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MG MGA - Oil preasure

All is runing well with my new rebuils however I have a slight concern with the oil preasure. Start up and driving around town is fine with 65 to 70 lb running and 25-30 idle. However running a long trip hot trip today at 70 MPH runinng preasure went down to 55lb and idle to 20lb. Does this seem a little low for a newer engine? Or am I just a lttle paranoid?
WMR Bill

Don't worry about it. That's OK
Bill Spohn

Just curious, what weight/type oil are you running?
Tx
GTF
G T Foster

Using Castrol 20-50
WMR Bill

IMHO, 20 psi hot idle seems rather low (although it should survive for a while longer). My engines in good condition always run 50-70 psi at road speed (depending on relief spring shimming) and usually 40 psi at hot idle (800-1000 rpm). I would have to slow the engine to about 400-500 rpm to get it down ro 20 psi. When I see 20 psi hot idle I figure it's time to change some crankshaft bearings or check for worn oil pump.

$.03
Barney Gaylord

I have to disagree with Barney. There is absolutely nothing wrong with 50 psi running and 20 psi hot idle, mine has been like that since the engine was rebuilt 6 or 7 years ago. I run 15/40 oil which obviously drops the pressure a little when hot but 20 psi is more than adequate. More pressure on the relief spring would take the pressure higher but does it actually need it? Not IMO I am afraid.
Bob (Robert) Midget Turbo

I often see references to "relief spring shimming" on threads related to oil pressure (which I always read). Can anyone clue me in as to how this works ?

Thanks,
Chuck
Chuck Mosher

If the oil pump is producing more oil than the engine bearings can allow to flow thru them, the bypass valve lets the excess oil flow back into the sump. It is a pressure relief valve. It will limit the oil pressure according to the strength of the spring that keeps it seated in the block. By shimming the spring, you increase the pressure that is holding the valve closed, resulting in higher oil pressure in order to unseat the valve and let the excess bypass into the sump. If you have 60 psi when cold, that would indicate that the bypass valve releases at 60 psi. When you have less than 60 psi, the valve remains closed and the bearings recieve all of the oil produced by the oil pump. If you are seeing lower reading at low rpm or idle speed it would indicate that your oil pump is not able to produce more oil that the bearings allow to pass thru them. This is usually a worn pump, or worn bearings, or both. However, if your bypass valve is not a good fit in the block it may not close off tight, allowing oil to bypass into the sump. This may also result in low oil pressure at low enging rpm.
Ed Bell

Thanks Everyone! Ed I did put a new releif valve in it. I wonder if I should of "seated" it?
WMR Bill

I don't know what the block seating face looks like so I don't know if lapping a new relief valve is of any value. Perhaps someone who has done this will shed some light on this subject.
Ed Bell

Chuck,

The oil relief valve is a bullet/cone shaped metal piece with a hollowed back. A spring fits in that hollow and then the assembly is fit into the engine and the nut is screwed on. Over time the spring looses its springiness. Mechanics will then throw a washer in the back of the hollow and put the spring on top of that.

I don't like the practice, I think it misleads buyers. To my understanding the only thing it benefits is driving the cold oil pressure up past 70 or so, I don't think it helps anything under that.

Bill,

See my note above, you can replace the spring with a new one, but that will only help your top end number not the bottom end number.

I think you may be seeing an issue with a pressure as low as 20 when hot.

1) How long did it stay at 20? e.g. did you run for a while at 70 mph, slow down and idle with the pressue at 20 and then drive for a minute or two at say 40 and then the idle oil pressure was back at 30?

2) Did you inspect the oil pump when you had it apart? Meaning measure the distance between the impellers?

3) I'd defer to others on this but the main cause for low oil pressure (other than the pump) is a looseness in the bearings. If its not the oil pump I'd suggest you may want to torque down all the bearing caps again...they may have relaxed with the heating/cooling of the engine.

Just a thot
T McCarthy

The oil relief spring should be 3" long. If less than that, replace it. The relief valve should be lapped to its seat in the block during the engine rebuild. OK to install a spring washer (1/4" I think) that has been flattened, under the spring to increase pressure.

George
G Goeppner

This thread was discussed between 15/09/2008 and 16/09/2008

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