At 05:20 AM 5/6/2000 -0700, Brad Pace wrote:
>.... what are some of the causes for the engine to have 65 psi oil
pressure at start up, but after 10 miles of driving it is down to 20 psi at
highway speed? ....
That's a pretty serious drop. My FIRST guess is loose bearings (worn with
too much clearance) on the crankshaft. If all of the bearings in the
engine are equally loose you can have pressure that low and still be
running quietly. If that much pressure drop is caused by one or two bad
bearings you will have a definite knocking noise.
For a little MGA education start here:
http://www.ntsource.com/~barneymg/mgtech/engine/en1.htm
Click on OF-101 for a complete description of oil flow throught the MG
B-series engine.
The short description goes like this. Oil from the oil pump only dumps
over the pressure relief valve when there is more flow than is reguired to
lube the engine. Otherwise all of the oil goes on to the filter and into
the engine block where the first moving parts to get it are the main
bearings on the crankshaft. From the mains it passes through drillings in
the crankshaft to the rod bearings, and from all of these bearings it can
escape back into the sump.
Except for the pressure relief valve, all other oil passages in the engine
eventually lead to an open exit point back to the sump. As such, the oil
pressure is determined by the viscosity of the oil and the small size of
the passages it must squeeze through. When the oil gets thinner or the
passages get larger the pressure drops.
That said, I have some first hand experience with pressure drop. One con
rod bearing being loose to the point of causing a knock will drop the
pressure by about 10 psi. Four bad rod bearings at the same time will drop
the pressure by 20 psi. One really bad main bearing that knocks at idle
will drop the pressure by 20-30 psi. All bad main bearings will drop the
pressure by 40 psi regardless of the condition of the rest of the engine.
If the crankshaft bearings are in good condition, then loose camshaft
bearings can drop the pressure about 20 psi, but it is rare for the cam
bearings to be the first to go. This condition might occur if more of the
engine was bad and someone had just replaced the crankshaft bearings.
Sloppy bushings in the rocker arms, or a worn rocker shaft can drop the
pressure 10-15 psi, but would be hardly noticeable if anything else before
it was causing a large pressue drop.
One other thing can cause a large pressure drop without a knock. If the
oil pump is badly worn and has large internal clearance it will have
severely reduced pumping capacity, with the results on pressure being the
same as using extremely thin oil, much reduced pressure when it warms up.
Now for some good news. The oil pump and all of the crankshaft bearings
can be replaced by removing the oil pan without pulling the engine.
(Removal of the front main bearing cap requires removal of the two bottom
polts in the timing chain cover). The rocker shaft and rocker arm bushings
can be replaced via removal of the valve cover without removing the
cylinder head. I can tell you how to R&R and ream the rocker arm bushings
in your home garage if necessary. I think you are in for removal of the
oil pan, the oil pump, and at least some of the rod bearing caps for
inspection. Moss parts can be had at 15% discount and free shipping.
May you have many happy experiences with your new MGA.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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