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MG MGA - Fluctuating oil pressure

1960 restored MGA 1600. New oil, temperature gauge. Oil pressure is at 40 psi upon start of engine and for some period during initial running. After engine warm up and at highway speeds, 55-60 mph, oil pressure is at a steady 20 psi.

My question is: Is this normal? If not normal, what is needed to be done to make oil pressure continuous at what I believe should be about 40 psi.
W. D. Derryberry

20 psi at highway speed is too low. I believe the original MGA owners manual says 30 psi minimum when driving (but they don't give a speed or RPM), but that even is low. I usually look for 45-50 psi min. at highway speed. Anything lower than 40 I would start to worry. Some people say 10 psi minimum per 1,000 RPM.

You could check the Oil pressure relief valve and spring. I believe the spring free length is 3.000"
If yours has collapsed replace it. You can verify your oil pressure gage by disconnecting the line at the engine and pressurising the line with compressed air at a known setting. The gage will read correctly with oil or air pressure. I would start by verifying the gage, easy to do, and tells you whether you really have a problem.
After this it gets tougher. It could be the oil pump
or a problem with the bearings etc. in the engine. Others will know more about this than I do. Good luck

Ralph
Ralph

"Is this normal?"
As you probably know, most engines experience a drop in oil pressure after they warm up. The amount of change is a fuction of engine condition (bearing clearance, oil pump condition, etc) and the type and viscosity of oil being used. You don't mention what type/weight of oil being used or mileage on the engine. A "restored" car may or may not have fresh bearings, etc.
In my experience, the numbers you describe are not that unusual for an MGA engine with some mileage on it. One question, however, regarding the steady 20 psi after warm up and at road speed. Does your pressure not drop off at idle and pick back up at higher rpm...at least a little?

Regards,
GTF
G T Foster

Hi,

Mine is a rebuilt 1961 MGA roadster (barely 200 miles on her) with oil pressure about 70 when running cold and at highway speeds. It drops only to 40-50 when idling.

Is this way too high?? I am experiencing a fair amount of bluish smoke on acceleration.

Duncan
DA MacFarlane

My 1500 engine is well worn-in and doesn't drop below about 45 psi at cruising speeds, down to 20 or 25 at idle. I think 20 psi at speed is too low from what I have read and been advised in the past.
Duncan, your blue smoke on acceleration reminds me of a similar symptom I once had and it was due to oil getting past the rings (wrong size). Maybe yours aren't "bedded-in" well yet. Another possibility is valve seals and/or too much clearance between valve stems & guides, but this shouldn't ocur with a proper rebuild.
Tom Heath

hi duncan,

that's what I got. it's definately high, but as everything is standard i've never felt the need to make any changes. mine has 6000 miles on the clock since rebuilt, so maybe it come down a bit after a while.

you could put an extra sim behind the bolt of the relieve valve, but personally i wouldn't change it.

regards,
Koen
Struijk

Did you bleed the air from the guage pipeline when you installed it? If not, you will have air in the pipe which could affect you reading.
Ian Pearl

Ian

Pressure is pressure. So long as there is no significant flow the gauge should read correctly.

Larry
69 C in restoration
Larry Hallanger

Larry,
If I remember correctly, last year this was discussed with regard to low oil pressure. After the gauge line was bled clear of air, the oil pressure came up fine. I understand the pressure is pressure statement, but I think that it does matter.
FWIW
Mike
mike parker

Mike,

agree with larry, pressure is pressure.

personally i've never bleeded an oil line, it always worked straight away. must admit it was always new or rebuild material. never tried it with used gauges or lines.

W. D. Derryberry,
few things to check:
-you sure the right bearing shells have been used?
(a workshop once installed standard size shells in an MGB engine of a friend of mine while crank wasn't standard anymore)
-is there exessive clearance on the rockershaft?
-is the relieve valve ok?

Good luck,
Koen
Koen Struijk

Air compresses under pressure. If it didn't there would be no need to bleed brakes. This is often used to advantage as in cushion tanks in a heating or cooling systems. It is best to bleed the oil line if it has been emptied.
John DeWolf

Bleeding the oil gage line will not change the reading, but it will cause the gage to react faster to changes in pressure. This is most evident when the engine is first started. It will take longer to show pressure if it has to fill the small tube with oil as it is compressing trapped air.
Ed Bell

I fully understand pressure, but I am relating what I recall from a previous thread. Some of the people had posted that oil pressure was always low; when they bled the line, it came up. As John states, air does compress. If the line were disconnected for some time, moisture could get in the line, and that always causes fun, especially if you have a freeze. While I hope we are beyond the winter season now, it can happen. If you have water in your oil line while hot, your oil line will not really get hot enough to boil off the water so it will freeze when the car is shut off and cools down. The hydraulic lock will cause a pressure lock, which might not be noticed when the driver starts the car. This will cause a non-responsive gauge, or a sluggish gauge.
mike parker

W.D. Back to basics. It would of use to know when the engine was rebuilt and how many miles it has on it. The term "restored" is somewhat imprecise as it means many things to many people and there is no standard rating system as to what qualifes as a "restoration". To some people, a little body work, a fresh coat of paint, new brake shoes and pads and a new interior is a "full restoration". To others, a full restoration means taking the entire car down the smallest pieces possible, examining each piece, rebuilding and replacing everything not in perfect condition and, after a lot of time, effort and money, assembling a car that is in better shape than any one ever sold through a dealer's outlet. I know of one prize winning MGA which is "totally restored" but in very poor mechanical shape. So, please tell us how many miles on the engine and, if it was rebuilt, how many years ago.

If you have a new water temp/oil pressure gauge, that is the first suspect. Attach a good, direct reading gauge to the block and see what readings it is providing. Bad parts are becoming more and more common today and anytime there is both a new problem and a new part, the new part is suspect until proven otherwise.

Normal readings should be at least 60 psi at road speed and at least 25 psi at idle. This is my experience with relatively new engines in excellent condition. Most newly rebuilt/properly broken in engines will read about 70-75 psi at road speed for a period of several years. After that, as they become more worn, the hot oil readings will drop somewhat lower. My experience is that about 45 psi is the point when I need to consider a rebuild. By the time the cruise oil pressure has dropped to 45 psi, you still have a basically sound, rebuildable engine, but the rod and main bearings will be worn down to the copper. Hence, you can run the engine for a period of time at 45 psi, but, your level of wear is increasing signficantly.

Hot oil idle pressure readings are funny. They are engine rpm dependent and even a few hundred rpms can make a significant difference in what the guage is reading at hot/idle. Anything below 20 psi at 1,000 rpm hot/idle is cause for concern. In fact, if you are only getting 20 psi at road speed, I would expect a reading of zero at hot/idle.

If the guage tests good and your new gauge is reflecting the actual running oil pressures, you either have an oil pressure relief valve which is sticking in the open position when hot (bad thing as it is releasing most of your oil back into the sump) or a very badly worn engine. The only thing to do, in either case, is to strip down the engine and examine it.

Les
Les Bengtson

Listen to Les here folks he is spot on. Nothing to do with oil viscosity, air in gauge lines etc, that's just wasting time. Check the gauge by screwing a known accurate gauge straight into the union coming out of the cylinder block. If that's no good check the relief valve spring and also that the valve poppet itself is free. If all that is OK it's engine out time as your oil pressure is far too low. I'm very surprised that you can't hear the big end bearings rattling bearing in mind the low pressure and the amount of wear that will have taken place. This has got nothing to do with the wrong size of shells having been fitted, if that was the case the noise would be unbearable. It's shaft regrind time new shells and oil pump. It would also be normal practice to rebore and fit new pistons otherwise you risk having to do the whole thing again after relatively few miles.
Iain MacKintosh

To all who submitted some very helpful suggestions, thank you very much. Here is the end of the story.

I did bleed the line from the engine to the oil gauge. Reconnected and the gauge did not register at all, 0. Tested the gauge with air and the gauge worked fine.

Then tested the oil pressure at the starter switch with an external test gauge. It registered 50 psi. Whew! I had been having images of bad main bearing and other unpleasant malfunctions.

I then used high pressure air to blow out the recently installed new oil line from the gauge to the starter switch connection. Rebled the oil line to the gauge. Reconnected the dashboard gauge and Eureka! Oil pressure went up to over 40 psi.

For your internal data base. Blow out a new line or an old one prior to installing, bleed and connect.

Thanks again for all the great suggestions.
W. D. Derryberry

Thank you for letting us know how you got on. Nothing worse than an unsolved problem, or an open ended post.

Good man.
Ian Pearl

W.D. Where is the oil pressure connetion at the starter switch? None of my MGAs have had an oil pressure connection at the starter switch. Rather, they have had an oil pressure take off at the right, rear of the block.

Your reading of 40 psi, presumably cold, is still very low. I would expect at least 60 psi, cold, from the properly functioning gauge. If you are only showing 40 psi, cold, I would expect your engine in in serious need of a rebuild--sooner, rather than later.

The concept of "blowing out" an old oil line, or a new one, is only a factor if the line has some form of "crud" in it. As others have noted, air compresses when the oil is forced up into the line and the only effect is about two seconds from when the oil filled line would give a proper reading from when the air filled line would give a proper reading. "Air in the line" is not a realistic factor.

Figure out what your problem is, and from you description, you have a problem, and correct it. My rebuilt engines show 75-85 psi when started. My old, well worn engine shows 65 psi when started (but drops much more at hot idle--25 psi). Thus, 40 psi at cold idle signifies a problem which need immeditate attention. Les
Les Bengtson

Les

There is a flex hose which goes from the block to a double ended mounting at the starter switch. From this mounting, there is a microbore steel pipe (with a coil in it) that goes to the oil pressure gauge.

Could be taht it's common for this to be missing, but it should be there
dominic clancy

Now can anyone suggest why my gauge sticks at 80psi? Is this the end of the travel of the gauge? If I tap the glass it often goes down 10psi, if I turn of the engine, a tap on the glass will free it to 0 psi
dominic clancy

Lee,
The flex line goes from the engine block to a double ended fitting attached right under the starter switch. The double ended fitting end opposite the flex line connection connects to the oil gauge pipe which goes to the gauge. See Moss Motors part435-530.
W. D. Derryberry

I don't think 40 PSIg at idle is a problem at all. If I recall, the owner's manual says 10 PSIg is OK at hot idle. Unfortunately my book is with my car at my shpo 20 mies away, or I'd look it up. Why wouldn't 40 be OK at idle before the oil got hot?
Chuck Asbury

Chuck. 40 psi, at hot idle, is excellent. 40 psi, when first started and warming the engine up at 1,200 to 1,500 rpm is, in my experience, rather low. My cars show about 70 psi at that time--the same as they show at cruising speed. After the engine and the oil is warmed up, they drop down to about 40 psi at idle. Hence, one needs to know the conditions under which the readings were taken and the oil being used as both can affect the results and the interpretations of said results. Les
Les Bengtson

I think that there is still a problem here. You used to get 40 psi at start up and your direct gauge confirms this by registering 50 which makes sense. Hydraulic lines are not friction free and the longer they are together with smaller diameter, number of bends and high viscosity oil then the greater the pressure drop. I dont think the 10psi drop is unreasonable for the dash gauge. Now I would want to know what the hot pressures are say after running 10-15 miles. Idle at 800 rpm should be 25-40 psi and running at say 2500rpm or above should be 50-70psi. I have rebuilt both my Bs and they read 25/60 and 40/60. I doubt you can achieve this especially as you confirm 40 on COLD idle. When the oil warms up the idle pressure could well drop 20psi or more and all this points to worn shaft and pump. The relief valve setting is not a factor here as the pressure drop figures relate specifically to temperature.
Iain MacKintosh

Aargh! Sounds as if I must rebuild my 1500 engine, too: 40psi cold; 23psi when hot. Just as well that I only have to complete the gearbox rebuild on the BGT, the MGA gearbox and the rewiring of the Morris Traveller. Another evening (!) bites the dust.

Shane
shane rj

This thread was discussed between 12/04/2005 and 18/04/2005

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