| Re: [GMCnet] Finally about to pull the 403 [message #208864] |
Sun, 26 May 2013 14:59  |
KB
Messages: 1262
Registered: September 2009
Karma:
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Senior Member |
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> If it's quiet at idle and starts to clatter at engine speed (not 1/2 speed) pace starting at about 1500 RPM it's probably a connecting rod bearing at the crank end. The extra slop can allow the piston to rap the ledge on the head once piston speed is high enough. Sounds worse than it is but will have to be fixed before ruining the crank. Now if you can detemine which one it is by plug wire pulling and sound pattern changing--- wondering if its worth lifting engine enough to pull the pan and rebearing that rod. However what caused it is the question and that could be an oiling issue needed full deep cleaning. Out of coach unfortunately.
The current theory is it's a rod bearing. I pulled all the spark plugs
and they looked fine. While moving, I listened with a hose and metal rod at the
alternator, water pump, tranny, and valve covers and couldn't locate it.
It does have exhaust leaks, which we suspected but couldn't prove
was the culprit. For most of the drive, it seemed to be related to throttle
position rather than RPM -- ie, when coasting down hill or at low throttle,
it'd stop. Since we have EFI with EBL, we have logs of it happening --
the knock sensor picked it up and backed off the timing.
Coming up our steep hill for the last 5 miles of the trip, it seemed
quiet under load, but knocked in between load and coasting. Once home
though, there was a steady knock at idle, and we can get it to
rap once or twice if we rev it. Could, in theory, be something to do
with belts/pulleys, but the metal flakes kinda discount that idea.
I considered pulling the pan and trying to see what's up, but
since that's also quite labor intensive, I haven't a clue what
I'm looking at, and don't know what caused it or how extensive
the damage, the easiest course seems to be an engine swap.
We also considered getting a cheap USB bore camera and looking
through the oil drain hole to see if anything obvious was visible.
Not sure how useful that would be though.
Guess we'll see what the oil analysis says and go from there,
but I'm not feeling optimistic. At least it got us home.
thanks
Karen
1973 23'
1975 26'
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Karen
1975 26'
San Jose, CA
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