Low/Zero Compression

[MisterMiniS]

I bought a rebuilt head that had some questionable craftsmanship on it (rocker shafts in funny positions and vice grip marks on them among other things). I checked the gaps between the cam and the eccentrics and they seemed right where they should be, but what I did not do was check the position of the camshaft with respect to TDC (giant mistake, yes).

I plopped the head on (no interference) and bolted the rest of the motor together. A compression test shows the following:

80 30 15 30 0 100

Right away, the 0 is really suspicious as my block was working fine before the new head, so I'm thinking stuck valve or something. As for the REALLY low compression numbers, again, previously working block (motor pulled hard, but was never comp tested) and a new head. I made 100% sure that the cam gear and crank were both at TDC, so that is not the issue.

My initial thoughts are that it is a mechanical timing issues such as the cam being installed incorrectly. Maybe 180 degrees off or maybe just a few degrees?

Anyways, I am going to pull off the valve cover when I get home and look at the cam position closely but I figured I would get your thoughts first. I have not done a wet compression test as I cannot image my rings going from a working state to something that results in 15 and 30 psi compression numbers. Could be wrong though.

Thoughts? Thanks.

[Madhatter]

you can't really install the cam incorrectly. The only difference you sometimes need to look out for is to make sure it has the hole for the locator pin on motronic cars, otherwise, it just slides on in.

You can time it wrong though as the cam rotates twice per 1 revolution of the bottom end. You will be able to see very easily as if you line up the crank with the timing mark, the cam gear should be at the TDC mark on the sprocket, AND you should roughly have both valves closed. If you have the exhaust on the cam lobe, you know it's out 180 degrees.

[packratbimmer]

Do you know how to do a leak-down?

[MisterMiniS]

you can't really install the cam incorrectly. The only difference you sometimes need to look out for is to make sure it has the hole for the locator pin on motronic cars, otherwise, it just slides on in.

You can time it wrong though as the cam rotates twice per 1 revolution of the bottom end. You will be able to see very easily as if you line up the crank with the timing mark, the cam gear should be at the TDC mark on the sprocket, AND you should roughly have both valves closed. If you have the exhaust on the cam lobe, you know it's out 180 degrees.

From what you said I am thinking that I have it 180 off. I will check that when I get home. As completely n00b as it sounds, I did not know that the cam rotates twice for every one crank rotation. It makes perfect sense, I just never thought about it. Thanks!

Do you know how to do a leak-down?

Yea. Never done one, but understand the logic behind it. If the above does not work, then I will likely head to harbor freight and grab a leak down test as it does seem like the next step in diagnostics.

[Madhatter]

I really shouldn't post before going to bed. hehe. What I meant to say is the crank turns through twice for every turn of the camshaft. Not the other way around. The crank sprocket is obviously smaller than the cam sprocket. Timing being off is the same though and could be your issue.

[MisterMiniS]

I really shouldn't post before going to bed. hehe. What I meant to say is the crank turns through twice for every turn of the camshaft. Not the other way around. The crank sprocket is obviously smaller than the cam sprocket. Timing being off is the same though and could be your issue.

Yea, I figured. I popped the head off and rotated the crank around once. Motor still not firing. When I was done it was late, so I havent done any diagnostics. But yea, rotated crank not cam.

[Madhatter]

where were the cam lobes when you had both timing marks lined up at TDC when you checked again?