I saw somewhere that compression on the inline 6 is supposed to be 175 or so new, and that of course there is variances between cylinders. When the engine was still running, I did the carb cleaner test looking for vacuum leaks, and found none on the intake, head, exhaust manifold, base of carb, vacuum ports, manifold bolts, etc.
Mine was running rough, so I did a dry compression test and got this:
#1 - 130 psi
#2 - 137 psi
#3 - 138 psi
#4 - 60 psi
#5 - 55 psi
#6 - 100 psi
The average was therefore 103 psi. The lowest, 55 psi, was 53% less than the average - well more than than the 15% variance recommended by some engine builders (I'm not an expert, so don't shoot me).
My highest reading was 138, and 15% of this is 21 psi. The number I don't think I'd want to be below would be 117 psi (138-21). Three cylinders, or 50% of the engine,are below this, so then I did a wet compression test (added 2 tablespoons engine oil) on the low cylinders, and got this:
#4 - 70 psi (up 10 psi)
#5 - 60 psi (up 5 psi)
#6 - 120 psi (up 20 psi)
Now, I know that adding any fluid to combustion chamber will raise any compression, so increases I saw in the compression are nominal at best.
I have the valve cover off, and checked torque on all head bolts. When cranking the engine, all valves bob up and down as you would expect. Engine is hydraulic lifters, and this doesn't have valve adjustment, so not sure what else I can do at this point.
I think that because the amount they went up was minimal, I am hoping to rule out pistons or rings. However, I know that if the compression values don't go up between tests, the problem is usually in the cylinder heads. I think that if the compression went way up, then the rings would be warn and bad, and I'd be doing a bottom-end job. As the compression values stayed kind of the same, it feels like a cylinder head valve problem.
Called the machine shop, and they are two weeks out, so I have some time to continue diagnostics. They said that there really wasn't any point in doing a cylinder leak-down test, as we know that the compression is so low in 2-3 of the cylinders. Should I do one anyway? I don't own a test tool to accomplish this, though.
Let me know what else you think I should test. I've sunk a lot into this engine already in new parts - distributor, carb, exhaust manifold, plugs, points, cap, rotor, coil, fuel pump - and testing every aforementioned system, and I just cannot make it run worth a damn.
Note: I did the tests with the throttle plate wide open, as I think you should. However, I just realized that the intake manifold had a line detached during the compression testing. I think that having the manifold not plugged won't affect the results I saw in just 3 of the cylinders, but could you more technical folks confirm? Wish I could find my old high school "the internal combustion engine" book from the 50s.
Mine was running rough, so I did a dry compression test and got this:
#1 - 130 psi
#2 - 137 psi
#3 - 138 psi
#4 - 60 psi
#5 - 55 psi
#6 - 100 psi
The average was therefore 103 psi. The lowest, 55 psi, was 53% less than the average - well more than than the 15% variance recommended by some engine builders (I'm not an expert, so don't shoot me).
My highest reading was 138, and 15% of this is 21 psi. The number I don't think I'd want to be below would be 117 psi (138-21). Three cylinders, or 50% of the engine,are below this, so then I did a wet compression test (added 2 tablespoons engine oil) on the low cylinders, and got this:
#4 - 70 psi (up 10 psi)
#5 - 60 psi (up 5 psi)
#6 - 120 psi (up 20 psi)
Now, I know that adding any fluid to combustion chamber will raise any compression, so increases I saw in the compression are nominal at best.
I have the valve cover off, and checked torque on all head bolts. When cranking the engine, all valves bob up and down as you would expect. Engine is hydraulic lifters, and this doesn't have valve adjustment, so not sure what else I can do at this point.
I think that because the amount they went up was minimal, I am hoping to rule out pistons or rings. However, I know that if the compression values don't go up between tests, the problem is usually in the cylinder heads. I think that if the compression went way up, then the rings would be warn and bad, and I'd be doing a bottom-end job. As the compression values stayed kind of the same, it feels like a cylinder head valve problem.
Called the machine shop, and they are two weeks out, so I have some time to continue diagnostics. They said that there really wasn't any point in doing a cylinder leak-down test, as we know that the compression is so low in 2-3 of the cylinders. Should I do one anyway? I don't own a test tool to accomplish this, though.
Let me know what else you think I should test. I've sunk a lot into this engine already in new parts - distributor, carb, exhaust manifold, plugs, points, cap, rotor, coil, fuel pump - and testing every aforementioned system, and I just cannot make it run worth a damn.
Note: I did the tests with the throttle plate wide open, as I think you should. However, I just realized that the intake manifold had a line detached during the compression testing. I think that having the manifold not plugged won't affect the results I saw in just 3 of the cylinders, but could you more technical folks confirm? Wish I could find my old high school "the internal combustion engine" book from the 50s.
