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MG MGB Technical - Help - oil pressure gone mad...

Driving home the other day I noticed that the oil pressure was all over the place, going from 20 to over 100psi depending on revs, load etc.
I assume the oil pressure relief valve is stuck shut to give the higher pressure reading, but don't know why it would read low suddenly (doesn't normally go below 40psi).
Thought I'd replace the valve and spring, but now I can't get the old valve out. I've just put the cap on without the spring and ran the engine hoping that heat and 80 - 100psi might shift it off the seat, but having disassembled it yet to find out. Anyone have any thoughts on what happened to give these readings and how I might get the valve out if it is the culprit?
Dave Smith

If you can't get the valve out i.e. it is stuck then there is a fair bet that it is the sticking valve that is causing your wayward readings :o). OTOH if the gauge is only moving suddenly and relatively large distances at a time, i.e. no small gradual changes, maybe the gauge is at fault. If the valve I would say because high readings are followed by low readings even when the high pressure has pushed the valve off its seat it is still sticking, not getting back onto its seat until the pressure has dropped greatly and the spring can finally move it. I.e. it is sticking at more than one place in the bore and may still be stuck in the fully open position. You could try it again when the engine is fully cold and the oil thicker, which may push the valve further than when the oil is hot, or even try a cold start with the cap off (hint: don't peer down the bore when doing this). Also before refitting the spring refit the cap until the threads engage, then slowly unscrew until it come free, then mark the cap and block. With the spring in you can pres down on the cap with the marks just on the unscrewed side of each other, knowing that you only have to turn the cap until the marks cross-over and the threads will be engaged. Much easier than trying the maintain pressure and turn not knowing when the threads are going to engage.
Paul Hunt

Hi Paul - I got the valve out in the end, cold start with cap and no spring. Lots of scoring on the valve, can't tell the state of the bore though. Cleaned it as best I could with rag, cotton bud etc but didn't get any swarf out. Eventually got the new one in - what a s*d to get in with that spring, had to take the exhaust manifold off in the end to make enough space.
Got it back together again and although the low pressure is gone I still get around 80 at idle and 100 psi with revs. Don't know where to go from here!
Dave Smith

Dave, it must be the gauge if you're getting
such high pressure (80 psi) at idle. If you've
got a spare mechanical gauge try cross-checking
the readings.

Is this the electric press. gauge? Then the
sender is probably bad. (The sender is also
ridiculously expensive.)
Ronald

Dave,

The excessive pressure with the stuck valve may have caused the guage gear to have come off the end of the quadrant - try another know guage before worrying about the engine internals.
Chris at Octarine Services

Chaps,
Thanks for all replies, I'm leaning toward the bust gauge theory too. My usual MG garage (Beech Hill) has a spare we can try.
As a precaution I'm going to swap the oil filter and cut open the old one to look for metal, as suggested by a friend in aircraft maintenance - it's a requirement at all oil changes it seems.

Chris - do you know if the gauge can be reset if it's slipped off the quadrant, or would it really need recalibration (although I can't imagine they were ever highly accurate in the first place)?
Dave Smith

Yes it can be reset - open up the guage and pull the curved tube out gently till the quadrant clears the gear wheel then adjust the pointer position and release the tube - it may take a few attempts to get it right.
Chris at Octarine Services

This thread was discussed between 31/10/2004 and 01/11/2004

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