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MG MGA - Oil pressue MGA 1500

Strange problem with my oil pressure reading. Running at 3000rpm, oil pressure about always. Then suddenly the pressure gauge needle starts to flutter really fast, dropping down to 55/60psi then back to 75psi. Then it might stabalize for a min or two then it does it again. engine temperature stays at 175f. I have already checked the pressure relief spring and valve and its okay. Any ideas anyone?
Paul Longbottom

This is caused by the oil pressure relief valve fluttering under certain hydraulic resonance situations. It is not harmful, just irritating to watch. The fix is to install a new pressure relief valve poppet and lap it into the seat. Sometimes just lapping the old one can banish the problem.
Barney Gaylord

Hello Paul,
It sounds like either the oil pump might be sucking a little air either from low oil level, or high oil level. High level can cause millions of air bubbles in the oil, as the crankshaft big ends splash it around. I believe Barny has a dipstick calibration method at It might be worth investigating. Another slim possibility might be too much oil gathering under the valve cover at high speed. You could start the engine with the valve cover off and look for any large stream of oil coming out, say from a missing plug, bad rocker arm bush etc. Maybe someone else has a better answer.


Thanks guys, Moss sell a 85psi pressure relief spring, give that a shot first with a new poppet...its a simple fix if it works. Not too sure about trying to lap it in though, with the limited access if any of the lapping compound gets into the engine it will not do it too much good. Let you know how it goes. Paul.
Paul Longbottom

The normal average equation is 10 lbs of oil pressure for every 1000 rpm. Too much oil presuure is as bad as too little. The oil must FLOW thru the engine and too much oil pressure will restrict its flow. Similar to adding oil to a funnel. More oil poured into the funnel will not pass thru until the oil in the narrow spout has made it thru. I would just lap the old one and try that first. Lapping the poppet is simply done (if I am correct) buy using a little valve gringding compount and a suction cup stick.IMO your oil pressurre is already a little high and increasing it will not help.
Gordon Harrison

Okay guys, I have lapped in the poppet until it was bright and shiney all around the angle on the end. I will give it a try later and see what happens, its raining here right now. I have only had the MG for 1 month and never taken it anywhere near 70mph until the other day, thats when I saw the oil pressure needle start to flicker now and again. I suppose when the oil gets thinner at a sustained 3500rpm, and the engine is really hot, the oil passes the poppet if its not bedded in correctly. Once it passes the pressure drops until the spring shuts it again, the it just repeats. I used really fine valve grinding paste then tried my best to clean out the seat after, but the bit that was still on the end of the poppet was down to fine dust so I dont suppose if any was still inside it would do much damage. Paul.
Paul Longbottom

A quick update....Barney you are a genius. you were 100 percent correct. the new lapped in valve worked a treat. no more pressure gauge fluctuations. Just has a good blast around the country lanes round where I live... and the needle was steady, except for the gear changes. Thanks for your advise everyone. By the way, the 85 psi spring from Moss UK arrived this morning...anyone need it? Paul.
Paul Longbottom

This thread was discussed between 05/07/2008 and 09/07/2008

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