slant 6 compression and missing

[Abodyrookie]

I am currently trying to get my '75 duster to running condition to be a daily driver. I had a compression test run on the cylinders about a month ago and found out that i had a cylinder running at only a 15 ale:. I have finally got it close to ready to get insured and tagged, but i want to fix that cylinder first. Any thoughts on what might be wrong, and how i can fix it. This is my first remodel so I am open to all suggestions.

 

[Serj22]

Could be a few things - burnt valve, blown headgasket, crack in the block, crack in the head, piston has a hole in it, rings are super super bad, etc...


I'd say you could check for a crack by using a block sniffer or something, or see if there's water in the oil, oil in coolant, etc... but I don't see a point. Whatever it is needs to be fixed, and it's not going to be fixed without invasive operations - taking the head off.

If you're sure it only has 15psi, that warrants taking the valve cover off at least. see if there's anything you can see up there, likely not, so take the head off. It doesn't take up much time to remove the head from a slant six. I do it what seems like every six months to do something. Once the head is up, look at the head gasket when you take it off - see if there's an obvious breakage at that cylinder. If not, look at the head - flip it over and see if there is a crack on that cylinder, or if the valves are burnt, dropped, etc... Then if none of those is the case, or you can't tell, put a straight edge on the head and see if it is flat. I've had mine warp before, and you look at the block and head and nothing's wrong, but then take it to a machine shop to find out it was warped.

That would be a good place to start then report back.

 

[krazykuda]

Do a wet and dry compression test.

First try the cylinder as is dry and run a compression test.

Take an oil can and put 2-3 squirts of oil in the spark plug and perform the compression test.

Record the readings for each cylinder and then compare if the readings are higher wet than dry.

If a cylinder is higher wet than dry, the problem is in the rings/cylinder sealing.

If the cylinder is low and not much or no difference wet or dry, then the problem is in the valves or head gasket, most likely the valves, then get a valve job....

 

[RustyRatRod]

All you have to do is remove the valve cover. Rotate the engine until number 1 valves are both closed. Remove number 1 plug. Screw a compression tester hose in number 1 hole. Connect air hose to the compression tester hose. If you get air rushing into the valve cover area where the valve springs are, you have a hole in number 1 piston, or terribly bad rings. If you get air rushing out of the tail pipe, you have a bad exhaust valve on number 1. Air rushing up through the carburetor and you have a bad intake valve on number 1.

Repeat for the next 5 cylinders. You might get slight bits of air here and there but when you find what is bad, you will KNOW it.

Since the rockers are all on a common shaft on Mopars, "I" usually simply remove the whole assemble, because this eliminates having to rotate the engine to close all the valves. Once the rockers are removed, all the valves will be closed.

Just personal preference. In the end, the rockers are gonna have to come off anyway.

 

[Aaron65]

On the cylinder with low compression, did you make sure that the valve adjustment isn't too tight? It could be hanging a valve open.

If that's the case, however, there's a good chance it's burned anyway. Something to check.

 

[BillGrissom]

It is common in an engine that sat for the valves to be stuck wherever they were. WD-40 and a few taps w/ a hammer usually get them moving. I would run it a while and see if compression improves. If gunky, some say that adding ATF to the oil helps clean things. Perhaps safer to run a diesel oil (Rotella) since has more detergents.

 

[nm9stheham]

It is quite likely that a valve is sticking on that cylinder as Bill says. /6's have pretty low valve spring pressure and so the valves tick easily. Start there.

It would be worth getting the valve cover off and see what amount of burned oil crud is built up in there. If the OP can provide a photo of the valve area under the valve cover, that would be good.

 

[Abodyrookie]

Thank you for all the advice. Irritatingly and sadly though, I ran into a snag with time so I have been unable to actually do anything extensive like that. Now I am having other problems. I was working on some wiring and I don't know what happened, but my gauge cluster touched some exposed metal and sent a nice shower of sparks over me and the car will not even turn over now. If you have any knowledge of what I might have done wrong, it would be helpful.

 

[BillGrissom]

Most likely you blew the "fusible link", which is there to protect for any such short circuits in the cabin. It is on the engine bay side, going into the plastic "bulkhead connector" near the brake master cylinder. Google Images for photos. Might also have just been a regular glass fuse in the box near the e-brake handle.