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MG MGB Technical - Fluctuateing Oil Pressure

I am so sorry if I am repeating this issue but I couldn't not find it under a search. On a long freeway drive sustaining 3400 RPM my oil pressre would move up to 45 then down to 30 PSI. This would happen with in minutes and I could watch it, quite puzzling. This past weekend another trip on a hot day pressure would drop to 20 at 3000 rpm the back up again, some what.
I think three possible problems could cause this; oil pressure relief valve, pressure sending unit at the block or pressure gauge.
Would appreciated ideas on what is going on and what to change first.
Geoff Kimler

I would concentrate on the sending unit. These are notorious for giving false readings and is the reason the factory switched back to a mechanical oil pressure gauge in the early '70s. If you have access to one, hook up a mechanical oil pressure gauge, to the block fitting, and check your oil pressure there. I'm betting that you will have found your problem. RAY
rjm RAY

Some fluctuation is very common, more like fluttering, Smiths fitted a sintered iron plug to the gauge during production to try and reduce it. That is caused by a combination of particular revs, oil viscosity, temperature, pressure, relief valve characteristics and all sorts. Changing pretty-well anything may reduce it, or simply move it to another combination of circumstances.

But if yours is settling at various levels for several seconds or more when the revs are the same that is potentially more concerning. First is to eliminate the gauge, regardless of whether it is mechanical or electrical - which were used from 67 (start of Mk2 and Abingdon pillow) but reverted to mechanical for the 72 model year. Next would be to remove, clean and check the pressure relief valve isn't burred or sticking, after that it's things like sump and oil pump off.
PaulH Solihull

Ray and Paul.
Thanks very much for your ideas. I forgot to mention the car is 1969, is it possible to fit the car with mechanical guage or is that to much to change?
I had folks from out of town for this past weekend so did not get a chance to check things out. Will report back when I do.
Geoff Kimler

My car had the same ailment one time. Pressure would fluctuate
during driving. One time it had dropped to zero while on the
freeway (yikes!).

Turned out to be the rivet that holds the tab connector at the
sender had worked a bit loose and I was getting intermittent
or poor contact.
Daniel Wong

Whilst I don't know the relative sizes I don't see why the 72 mechanical gauge shouldn't fit the earlier engine and dash, although the 72 engine was an 18V of course and not the 18G.
PaulH Solihull

To follow up, I did put an external test guage on the car to bypass the sending unit. I warmed up the engine before installing and right a way pressure went to 65 and stayed steady at 3000 rpm for several minutes. At idle it dropped to 45! After letting it idle 10 to 15 minutes if was 35. So looks like the sending unit is the porblem. A new on is on order!
Geoff Kimler

>Whilst I don't know the relative sizes I don't see why the 72 mechanical gauge shouldn't fit the earlier engine and dash,

It fits -- I put a '72 gauge in my '69. Incidentally, my mechanical gauge flutters at certain rpms....
Rob Edwards

As written recently elsewhere "They (the mechanical gauges) all do that sir". Well, not all of them do, but whether they do or not is down to the individual car.
PaulH Solihull

This thread was discussed between 12/07/2011 and 18/08/2011

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