69' 351W compression isssues help!? - Mustang Evolution

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Old 12-18-2011, 02:15 PM   #1
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69' 351W compression isssues help!?

Alright. Well i have a 1969 mustang with an original 2V 351W. I had a 2 barrell on it (stock) and it was nice and torquey. Well i blew my water pump and when i had the shop do the repairs i had them throw on a 4 barrel intake off a 69 cougar and a brand new holley 600. Well they ran a vacuum or compression check and told me my number 8 cylinder is like 70 percent off its compression. Now i can put my foot all the way into the pedal and not break 60mph and have lost all torq. I want to fix this myself but would like some suggestions on what it could be.
My first thought was that cylinder had a blown piston ring.
Another thought was the number 8 cylinder valve spring on the head was fkd.
could even bee a partially blown head gasket.

Im just reluctant to delve into it because I dont want to find out it needs a overbore lol. Tell me your thoughts. Thanks.
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Old 12-18-2011, 02:19 PM   #2
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69' 351W compression isssues help!?

Do your own compression check..also check the timing, the shop may have messed something up...I'd find another shop...but then maybe when the water pump went, it got hot and messed something up..
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Old 12-18-2011, 03:06 PM   #3
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69' 351W compression isssues help!?

Given that this was a "leaded gas" era engine and is 41 years old.... I would suspect valve seal erosion, or perhaps a blown head gasket or cracked head from the water pump incident that triggered all the repairs...... Sounds like the water pump episode was more catasrophic than you mentioned........ It isn't uncommon for an engine to lose ring tension, and hence; compression, if it's been over heated.
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Old 12-18-2011, 03:26 PM   #4
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69' 351W compression isssues help!?

You guesses are right on could be any of those, however generally with a bad ring you would have blue smoke coming from the exhaust from burning oil. And usually when it time for a rebore other cylinders would be showing signs of compression loss.

So if you dont have any smoke or oil residue on the 8th plug, its probably not the rings.

& if you dont have any coolant in the oil, it would point me to believe you have a faulty valve of some kind (bad seat or partially broken spring)
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Old 12-18-2011, 03:51 PM   #5
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69' 351W compression isssues help!?

It is common as the motor pulls its vacum through that cylinder. Probably getting close to time for a rebuild.
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Old 12-18-2011, 04:23 PM   #6
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69' 351W compression isssues help!?

If it's a piston ring ... that spark plug would be very oily. Which that in turn would cause the plug to foul out.
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Old 12-18-2011, 04:49 PM   #7
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69' 351W compression isssues help!?

Most likely a combination of issues from ring seal, to valve seat erosion, to a bad head gasket.
As for the water pump and prior heat issue, it all depends on just how hot it got.

A grossly worn camshaft lobe will also make for a weak cylinder pressure. With the ZDDP depleted shelf motor oils of today, that is a very possible issue.

As for cranking compression, anything less than 90-100 PSI for any given cylinder, and that specific cylinder will hardly fire.

What you should be doing to test how well things are sealed up(rings and valve seats) is a leak down test. Buy a dual gauge leak down tester from Summit for about $60, get an air compressor which can give you steady 100 PSI of regulated air and then start leak down testing at least the suspect cylinder.

If cylinder walls are microscopically coked, then you can buy a can of AC Delco Top Engine Cleaner and follow directions on the can. It will most certainly help ring seal, but it's still a Band-Aid.
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