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5.0L (1979-1995) Mustang Technical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.

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Old 04-26-2013, 10:53 PM   #1
Dean Paradise
 
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Default 5.0 Compression Issue

I am considering purchasing a vintage Mustang which has a 1980's 5.0 Engine. 7 cylinders have a compression value of 75-77psi. One Cylinder has a compression test of 50psi. (Yes we checked it 3 times.)

1. Can anyone give me advice about the meaning of that result?
2. Could the valves be adjusted to improve the compression?
3. Is that something to worry about or not?

Thank you very much for your response.
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Old 04-26-2013, 11:24 PM   #2
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most definetly be very worried either the pistons are shot the valves are leaking badly or it could be a jumped timing chain. does the compression change when you add a little oil to the cylinder? this test will tell you if its the rings or valves if the compression improves significantly with oil added its got bad rings if not its a valve or valve timing issue. glad i could help muddy the waters for ya! im kidding good luck with it. what sort of vintage mustang are we talking about?
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Old 04-27-2013, 12:23 AM   #3
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Id say plan on rebuilding it if you buy it. Compression numbers should be over double that on a fresh motor.
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Old 04-27-2013, 04:51 AM   #4
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Those are pretty low numbers, Id consider looking elsewhere unless your getting a great deal on the car. The engine is tired , very tired. A compression test with that low of numbers on all the cylinders likely means the engine sat for a long time and the rings are locked up in the pistons. Ive seen some engines sit for a long time like that and spraying some pb blaster in to the cylinders and let em soak over night has restored most of the compression. If you buy it be prepared to do some work to it.
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Old 04-27-2013, 10:48 AM   #5
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Default 5.0 Compression- Short Block

Thanks guys for your response. There is one other question for you guys.
I forgot to mention that the engine is a late 1980's SHORT BLOCK. After 1985. it is a ROLLER engine.

Do those engines run at a lower compression?

I appreciate your response. The guy is asking top dollar for 69 coupe. It has a killer paint job and a killer interior, but I am very concerned that this is a tired engine. Do the short blocks run lower?

Thanks for your response.
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Old 04-27-2013, 01:17 PM   #6
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No, short block just means block + internals. Long block = block + internals + heads.
Compression numbers should be in the 160-170 range across the board for almost any internal combustion 4 stroke car engine.
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Old 05-01-2013, 01:14 AM   #7
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Sounds like a very worn out engine to me..........
Part that worrys me is the cylinder that only makes 50# compression, all 8 should be within 10% of each other.....
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Old 05-01-2013, 03:14 PM   #8
88 orangepeel notch
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Did you check with a good, name-brand tester? Cheaper compression testers are not reliable. I doubt a good tester will get the #'s you want, but I'm surprised that car even moved with that weak of a motor.
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Old 05-02-2013, 09:35 AM   #9
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Quote:
Originally Posted by 88 orangepeel notch View Post
Did you check with a good, name-brand tester? Cheaper compression testers are not reliable. I doubt a good tester will get the #'s you want, but I'm surprised that car even moved with that weak of a motor.
I had a mechanic check it while I watched. I had the same issue on another car with another mechanic. You cannot judge an engine by listening to it.
The seller wanted me to pay for another test, at another shop, rather than acknowledge the problem.
I passed on the car.

Thanks for all the suggestions.
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Old 05-02-2013, 09:58 AM   #10
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Good decision , after 3 tests with the same result he wanted you to fork out more money to have it tested again? I would have told him to go pound salt also....
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Old 05-02-2013, 09:58 AM
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