>From: jean@spock.usc.edu (jean hertzberg)
>I have been experiencing strange oil pressure readings in my '68 MGB.
>Prior to the rebuild the engine had zero pressure at idle, which
>would come up to better than 50psi above 3K rpm. ...for seven years
>...the oil pressure began occaisional drastic fluctuations
> Ok, so now, after the rebuild, I have 35-55 psi at hot idle, and
>50-70 psi above 3k rpm. The fluctuations within these limits are
>random, and do not correlate with temperature, rpm, road grade,
>precise sump levels, or whether I've wiggled the electrical
>Are these electrical gauges notorious, or
>could the oil pressure really fluctuate like that?
Maybe, Yes.
And: >more gory details
>block stuff only. A lot of head work done about five years ago,
>including valve guides, seat grinding, surface mill, and a few new valves.
>The rocker bearings are not showing wear.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Key statement.
>new parts put in the block: new oil pump, main, rod
>and cam bearings, timing chain and tensioner, all new lifters, and rings.
> The machine shop work: removal of that hideous nut on the end of the
>crank, and subsequent dissassembly and cleaning, cylinder hone, crank polish,
>cam "reworking", top surface milling, and cam bearing installation.
****>>>>I'd been dealing with (Garnett Auto Parts, by the way,)
Hey! I hope it's a good shop.
>The pressure relief valve parts are about 8 years old, and ...
>the inverted (screw threads down) replaceable oil filter cannister
OK. This is a pretty well worked over engine. The bottom end stuff did
it's job, pressure is up. I see 3 main possibilitys here-
1) Rocker bearings can cause this, as the arms shift back and forth.
If, as you say, they, and the springs really are in good shape, you're
probably ok here.
2) The relief valve and spring. They're cheap enough, after everything else,
and may be a bit weird. It sounds like they got inspected during the
rebuild? (Easy enough to get at, if not.)
3) Given everything else you've done, and the broad range of conditions,
I'd suspect sending unit too. I'd really try to find a substitute sender,
or a purely mechanical guage for diagnosis before buying a new sender.
Sounds like the diaphram or transducer may be on the way out. I have had
some success salvaging pressure switches, by cleaning them, and
inserting an appropriate probe to carefully work the diaphram back and
forth. This has freed stuck hydraulic brake switches, and might help
your sender. At $100, it's worth trying.
________
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