #4 plug look like. Black? Wet w/ fuel or oil?
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Re: 84 low compression (Pete K)
:iagree:
You could redo the compression test, "wet", by squirting oil in the cylinders. This will indicate a ring problem. I agree that the cylinder leak down test will be more telling. If the other 7 cylinders were relatively high and within 10% of one another and your discription including, "backfire" makes me think of a burnt valve. You might want to just go ahead and pull the heads and look at the valves and gaskets. On an engine this old, it is likely it would benefit from everything an overhaul would provide. Good luck, and...
RACE ON!!!
EDIT While I was origonally typing this, ZylaRace posted. My "I Agree" was intended to reflect agreement with Pete K's post.
A compression test with the rocker arms removed, ain't gonna work. Where is the air to be compressed going to come from if the intake valve doesn't open???
[Modified by CFI-EFI, 8:48 AM 5/2/2004]
You could redo the compression test, "wet", by squirting oil in the cylinders. This will indicate a ring problem. I agree that the cylinder leak down test will be more telling. If the other 7 cylinders were relatively high and within 10% of one another and your discription including, "backfire" makes me think of a burnt valve. You might want to just go ahead and pull the heads and look at the valves and gaskets. On an engine this old, it is likely it would benefit from everything an overhaul would provide. Good luck, and...
RACE ON!!!
EDIT While I was origonally typing this, ZylaRace posted. My "I Agree" was intended to reflect agreement with Pete K's post.
A compression test with the rocker arms removed, ain't gonna work. Where is the air to be compressed going to come from if the intake valve doesn't open???
[Modified by CFI-EFI, 8:48 AM 5/2/2004]
Thread Starter
Heel & Toe
Joined: Apr 2004
Posts: 21
Likes: 0
From: Humboldt tn
Re: 84 low compression (CFI-EFI)
Thanks to all, I will pull the valve cover and look at the springs etc for #4. The other seven cylinders read between 135 and 150 psi. The plug was damp but not wet and was blackened but not too bad. Is the leak down test something I can do? If so how do I do it? Thanks again.
Race Director
Joined: Feb 2004
Posts: 12,517
Likes: 19
Re: 84 low compression (tenntomt)
A leakdown tester costs between $50 and $100. Try to borrow one. What you do is screw into spark plug hole and add 100 psi to tester. The guage will show psi in hole . If for example guage shows 70 then you have 30% leakdown. Next listen to air . Where it comes out shows oproblem. Example, If air comes out throttle body you have an intake valve not seating. This is an easy way to isolate problem.
Drifting
Joined: Dec 2000
Posts: 1,851
Likes: 5
From: Liberal Kansas
Re: 84 low compression (Pete K)
My first thought was 'was the engine running right when it was parked 5 years ago?
We had a similar situation on a vehicle that had been parked for a few years. Low compression on one cylinder and popping and banging out the exhaust and in the intake. Speculation (swag) was that either a valve was open while parked and surface rust formed on the valve seat area or the seat itself or a lifter plunger was stuck and wasn't pumping up. The longer it ran the better it got until it was fine.
Depending on the severity of the problem a little upper valve lube (mystic oil or atf) and an engine crank case flush might help.
We had a similar situation on a vehicle that had been parked for a few years. Low compression on one cylinder and popping and banging out the exhaust and in the intake. Speculation (swag) was that either a valve was open while parked and surface rust formed on the valve seat area or the seat itself or a lifter plunger was stuck and wasn't pumping up. The longer it ran the better it got until it was fine.
Depending on the severity of the problem a little upper valve lube (mystic oil or atf) and an engine crank case flush might help.
Thread Starter
Heel & Toe
Joined: Apr 2004
Posts: 21
Likes: 0
From: Humboldt tn
Re: 84 low compression (Lionsden)
Think I'm calling it quits. I've invested double what the car is worth and have nothing to show for it. I don't have enough mechanical skills to work on the problem and its not worth investing more money into it. Thanks for all the advise. :mad
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