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This isn't on a military vehicle so I hope this is OK, but I have a 1955 F head motor that has run fine for quite a while, but it started developing a knock under load. Especially in third gear while giving it gas. It slowly worsened to the point that the motor is undriveable now. Just barely made it up the hill because the clattering was so bad it was sending chills up my spine......

After changing plugs and wires, I noticed that number 4 cylinder didn't seem to be firing at all, confirmed by pulling the wire while running - no change.

Pulled the plug and it does have a spark

Oil pressure stays about 40 psi while at running speeds.

Checked for spark on the rest and they all appeared to be good.

Pulled the valve cover and the intake valves appear to be doing OK. The clearances all measure out pretty close to spec.

Thought maybe the exhaust valve was stuck or dropped a retainer, but I removed the inspection plate an they all appear to be performing as expected. I have not compression tested the cylinders yet.

What am I missing? I really don't want to pull the head if I don't have to, but I'm not sure what else it could be. Any help would be greatly appreciated.

Gene

What am I missing?

The short answer to that is The Compression Check!

As a old time professional mechanic I would not have played with all the things you did but would have instead immediately did a vacuum gage check followed by a compression and a leak down check.

These three checks will answer just about any question you have.

A great place for you to start is on page 19 and/or 88 of the civvy jeep universal service manual SM-1002. Great troubleshooting chart there. Page 20 & 85 details the compression checks and page 23 details the vacuum gage checks.

When describing engine noises try to be as accurate as possible. A heavy metallic knock is usually a rod or main bearing and will usually be accompanied by low oil pressure. A rough running knock or ping or miss is usually an issue with that particular cylinder/piston/valve and can normally be isolated by pulling one plug wire at a time while the engine is running. The one that when pulled causes no drop in engine performance or smoothness is the trouble cylinder.

http://www.maximausa.com/technical/leaktest.html
http://www.smccd.net/accounts/mcafee/La ... sheet.html
http://www.off-road.com/trucks4x4/artic ... ?id=227292
http://forum.helluvafast.com/showthread.php?t=2165

wesk wrote:

What am I missing?

The short answer to that is The Compression Check!

Thanks Wes!

I have to agree with you on all counts! My first thought was compression test, but I kind of live out in the boonies and finding a compression tester has been problematic.

But a 1/2 inch wrench I have! So I started removing things

wesk wrote: A great place for you to start is on page 19 and/or 88 of the civvy jeep universal service manual SM-1002. Great troubleshooting chart there. Page 20 & 85 details the compression checks and page 23 details the vacuum gage checks.

That's some great info there! This little CJ is kind of my fiance's baby so I have inherited the title of "mechanic" from her son.

wesk wrote:When describing engine noises try to be as accurate as possible. A heavy metallic knock is usually a rod or main bearing and will usually be accompanied by low oil pressure. A rough running knock or ping or miss is usually an issue with that particular cylinder/piston/valve and can normally be isolated by pulling one plug wire at a time while the engine is running. The one that when pulled causes no drop in engine performance or smoothness is the trouble cylinder.

Yep, that's how I isolated number 4 - definitely something wrong there.....

wesk wrote:http://www.maximausa.com/technical/leaktest.html
http://www.smccd.net/accounts/mcafee/La ... sheet.html
http://www.off-road.com/trucks4x4/artic ... ?id=227292
http://forum.helluvafast.com/showthread.php?t=2165

I'm off to all these sites! Thanks for the leads!

Gene