Re: [Amc-list] Help diagnosing 4.0 loss of compression

[Frank Swygert <farna@xxxxxxx>]

I'm thinking more along the lines of a broken camshaft, most likely at 
the gear hub. There's no way that half the valves stuck or cam lobes 
suddenly wore down to the point that the engine wouldn't run, or all the 
rings failed at once.

The problem almost has to be bent valves due to a timing chain or cam 
breaking. I've seen the hub end break off a cam and cause loss of 
compression in multiple (but not all) cylinders. The last time I saw a 
problem exactly like this was in a Renault GTA. Timing belt broke, at 
least one valve in every cylinder was bent and not sealing.

Since the distributor is turning, it can't be a broken camshaft. It 
could have jumped timing though (by a LOT of teeth -- said chain "seemed 
tight"), stripped the key in one of the gears, or if it's a composite 
cam gear it could have stripped or slipped. I'd pull the head first and 
see what the valves do. If they appear to be opening and closing about 
right then I'd check the timing via the marks on the balancer. If the 
valve heads are bent much some will have more stem travel than others, 
and that should be obvious by rotating the engine. If neither test above 
was conclusive, pulling the head would be the next step. That should 
tell 100% what the problem is, if nothing else you can then see the cam 
and check the lobes and rotation. Still nothing? Pull timing cover. If 
no obvious solution to the problem under the timing cover tell customer 
they need a new engine. No point in tearing it down further, as it would 
need nearly a complete rebuild to reassemble.

-- 
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
http://farna.home.att.net/AMC.html
(free download available!)

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