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FE390 Compression Results - Diagnosis?

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Old 01-31-2018, 06:00 PM
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FE390 Compression Results - Diagnosis?

Just checked the compression on my 66 F100 and hoping to get some opinions on what the readings mean. I have my own opinion,but no real experience with this sort of thing.

This is supposedly a 390, supposedly from a 1968 GT, no real reason to doubt it. I have not seen any blue smoke, but it does have a dead cylinder, as you can see, the dead one is #2. Compression was checked with all spark plugs out and throttle completely open.

Here's what I got - what do you think?

Cyl Dry Wet
1 114 130
2 40 50
3 102 102
4 120 125

5 120 120
6 130 142
7 94 100
8 126 135

Thanks!


-Bob
 
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Old 01-31-2018, 09:36 PM
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Anything below 120 is getting pretty damn low in my opinion, I believe 160ish is the norm for a respectable used engine. So you definitely have some sealing issues in pretty much all your cylinders. So pretending you didnít already have a dead cylinder, a rebuild is highly recommended
 
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Old 01-31-2018, 10:44 PM
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its time for a rebuild as 70fe250 said,those numbers are pretty low,that one cyl with 50lbs wet has a bad problem inside,
 
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Old 02-01-2018, 06:39 AM
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Thanks for the replies. That's what I was afraid of.
 
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Old 02-01-2018, 10:33 AM
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Cylinder 2 has a problem, cylinder 7 is to low to run, cylinder 3 is border line for a very worn engine or valve problems.
Pull the valve cover and check valve lift, you could have some worn cam lobes, do a leak down test to see if it's loosing compression around the valves, rings or both.
68 engine won't have hardened seats and it may simply have some sunk valves and only need head work.
 
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Old 02-01-2018, 11:07 AM
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Because there's not much difference between wet and dry, I suspect all your valves are leaking. Most likely, the exhaust valves have sunken, maybe even to the point of that dead cylinder exhaust valve riding on the bottom of the hydraulic lifter making it stay somewhat open.

Depending on how much oil you put in the cylinder, the difference in wet vs dry could be that the compression ratio has increased because of the space taken up by the oil

My humble opinion: You more than likely need a top-end rebuild. A low-budget solution would be to throw a set of good used heads on it. A leak-down test will tell you a lot more, especially if you listen for air escaping into the intake or exhaust.
 
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Old 02-01-2018, 03:34 PM
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Originally Posted by Destroked 450 View Post
Pull the valve cover and check valve lift, you could have some worn cam lobes, do a leak down test to see if it's loosing compression around the valves, rings or both.
68 engine won't have hardened seats and it may simply have some sunk valves and only need head work.
Originally Posted by Krewat View Post
Most likely, the exhaust valves have sunken, maybe even to the point of that dead cylinder exhaust valve riding on the bottom of the hydraulic lifter making it stay somewhat open.

My humble opinion: You more than likely need a top-end rebuild. A low-budget solution would be to throw a set of good used heads on it. A leak-down test will tell you a lot more, especially if you listen for air escaping into the intake or exhaust.

Thanks! that makes senses, and it's something I can check out.

Replacing the heads would be a workable solution if I can find some used heads that I can trust. I don't really want to throw tons of money into this particular truck via rebuilding the engine.


I'll pull the valve covers and see if I can tell how the cam lobes are doing, and then maybe get a leakdown test done. I don't have a compressor, so have to get a shop to do it for me.
 
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Old 02-02-2018, 09:45 AM
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Was this a car 390 or truck 390 ?
120 is about where a Truck 390 would be
But either way #2 and 7 are issues !
 
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Old 02-02-2018, 12:11 PM
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He said it was supposed to be a 68 GT engine which would be from a car.
I knew they made a 390GT engine for the 66-67 models but wasn't sure if they continued them into the 68-69 models.
Didn't said anything before because I didn't want to get things off topic.
 
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Old 02-02-2018, 12:25 PM
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whats the compression for a 68 gt 390,i had one 30+ yrs ago i think it was a very high compression engine,
 
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Old 02-02-2018, 01:00 PM
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According to my Motor's manual, 10.5:1.

Be careful if the engine was determined to be a "GT" by the casting numbers. it really could be anything.
 
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Old 02-02-2018, 01:23 PM
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Originally Posted by Destroked 450 View Post
He said it was supposed to be a 68 GT engine which would be from a car.
I knew they made a 390GT engine for the 66-67 models but wasn't sure if they continued them into the 68-69 models.
Didn't said anything before because I didn't want to get things off topic.
66-68 for the 390GT engine. The camshaft, valve train, pistons and carb (Holley 600cfm) are what differentiates a GT from a regular 390 4V passenger car engine. In '69 they de-carbed and de-cammed the 390GT and called it the 390IP (IP for improved performance).
 
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Old 02-02-2018, 02:00 PM
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Yeah, it came out of a car - still has the car-style oil filter mount. Is it really a GT (or even a 390)? who knows? The heads are C8AEH, correct for a GT, but also for other engines, too.


I knew # 2 was hosed, but #7 surprised me, it responds about as much as the other 6 cylinders when I pull the plug wire off. For the compression to be that low, don't those two cylinders almost have to have valve (or Cam) problems?




I was a bit surprised at the overall readings because it doesn't smoke. If the rings are really bad, shouldn't I be seeing plenty of blue smoke?
 
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Old 02-02-2018, 02:59 PM
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66-67 GT heads had the 14 bolt hole pattern using special manifolds
Simular to the 16 bolt pattern used on 428CJ heads
Not sure about the 68-69 390GT-IP heads


If the rings are bad, yes you should see smoke
If the valves are leaking or sunk, No it may not smoke.
 
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Old 02-02-2018, 03:57 PM
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Originally Posted by Destroked 450 View Post
66-67 GT heads had the 14 bolt hole pattern using special manifolds
Simular to the 16 bolt pattern used on 428CJ heads
Not sure about the 68-69 390GT-IP heads


If the rings are bad, yes you should see smoke
If the valves are leaking or sunk, No it may not smoke.
1966 390's (2V & 4V GT) installed in intermediates all received C6AE-R heads drilled for 14 bolt exhaust manifolds.

1967 390's (2V, 4V & 4V GT) installed in intermediates all received C7AE-A heads drilled for 14 bolt exhaust manifolds.

1968 390's (2V, 4V & 4V GT) installed in intermediates all received C8AE-H heads drilled for 14 bolt exhaust manifolds.

1969 390's (4V IP) installed in intermediates all received C8AE-H heads drilled for 14 bolt exhaust manifolds.

As you can see 14 bolt heads and exhaust manifolds were used in a lot more applications that just 390 GT engines. But for some reason guys look at the 14 bolt holes and associate them as "GT" only.
 
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