help confirming engine diagnosis


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Posted by Al Evangelista on June 18, 2000 at 07:44:22:

Gang,

After a few years of occasional around-town driving my 52 M37 suddenly stopped firing on one cylinder. I had driven the truck for a mile or two, stopped for gas and coffee, drove another mile then all of a sudden noticed that the engine was making a different sound and there was a slight loss of power. Under this condition the engine also has a tendency to backfire (through the exhaust) slightly under acceleration.

The vacuum reading fluctuates slightly between 18 and 19 at warm idle. Compression readings (engine warm, all plugs out) are as follows: 115,109,104,109,39,124. I have taken two compression readings and the numbers are pretty consistent. A few hours after injecting a few tablespoons of motor oil in #5 cylinder the compression reading was around 100. I am not so sure that I was able to get the oil onto the top of the piston only so I am not sure that this test conclusively indicates bad rings.

My conclusion is that the engine could use rings, The head gasket is probably OK and that the problem with #5 is a bad (burned, cracked?) exhaust valve. The exhaust valve theory is supported by the relatively steady intake manifold pressure and acceleration backfire.

My planned approach is to pull the cylinder head and have a look. Since the engine had been running well for the past thousand miles or so I have concluded that it is not a matter of valve adjustment. I am on a limited budget and can't afford a total rebuild.

Does this diagnosis make sense? I have never done a flathead valve job but am willing to give it a try. Does it make sense to do the valves and not the rings?

Any wisdom / lessons learned in this department would be greatly appreciated.

-Al


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