MGB MGB High Oil Pressure

[Lynn Kirkpatrick]

I have a '68 MGBGT with a '74 engine swapped by the PO. I was told it had about 35,000 miles on it 12 years ago, so it still has less than 40k. A year or so ago the dash oil pressure gauge started showing 20-40 psi when warmed up. The engine sounds good running.

I T'ed in a Stewart-Warner gauge and can see them both. The S-W shows 65-75 psi all of the time (idle, @ startup, hot). The dash gauge shows about the same at start but drops to 20-40 when warm. So there is a difference, which could be the dash gauge or the sending unit. I checked the S-W gauge with a calibrated one. It is within 3 psi at 80.

I haven't done anything that would cause the oil pressure to go up that I know of. Can something happen internally to cause it? I don't put many miles on it, but change oil yearly. The oil looks clean coming out.

I've read where the high oil pressure could overload the pump drive shaft, causing it to fail. Since it has been like this, I think, since I've had it should I not worry about it? I could use thinner oil (10w-30?) that should theoretically lower the pressure, but is the gauge located "a long way from the bearings" and lower viscosity could come back to bite me?

Any info would be greatly appreciated.
Thanks

 

[beachbumbarry]

Hi Lynn, Just a stab-in-the dark, but the oil pressure relef valve could have a problem. If it is stuck all of the pump pressure goes to the engine. The pressure should be lower at idle. You could pull the relef valve and look at it.
BarryE

 

[Lynn Kirkpatrick]

What could be checked? I remember seeing a length for the spring specified, maybe in my Bentley manual. Would I use a magnet to pull out the "Bullet"? Check it for scoring?

Is the relief downstream from the all of the bearings? For instance, the pump supplies oil to everything and the oil flows through the relief valve last, controlling pressure. (?) I know that bearing clearances will cause some back pressure, but is the relief kind of like the sheriff, making sure there is a max that isn't too high?

If the relief valve looks OK, should the spring be shortened a teensy bit to drop the pressure? That seems risky. Would lighter oil do the same thing?

The hard part here is that I haven't changed anything, and I don't know how long this has been going on. If it has been for 30,000+ miles I'm inclined to not touch it except maybe changing viscosity.

Thanks

 

[fogliner]

Hi Lynn;

I saw this video on MGB oil pressure etc on University motors youtube channel,thought it would help.
John says the oil gets to the pressure relief valve right after the oil pump in the circuit.
Here's the link and its right at the beginning of the video when he does his schematic thing..Take care..
Mark

https://www.youtube.com/user/Universitymotorsltd#p/u/88/a7mVVvWnR34

 

[Hap Waldrop]

75 psi of oil pressure is not excessive and would not damage anything in the engine. I also I think the pressure relief valve is workingproperly form your description, if it was stuck shut, very unlikely, you know it real soon, it proably peg the gauge and blow the oil filter up, and it if it was stuck in partial or open postion you would have nearly no pressure. Hope this helps

 

[Lynn Kirkpatrick]

Hap
Thanks, the explanation helps a lot. That's the great part of these forums.

Next I'm going to try to troubleshoot why the dash gauge is reading way low. I think that I tried to measure ohms between the sending unit connection and the block as i varied speed (oil pressure), but I can't find the numbers.

I'm thinking that when everything works, varying sending unit resistance shows up as varying pressure at the gauge. Make sense? If the sending unit moves with pressure I'm guessing the dash gauge is the problem. If the sending unit is using some other electrical principle I'm in trouble.

Does this make sense? Has anyone else done this?

 

[billspohn]

What can happen with these engines is that owners decide that if one packing piece (standard on some of the MGBs) is good, another one is better. Tap the little bucket on something and see how many spacers pop out. You can always remove one to bring pressure down.

Hap is right, 75 psi shouldn't be a problem on a street engine. Start running it to 7000 rpm though and you'd see some wear on the gears pretty quick.