Posted by Paul Cook on Tuesday, July 06, 2004 at 11:55AM :
In Reply to: M37 idling problem.... posted by Nickathome on Monday, July 05, 2004 at 11:00PM :
I visited with friends (identity protected) this past Saturday. They were working on a mystery malady on a M-43. Start, yes; Run above idle, no.
This truck is one that is only driven in parades. An M-43 is not much of a trail truck. A while back, the engine problem appeared and there were several attempts to resolve it as described below.
There had been some work done to the engine, what I call a diagnostic overhaul. After determining that the ignition was working and the new spark plugs were firing - and in the right order, they decided there was a problem with the valves. The engine would run with #3 & #4 plugs removed - not well, but enough to reinforce their belief that it was valves in the #3 or #4 cylinder. The diagnostic overhaul is what I call removing the head to look at the valves. They also pulled the valve covers to observe the operation of the valves. They could see the valves working properly.
They had it all back together when I got there. The problem persisted.
Even though I was the oldest one there, I was selected to crank the engine with a hand crank (You all have one, don't you?) while one of them used a bent piece inserted through the spark plug hole to feel the valve movement. The valves operated correctly.
They decided to let the engine idle with the two plugs removed hoping that the valves would loosen up. They had become fixated on the valves, even though we were able to feel them opening and closing.
I noticed that there was a slight bit of moisture vapor coming from the tailpipe. I was surprised that there were no exhaust pulses - something I expected from a six cylinder running on four.
We put a hand over the tailpipe to see how much pressure there was and found the tailpipe end was cool. The youngest of us climbed under and found the front of the muffler was hot and the back was cold.
This prompted us to drop the exhaust pipe. The engine ran perfectly - smooth and strong.
Apparently, a baffle inside the muffler had failed ("It ran when parked.")and blocked the exhaust flow. This extreme back pressure was pressurizing the exhaust manifold. The truck ran with the two spark plugs removed since the exhaust back pressure escaped when the exhaust valves opened in the cylinders without plugs.
(This is for you, Norm.) There's no one better than an old man with "exaust system" flow problems to spot the same problems in an old truck.
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