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MG MGA - Oil Pressures

Hello all,

I'm wondering if I need to start thinking about a bearing replacement.

The pressures in my car are running

First 15-20 minutes: 60 psi idle - 80 psi at 3,500 rpm

Warmed up: 40 psi idle - 60 psi at 3,500 rpm

Those are obviously OK numbers but after a long highway run, say 20 miles at 65 mph, the pressures start to fall off to 55 psi at 3,500 rpm and 30 psi at idle just after getting off the highway. They rapidly stabilize back at 40 to 60 after that run...say within 3 or 5 minutes.

Anything to this?
T McCarthy

Don't think about it, just drive. Your numbers are well within normal operating ranges
J Heisenfeldt

My presures have been outstanding as they should be with only 2K miles on the engine. Great pressure at start 70lbs and 45 at idle. today though on a 40 mile cruise I noticed that my pressure started dropping until it reached 30lbs at 3500 RPM It held there for the duration of the trip. After a short pause to fill the tank ,the oil pressure went back up to 50lbs and then again slowly dropped off at 3500 rpm to 30 lbs. This does not seem normal to me.
David Holmes

Hi T Mc - your pressures are a good 10 to 20 better than mine - so I think you are OK for a while yet.
David - your hot pressures go alarmingly low - lower than mine on a 80000 kilometre (50k mile) engine. Sounds as though the oil is thinning out too much when hot - what are you using? cheers Cam
Cam Cunningham

Cam, 10-40 Castrol. Do you think that would make such a difference? Maybe use just 30 wt. ?
David Holmes

I use 20W50 in hot weather to give a little higher pressure with hot oil (switch to 10W40 for cold weather to avoid low pressure on start up). I have found that Castrol in particular seems to thin out and drop pressure at high temperature more so that some other brands. Ditto for some of the cheaper "house" brands at discount stores. For decades I have been using NAPA oil which I believe is made by Valvoline. It seems to be better in this respect, holding good pressure at idle with hot oil. (I also like NAPA Gold oil filters made by WIX).

The observation that pressure drops as much as 10 PSI with a long hot run is correct, recovering after a few minutes of rest or more casual running, nothing to worry about. If you stop for fuel the oil pressure will recover by the time you get the tank filled. If oil level is low and you add one quart you get instant pressure recovery from the oil temperature drop.

An engine in good condition should run 50 psi (or more depending on relief valve spring force) at road speed, and about 40 psi at idle with hot oil. If I see 40 psi at road speed I begin to worry. If I see 20 psi at idle it's time to change some crankshaft bearings, usually the rod bearings being the culprit.

Cam bearings seldom go bad. Worn rocker arm bearings can cause about 5 PSI drop in pressure. The steel rocker shaft wears about twice as fast as the bronze rocker bushings, so the shaft may need to be changed twice for one change of bushings.
Barney Gaylord

David - Like Barney, I use 20w50 - usually Duckhams .
Barney - looks like I need to change the big end bearings soon then - cheers Cam
Cam Cunningham

TMc & Dave - Just for comparison - I drove around 100 miles in my car today. 60 psi at start - on to motorway for about 45 minutes at 3500rpm - ambient temp around 18C/water temp around 180F - pressure slowly dropped down to just over 40 psi after about 25 mins running where it stayed. So oil is thinning but obviously big end bearings showing some wear.
Cam Cunningham

This thread was discussed between 18/07/2008 and 20/07/2008

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