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MG MGA - Oil Pressure slowly falling

So I start the A this morning and go for a drive. Hot idle is 15 and 3500rpm is 40. Over the next 38 miles on my driving loop the oil pressure slowly and linearly decreases. By the time I get home, idle is 5psi and 3500rpm is 12psi. The car ran great at 175F the entire trip. Old engine with alot of internal sludge. New Fram PH3600 spin-on w/10W-40. So, I let the car cool for an hour. Temp gauge is down to 130F and when I start the car and rev it up pressure jumps back up to 40.
I have started to search the archives, pressure relief spring, inaccurate gauge, and worn bearings seems to be the causes. I have only had the car a short time (first real drive) so I have no history on the performance.
Any other new possibilities?
This is a 'fun' car to tide me over while my other body-off resto is finished. I'd rather sell that tear into the engine.
Scott Steinhauer

I've been told to run 20w50 in mine and have done so for 22 yrs.Would not hurt to change it and give it a try,since you said it has alot of sludge
gary starr

Scott, it does sound like an engine with a lot of wear on the bearings and oil pump. As suggested, try some 20-50, should help the pressure, but in the long run it probably will need the bearings replaced and the oil pump checked for proper clearances. One other thing, is to check the gauge by temporaraly hooking up another gauge. It could just be the gauge out of calibration. Easier than rebuilding the engine and less pain than selling your fun car.
Bill Young

Oil pressure questions should always be verified through the use of an accurate mechanical gauge. Idle pressure at 5 and 3500 rpm of 12 would be to low to last at all. If these are true you may already have a damaged engine. Your three guesses and the pump are most likely.
#1 verify with a good gauge.
#2 Pressure relief is externally accessible see:
http://mgaguru.com/mgtech/engine/of111.htm
#3 It is an easy engine to fix. If you don't want to pull and fix properly the rod and main bearings can be checked with plastigauge or replaced without pulling engine. If new bearings show too much clearance they can be shimmed until they read ok.
#4 While open check the pump also. http://mgaguru.com/mgtech/engine/of107.htm
Don't run it any more until you check it. If it runs without bearing noise it can be helped. Once you spin a bearing it must be torn down.
R J Brown

A lot of sludge and fresh thinnish oil can block up a filter quickly, change the filter and see if you get some of the pressure back. Keep doing that if the filters keep blocking. Drain the oil and see how nasty it is; if not bad, keep the oil to put back for a couple of hundred miles. Tilt the car so the drain hole is a bit high, then pour a quart of kerosene in the engine (don't start) and run various wires and such in the drain hole to scrub around the bottom and get sludge out. Tilt the car so the drain hole is low and let it drain.
Cut the "clogged" filter open and examine with magnification; wash it in kero, then filter the result through a coarse cloth- examine for metal. Or go all out and send a sample off for oil analysis - like $20, though some companies do it free if you are using their oil - and they know, so don't try to cheat! Metal means dead bearings and the next step is disaster. "Good" bearings fail in old engines by corrosion, and the failure is catastrophic. Analysis will also show fuel or coolant contamination, in addition to abnormal wear.
FRM
FR Millmore

There's a bunch of stuff on this in the archives. Consider a new oil pump, rod bearing,and oil pressure valve/relief spring. Good luck.
Bill Haglan

Update:
The oil is still clean after 64.8 miles:} I started the cold engine and the pressure is back to 20(1000rpm)/50(3500). I think the gauge is sticking at higher temperatures. Once it reached 170F the gauge read 40(3500rpm) so I pulled over and let it idle and the gauge stayed at 40. I drove home and shut it off while it still read 40. I had to flick the coils behind the dash and all of a sudden it dropped to zero.
I'll try RJ's #1 and #2.
Scott Steinhauer

The relief valve may be accessible without pulling the engine, but be prepared for a lot of swearing when re-installing it. If the spring has the proper tension, it is very difficult to get the cap threads started while compressing the spring. It took me about two days of swearing to get get it installed with a new spring. This is one of the things that is much easier for our British friends to do, because the steering components are not in the way.

Jeff Schultz

I had an engine that did the same exact thing. Oil pressure would start out okay, and then after an hour or so of running it would drop to 20 at normal speed, and very little at idle. It looks like this now:

http://rawlinsbrothers.org/mga/images/seizure.jpg
Del Rawlins

Yikes Del,
Please explain what happened. It looks like the piston split. Did the engine seize due to heat and/or lack of lubrication?
Scott

Hi Scott. I agree with the other's comments and diagnosis. You might want to purchase a can of proprietary engine flushing compound to try and rid your engine of the accumulated sludge. This thin flushing oil is added to the crankcase, and the engine is allowed to idle only. After 10 or 15 minutes the old oil, hopefully with the sludge in suspension, can be drained off. The oil and filter are then replaced. 20 W 50 is a good choice for MGA engines. I did this on a friend's engine that was very heavily sludged, and it brought the oil pressure up considerably. If your engine is badly worn, you will need to rebuilt it. Cheers, Glenn
Glenn

I haven't taken it down any farther than that, probably won't until I have the rest of the car finished. That is a spare engine, the original is going back in the car. Pistons number 1 and number 4 are at different heights, so either the rod let go or the bottom of the piston let go from the rod.
Del Rawlins

This thread was discussed between 01/04/2007 and 02/04/2007

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