Falling Oil Pressure

oil
mg
gauges

#1

I have recently acquired a 1975 MG Midget. The previous owner drove it VERY infrequently. I gave it an oil/filter change when we got it. For the most part it runs well and starts right up, but during the last few trips, I’ve noticed an issue with the oil pressure gauge. Upon cold starting, oil pressure is around 65 psi. As it warms up and I drive around town, it falls to near 25 or 20 psi. This is usually over the course of 30 minutes or so and occurs slowly. I’ve seen it drop to around 10 when idling at stops. When accelerating away it will pop back up. No leaks that I can see and the oil level looks good before I start. After letting it sit for a while, starting will give the same 65 psi or so. Running Valvoline VR1 20w50 oil. Any ideas about where to start the diagnostics? Is it possible the gauge is just faulty? Better yet, is this normal?


#2

This seems normal to me. If there are any English car fans out there, please weigh in.

The thick oil you use forces the pressure relief valve in the oil pump to open at the limit of 65 psi. Once hot, the low idle rpm drops it to 10 psi, that’s OK, and then rises as the rpm increases. You need about 10 psi of pressure for every 1000 rpm’s. 3000 rpm should need about 30 psi, hot. 5000 rpm, 50 psi. It sounds like you are OK for now but the engine may be more than a bit worn, allowing the pressure to drop as it warms up. Don’t use thinner oil than 20w50!

The gauge should be compared with another, known and calibrated gauge screwed into the pressure gallery to verify the gauge in the car just to be sure. If it matches, you should be OK.


#3

I had a similar problem with a 1977 Datsun 200sx. It could be the oil pressure sender.


#4

What engine does that have? Is a 20 weight what the manual calls for? I don’t know if adding a thickener like a Lucas oil treatment would help or not. Sounds like something internal like an oil pump might need to be replaced. I would not drive it until you can fix this problem, otherwise you risk messing up the engine.


#5

It has the stock 4 cylinder 1500 (1493cc, but who’s counting). The 20W50 is what the manual calls for. The speedo is mostly non-functioning and the tach waivers a bit so I suppose it wouldn’t surprise me if the pressure gauge is also faulty. Temp appears to be working, but it usually reads in the cold then normal range.


#6

I’d hook up a mechanical gauge to it and see if the readings are real. If they are correct, it sounds like a motor with slightly worn bearings. A bad oil pump would give low pressure to start with, and they’re typically one of the last things to go, anyway.

edit - here’s a lot of info on MGB oil pressure. Have you joined an MG owner’s club(s)? They’re very enthusiastic and knowledgeable.
http://www.mgbmga.com/tech/mgb30.htm


#7

Thanks a bunch. I’ve been on the online forums, but there’s only a handful of us here in Central Oregon so not much “local” help.

FYI - From what I’ve heard from other MG owners is that it is either A) normal, B) a faulty gauge/switch or C) a serious problem - to be checked with a mechanical oil pressure gauge (the Midget’s is electronic - wait, a vehicle with Lucas electric having an electrical issue? I don’t believe it).


#8

It is a sign of normal wear, but it is not a serious problem. If the pressure begins to drop to zero at idle, or if you begin to hear knocking sounds, you need to at that time begin to think about rebuilding the motor, but you really require little pressure to protect the engine at idle.

Crankshaft bearings and particularly rod bearings are subjected to lateral forces when the cylinders fire. The oil system maintains a pressurized fluid barrier between the surfaces to protect them. But at idle, in that little motor, it needs very little pressure to keep safe. When you wind it up, when you accelerate, the lateral forces increase and that’s when you need more pressure. If I understand you correctly, you have that.

I love the Midget, by the way. You can have an absolute blast without risking life & limb, and for a miniscule cost. Just don’t get hit.


#9

The gauge is direct reading if it’s like most MG’s there is no sender…But it sounds fine to me…As Mustangman said, 10 PSI for every 1000 RPM and you are good to go…