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MG MGB Technical - OIl Pressure drops, as temp rises.

A week or so back I thought I had a problem with fuel geting into the oil and whereas this may still be true Ive realised that the initial problem is that as the car warms up the oil pressure drops. That is when first started cold the pressure shows at 65lbs by the time Ive reached work (only a 2min drive - small town) my pressure is down to 50 lbs. Coming home I usually stop to get the mail and show 65lbs work to post office and as low as 30 by the time Im home - total of less then 5 mins driving time.

Whereas the engine is about to come out for an overhaul (DPO was a high school student and you can picture the lack of TLC spent on the poor girl), I just wonder if anyone has seen these symptoms before and what I should look for in particular??

Simon
Simon

Curious as to th 30psi. Is that at operating revs, ie 3000+? If so you might have some serious problems. I've had an MGB for years and in all the ones I have driven the oil pressure goes down as the car warms. Normally from 50/75 at idle/warm to 30/60 after it is fully warm, i.e ten minutes or more. If you have fuel mixing with your oil, it will thin it out. I would expect the pressure to drop across the board and not just as it warms up. BTW, what makes you think there is fuel in the oil?, and what do you think may be causing it?
J Arthurs

HAVE HAD THE SAME OIL PRESSURE READING FOR YEARS-IN THE HOT WEATHER ESPECIALLY--TO START---65-70 PSI @ 3000 + RPM FOR
FIRST FEW MILES-THEN GRADUALLY DROPPING TO 50-55
PSI AT NORMAL WATER TEMP-@ 3000 + RPM---IDLE ALWAYS ABOUT 25-30 REGARDLESS OF TEMP-RUNS GREAT---BUT IS THE 50 PSI A PROBLEM ? USING CASTROL 20-W50-190 THERMOSTAT----
robert snider

Simon: the bearings are very worn, at a minimum. Do you hear a rattling sound on acceleration? You are right to go for an overhaul at this time. I suggest you purchase Peter Burgess' book "How to Power Tune MGB Engines" from the usual suppliers or from the large book chains. Aside from power increases, the book gives good advice on general rebuilds.
Terry

Bob, If it doesn't drop below 50 psi it should be ok, that is the minimum psi figure listed in
the shop manual. Clifton
Clifton Gordon

Simon, I had a similar problem on my 74 before doing an engine rebuild last year. Oil pressure would drop down to around 30-40psi at 3000 rpm and 20psi at idle, may be lower.

When I dissembled the engine I had one very badly scored main bearing and one equally scored cam bearing. This was probably due to the fact that I took the car to the race track one day and while driving the car at high revs probably had a short time of oil starvation when driving quickly around some very sharp bends. Or it could be that the car had done 115,000 on the original engine without any rebuild.

Anyway expect to see some scored or worn bearings. Just have them ground by your local machine shop and you'll be back on the road with a new lease on life.

Andy 74 B GT
Andy Preston

To answer J Arthurs questions- yes the 30psi is at 3000rpm and on a longer trip (which was when I firdst noticed the problem) it dropped to around 20psi. Prior to this it never ran lower than 50psi even at idle. As for suspecting fuel in the oil - I did an oil change on both my cars (the MG and a 89 Mustang) on the same day using the same oil (Mobil 1 30grade) after one trip in both vehicles (from and to the same place) the MGs oil was noticably thinner than that in the Mustang and it showed none of the traces normally presenmt with water in the oil. The only other thing that could be getting into the oil is gas.

Terry Ive not notice any rattling sound on acceleration but then again the lousey fuel pump the DPO installed makes so much noiseI probably wouldnt hear it., but bearing is about what I suspect with a drop in oil pressure.

Andy I intend to stip and do a total rebuild - I have no idea when the engine last received any attention (heck Ive had the car since mid June) and had planned to do this over winter so I'll just start a couple of months early hihi. At the sametime I'll make a start on the bodywork that needs doing too.1

Thanks everyone for the information and confirming what I was starting to suspect. Hope that one day I can help you in return.

Simon
Simon

I Had a similar problem in my 71 MGB. When i first got it it ran about 55-60 idle/75 running, and then when it warmed up it would run about 40-60.

I took it in for an oil change ... now this is where the fun begins... I just assumed they would use the same 20-50 oil the manual recomends for this car.. they didn't they used a 10/30 synthetic.. since i only drive the car in the summer.. that shouldn't be a big problem...
well the oil pressure droped it still starts about the same... 55-60/75.... but now it runs at about 40 when @3000, and 20 when idle. I have also tried a straight 30 weight oil. and it is also much lower than the 20W50 oil the manual reccomends. Once i started using the 20W50 oil again is was back where it was supposed to be 45-50 at idle, and 70-75 @3000

Kurt
Kurt Breitsprecher

I don't want to sound like a smartass, but I'd recommend walking to work and back rather than starting the car from cold 3 times a day for a total 7 minutes of driving. If you need to use the choke (or especially if you have the ZS auto choke) in any of those starts, you will get fuel in the oil little by little because the fuel won't evaporate from oil during such a short drive. Excess fuel will also wash the oil off the cylinders, increasing wear.

If your engine is already shot (like the oil pressure may indicate), it won't hurt it more to continue driving 2 minute trips. But if you decide to rebuild your engine, and continue using the car for your 2 minute commute, I'd recommend waking up 15 minutes earlier in the morning and taking the "scenic route" to work and back, allowing the engine to warm up and evaporate any fuel that may have dissolved in the oil, and also dry the possible water condensation in the crankcase.

Kari

This thread was discussed between 25/08/2002 and 29/08/2002

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