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MG MGB Technical - Oil pressure drops when I accelerate!

Its a 74 and ran fine for years as a daily driver. It sat for a month and it had low oil pressure due to low oil level but I quickly added enough to bring the level up.
Now, the pressure drops when I accelerate. I can actually get it to drop low enough to ping (more like ching). Then I let off the gas and the pressure rises back to normal. This B used to have great pressure (25 or so at idle 600rpm) and at more than 900rpm it would rise to 70. What do I check? I am driving it daily again and worried so I keep the pressure up by driving carefully and watching the pressure.

I haven't been on here at the BBS in some time. My kids went to college and I canged jobs. The wife with cancer and so on. Life is tuff when you get old. I hated turning 40.

I gave the B to my son about a year ago and he and I shared it until he went off to school. It ran fine all that time and we both took good care of it. I want him to come back to the nice car it was. He and I built it together over the years.

What to do?

kids1

Interesting question. Most of us watch the tach, not the oil pressure gauge, when we are accelerating. I do remember an MGA which would have a notable drop in oil pressure, noticed on fast curves, if the oil level was one quart, or more, low.

Oil pressure does not cause pinging/pinking. It can, however, cause rod knock, but this sounds nothing like pinging. (Pinging sounds like shaking a can full of marbles. Rod knock is a much heavier thunking sound.)

I would suggest that you top up the crankcase with oil, preferably Castrol 20-50. Then, install a second oil pressure gauge into the passenger compartment to allow you to cross check the readings of the dash gauge. If, under acceleration, you are noting a drop in oil pressure, you need to find out why the oil pump is cavitating and correct it.

Les
Les Bengtson

Just a thought have you checked to see if the oil pan is dented?, maybe enough to knock the oil pickup high enough that it sucks air under acceleration.
Charles O'Brien

My first guess is low oil in the sump. This would allow the pump to suck air. On that thread, it's possible that oil return is slow or partially blocked. This would allow oil to pool around the rockers. All the pushrod passages would need to be pretty well blocked, but it's possible. Another thing that could cause a pressure problem is the relief valve. Either way, the noise is probably some bearing screaming for oil; I wouldn't replicate the sound. I'd overfill the oil by 1/2 a quart and see what that does. If you drain the oil, hold a magnet in the oil stream.

What's the highest the oil pressure will go?
Kemper

I would change the oil filter first - a partial blockage will cause starvation as the revs increase.
Chris at Octarine Services

Sorry to hear about your recent hardships, Kids1. I pray everything works out for you and yours.

What Chris brought up is interensting, try it. Its only a few bucks to change out the filter.

I also had the same issues when I first rebuilt the engine in '99. It seems that the pickup screen somehow became loose and started to suck up air. Don't fret. You can drop your pan w/o having to take out your engine to check the pump and screen. The worst issue would be the four front bolts on the pan. If you undo one engine mount and lift the engine slightly you should be able to get to them. I also learned to use thicker washers on the same four bolts to make them easier to get to later if need be.

Good Luck,

Luis
Luigi

I had the oil changed in my '79 and got a Mobil oil filter. The OP dropped radically. Changing to a Fram PH43 restored the pressure to 70 psi again.

HTH

Jim
jm McHugh

To me, that says the Fram filter is restricting oil flow.

Barney has some things to say:
http://mgaguru.com/mgtech/engine/of101.htm

Specifically about Fram (at the end):
http://mgaguru.com/mgtech/engine/of100e.htm
Kemper

Since the oil filter is up stream of the pressure gauge, I would say the Mobil filter was restricting flow.
Lee Bradley

The answer seems to be the rubber line to the pressure sending unit. It comes from the block just behind the distrib and runs to the passenger side firewall. It ruptured and under idle is fine (sort of) and under pressure, it opens and spews oil. A simple fix.
Thanks for all the comments.
kids1

Kids 1: Thanx for sharing the outcome of your dilemma It is noteworthy to all as MG problems arise and get resolved. All to often the problem is resolved and people do not post the cause and ultimate outcome of the event.

cheers

Gary :>{D
gary n. hansen

By the way, I should have said that I use the W917 filter. Been using it for years with alot of luck.
kids1

The line from the block to the firewall should not be a simple rubber line. This is a braided flexible with taper joints and fittings at each end. Fit one quickly or you risk emptying your sump.
Iain MacKintosh

This thread was discussed between 17/10/2005 and 24/10/2005

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