MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Oil Pressure

Now the weather is getting cold, I have noticed an odd behaviour. When everything is cold, the gauge takes a good few seconds to kick and then settles at 50psi. When the engine gets really warm (after,say, 3 or 4 miles) it has risen to 60psi.
I would have thought it would drop when warm, not rise.
Should I be worried?
Graeme Williams

Graeme, my MKII does the exact same thing, almost no pressure on start-up and then a slow rise in pressure till it gets to the 60+ range as the engine warms up (I never gave it much thought as its always in the 60+ range when driving). Its been this way for many, many years, so I don't think that its the sign of an engine problem. Hopefully there will be confirmation that it's a non-issue.
Nick Kopernik

20W50 oil will do that when ambient temperature is down round 40dF (5dC) or lower. Thick oil does no flow well, being slow to go through the passages to the main oil gallery, so it gets dumped over the pressure relief valve back to the sump until the oil warms up some. With ambient temperature closer to 0dF (-18dC) oil pressure might be 10-psi on cold start, and may need several minutes to get the pressure up to normal.

When driving a lot in cold weather in northern Illinois Winters I switch to 10W40 oil in the fall and back to 20W50 oil with first oil change in the Spring. I don't think this is particularly detrimental to the engine, just don't drive it too hard with cold oil.
Barney Gaylord

Thanks for replies everyone! I am not so concerned about it now. It was just in my mind that with oil cold, pressure is high and then drops off as it warms!

Graeme Williams

I think its just the cold viscous oil in the line to the gauge that retards the reading. Barney's theory about the RV limit being reached may be right too, but the guage would show it surely?
Art Pearse

Art: the gauge seems very slow to react and when the engine is turned off, it drops slowly too. I had wondered about whether there was any muck in the tube.
Graeme Williams

The original connection fitting between the oil pressure gauge flex hose and metal pipe has a small orifice in it that slows the flow of oil to the gauge. If there is air in the signal pipe, the gauge reading will be slow to respond as the air has to compress or decompress.


If the gauge is slow to respond, consider bleeding the air out of the signal pipe (similar to bleeding air from the bake lines). Detach the pipe from back of gauge, run engine until liquid oil comes out of the pipe (with no air), then switch off the engine, and reattach the pipe to the gauge.

Oh.... Purpose of the small orifice in that fitting is to damp out vibrations in the gauge that may result from pulsations from the oil pump. And by the by, some replacement fittings have large through bore, no orifice.
Barney Gaylord

Thanks Barney. I have just spent a few hours working upside down under the dashboard so here we go again!
Graeme Williams

Interesting one Graeme, I have had this weird thing for the 19 years I have had my "A". I typically and slowly get say 50 psi on a cold engine start up and it will be some miles before I get my 60+ hot reading, any time of year. This then can't be anything to do with the oil gauge pipe and the P/R valve won't blow at 50 psi either.
Personally I think the oil pump has a struggle sucking the thick oil through its wire strainer and small bore pipe leading into the pump. As a result the pump is probably cavitating, which means a reduced flow and therefore a lower perceived gauge pressure, until the oil warms enough to achieve a full flow into the pump.
Another reason to warm up an engine gently...
Anybody agree with me?
Pete
PeteT

Pete, I wait till the temp gauge reads 180 before driving off from a cold start, and will not rev the engine too high for a few miles after that. No proven reason for this, just some 40 years of driving an "A".
Nick Kopernik

Nick

Despite often doing this myself I have been told it is a bad idea to run an engine too long without any load and actually can lead to increased top end wear. Interestingly I might have done this in the last couple of years and worn the guides. I will know when i do a few miles with my re valve guided head come the spring.

Paul

Paul Dean

Paul,
Change of subject, but see if you can get bronze lined cast iron valve guides fitted. Bronze guides have to be made a loose fit to cope with the bronze's inward expansion within the cast iron head. Half worn out before you start or the valves seize.

Nick,
Its a plan but not really relevant to this odd cold oil pressure issue. Would be interesting to know if you have got a big mileage out of that engine using you warm up idea. Can we assume the choke is back in ASAP?

Pete
PeteT

Pete, yes, the choke goes in as soon as the engine will maintain a reasonable idle. I've owned my MKII for almost 38 years now and have consistently used the same warm up procedure. The only engine work performed in all that time was a new street cam and a rebuild of the head to deal with unleaded fuel, both of which were done two years ago. Rings, pistons, crankshaft & bearings all original; not too bad for an engine with 85,000 miles on it.
Nick Kopernik

The oil pressure issue is temperature related. I wonder whether turning it over for a short time with the ignition off is't a bad idea. That can build up the pressure without the engine load on the components. Prob not good for battery charge!
Graeme Williams

This is very much secondhand info but I have heard 2 explanations as to the alleged increased wear danger.

Firstly too little cool oil gets to the top of the engine to provide good lubrication.

Secondly with no load the moving parts can 'rattle around' causing wear which is particularly bad at higher revs. Taking this at face value the first point is an argument for keeping the revs up and second one is an argument for keeping the revs down!

I don't really know whether any of this is true but I have heard it over the years from different sources. It may though just be an old motor mechanics tale.

Pete - Thanks for your comments about type of guides. How common/available are bronze coated cast iron ones?

Paul
Paul Dean

Re bronze guides.
Yes bronze has a higher coefficient of expansion than CI. In both cases, there is a tendency for the hole to get smaller based on the head's temp being limited by the cooling jacket and the guide being subjected to the higher temp of the valve stem. I did a calculation based on very worst case scenarios (exhaust valve).
The valve stem could expand 1.5 thou and the bronze bore could shrink by only 0.5 thou, meaning you are OK with a 2.5 thou cold clearance. At worst , bronze needs only another 0.5.
I can share my spreadsheet with anyone interested.
Art Pearse

Hi Paul,
Sorry availability of bronze lined cast iron guides unknown. The firm that did a fantastic job of putting right the original dreadful 'unleaded' head job, sadly shut up shop, so cannot ask.

Graeme,
Turning over the engine every so often is a good idea to keep a film of oil over rubbing parts but it won't effect this weird increase of oil pressure as the oil warms up.

Pete
PeteT

I gave the Frog a run today and noticed the same issue -the oil pressure increases by 5 psi as the engine warms. Both cars run on Penrite oil.
Graeme Williams

This thread was discussed between 23/12/2016 and 11/01/2017

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.