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MG MGB Technical - Oil Pressure

Picked up my car (1970 mgb roadster, rhd) today and noticed a rather
perculiar thing getting it home.

I was doing about 50 mph at the time and the oil pressure guage (direct read
type) suddenly dropped, quite slowly, to zero.

There was no change in engine sound or revs/power and after I had stopped to
check it over there was no oil leaks nor odd smells and the sump was full.

Later I unscrewed the union onto the oil guage capillary and checked to see
if there was oil in it (there was). After resealing I restarted the car and
was getting a 25 psi reading and the reading did not change markedly with
revs.

Among other things the engine has been out for a replacement clutch and has
been off the road for a major rebuild (body/paint) for about 9 months.

The engine was rebuilt about 18 months ago and the car has had limited use
since.

Now is this as I suspect an air bubble in the capillary (ie guage problem)
or something more serious (ie engine problem)

regards

Tom

Thomas Edge

I would worry about this. I don't believe air in the capillary has any effect on the gauge. When I installed mine, I didn't bleed it or anything, just plugged it all together and it worked. Damned if I can think what might have happened though.

Mike
Mike Howlett

It's not an air bubble. The gauge will read correctly irrespective of whether there is some air in the tube or not. In any event 25psi is not sufficient and the fault must be rectified before load is put on the engine ( don't drive the car on the road)

What was the pressure before engine removal? Could some dirt have got into the system say at the oil cooler pipes ? I suspect it is dirt and one of the first checks I would make would be to oil pressure relief valve. Remove the cap nut, spring and plunger and carefully clean the components. Clean out the bore with a piece of lint free cloth then refit the plunger. Using a small diameter drift tap the plunger to make sure that it seats firmly and then refit spreing and cap. Is the oil filter clean. That's about all you can do without dismantling the engine but it may be worth fitting a gauge directly to the block as a final check. These gauges are not terribly accurate and the braided flexible can also collapse internally making it act as a one way check valve.

Hope you have luck
Iain MacKintosh

I suspect the guage or the tube or crud in the oil but the sensible course would be to assume that you have a problem in the engine.

25 psi after restart is low unless the engine was still hot. I suggest that if you do not get at least 55 psi when you start from cold - assume the problem is with the engine and that you have been running without lubrication/primary coolant.

If you do get a decent pressure on start then my order would be 1; new tube, 2; new filter/oil 3; new guage.

Worst case scenario; remove sump, confirm pump is tightly fitted with the correct gasket (all with engine in).

Good luck

Roger

RMW

Did U change the oil filter?

What make?

I have just had a Purolator block most of the oil flow - one way valve became a no way valve!
Chris at Octarine Services

I had similar on a Unipart filter last year. No pressure, then it blew past the oil filter seal and dumped a few pints of fresh oil all over my drive.
Steve Postins

Thomas I had the same problem with purolator oil filter straight out of the box(no pressure).Change the filter but first fit a test oil pressure gauge to make sure. Air in the line makes no difference to the reading but a bit of crap might. By the way I have started using BALDWIN oil filters as their anti drain back valves work even if left for weeks. Denis
DENIS

Thanks for the input guys

I checked the obvious things, no problem and so I picked up a cheap accessory oil pressure guage this morning and tapped it into the oil pressure hose.

After priming it shot straight up to 60 psi (with the existing hose) which pretty conclusively shows it is the guage itself as opposed to the engine/hose/filter (although it could be the capillary tube).

I also noticed this morning that the oil pressure guage wasn't returning to 0 so I guess that proves it.

I breath a big sigh of relief that it wasn't more serious......

Should have a replacement correct guage fitted next week (waaagh more money).

Regards

Tom
Thomas Edge

Thomas, If you weren't spending money on your car you would be buying beer. God made MG's to try and save a few of us from our own free choices. Ed
E kulick

Thanks Ed except I'm a non-drinker anyway!!

regards

Tom
Thomas Edge

It must be horrible to wake up and think your the best your going to feel all day. Denis
DENIS

Denis, you are SOOOOOOOO right. More beer makes the next day so much better!

Cheers

Too nny nny ony. HIC.
A G Oliver

Denis isn't that a Frank Sinatra quote??? or a Dean Martin????? lol......... Mike
Mike

Mike I think your right, it had to be someone smarter than me. With me its just an excuse. All the best
Denis
DENIS

Not only does air in the capilliary *not* have any effect on the absolute gauge reading - 60psi in air is the same as 60psi in fluid, personally I feel that bleeding the tube of air actually contributes to a slow-rising gauge as the pressure will be transmitted from one end of the tube to the other much faster in air than it will in cold oil in a small bore tube.
Paul Hunt

Paul

Not necessarily true. If the oil line is free of air there is very little motion of the oil required to activate the gauge since oil at these pressures is basically not compressible. Air, on the other hand, is very compressible and will result in more oil flow in the "lower" end of the oil pressure line to activate the gauge. Since it is resistance to flow that limits gauge response time the larger the air "bubble" the slower the gauge response.

Larry
Larry Hallanger

True, but it is only oil flow in a 'bled' pipe that moves the gauge. Whether the time taken to register a given pressure in a tube that is full of oil moving a short distance is more or less than the time taken to move a short length of oil a longer distance because the remainder of the tube is filled with air, is a matter of conjecture. Ultimately I doubt it makes any significant difference at all.
Paul Hunt

Folks

In a correct working system the pressure is controlled by the relief valve. I had a previous vehicle with a B engine in it and after a long time of no running found when i fired it up that the relief stuck partly open giving low pressure. After removal and cleaning and checking for smooth sliding of the valve tried again and everything was back to normal.

Gary
Gary Roberts

This thread was discussed between 08/10/2004 and 21/10/2004

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