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LOW OIL PRESSURE


GSW38
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I HAVE A 1938 BUICK MODEL 41. ENGINE RECENTLY REBUILT, ON START-UP THERE IS 45 PSI OIL PRESSURE AS ENGINE TEMPATURE RISES OIL PRESSURE DECREASES AT IDLE LESS THAN 10 PSI AT45 TO 50 MPH OIL PRESSURE IS ALMOST 25 PSI. WHAT CAN BE DONE TO INCREASE OIL PRESSURE AT RUNNING TEMP.? DO THEY MAKE GEARS FOR THE OIL PUMP TO CONVERT IT TO A HIGH VOLUME PUMP?

THANK YOU GW1938

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Actually, the pump is doing exactly what it was designed for if it produces over 45 psig. It is more likely that your oil is getting too hot and loosing viscosity.

First, you might try a different oil. But the real problem is that part of your engine cooling jacket may be plugged & the oil is picking up excess heat.

If an oil change does not help, the next step would be to pull the head and clean the cooling channels in the head and the block.

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My oil pressure is similar with 20W-50 dino oil. My engine was rebuilt with a nos oil pump but I did not replace the bearings and took the shims out to get proper clearance. I thought it was normal. In temperatures over 90 deg after a long run the oil pressure drops as low as 20-25PSI at 60MPH, and 5-10PSI at idle.

I switched to Mobil 1 20W-50 synthetic a few years ago and it now runs about 5PSI higher when hot.

Steve D

1938 Special 40-41

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I have tried different oils with no increase in oil pressure.I do not think the cooling channels are blocked because they cleaned out with rods compressed air then entire engine block was boiled out after all machining was done, thanks for the change oil suggestion.

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I know new pump gears were installed but I do not know if the clearence between the gears and cover plate was checked.I have been told that there are high volume pump gears made for this oil pump but I do not know where to buy them.

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Any higher volume pump I have seen, have wider gears than a normal pump, which means the gear chamber would also be wider. Hence, you would need to change the whole pump. Higher volume by no means, means that you will get higher pressure. The spring pressure on the regulating/bypass valve determines the pressure, not the volume. I've taken pumps apart before that have had stuck bypass valves or that have had chips or trash under the valve holding it partly open. Look there first. Dandy Dave!

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As always, I agree with Dandy Dave. There are a number of people who think they know what they are talking about on the internet and then there are those who are experts. Dave is the latter.

Assuming your bearing clearances are all correct (and they should be if you had it re-built correctly) then the place to look is the pump and the relief valve.

Stuck binding oil bypass/regulation valve, broken bypass/regulation valve spring and gear/gear pocket wear all need to be looked at but cover plate clearance and most importantly a worn cover plate or incorrect pump gear axial end clearance are where to look.

I have seen numerous 'rebuilt' engines where the oil pump cover plate is badly worn, yet the engine re-builder never addressed this. Some engines you can just flip the cover over and run on the un-worn outer machined surface. Milling the worn cover to clean it up is the best bet. Make sure your cover bolts are not too long now with the thinner cover plate if the bolt holes are blind in the pump housing.

Of the newly 're-built' engines I've been asked to look at because of poor oil pressure, one was a missing cam bearing that was never re-installed, one the oil pick-up tube was left out and the others have all been either stuck bypass/regulation valves or worn cover plates that were never re-conditioned and set to proper clearance.

Sadly, the fellow with the missing oil pick-up tube had done too much damage to correct without a complete second re-build and the person that did the initial work was no longer in the business nor available for a refund.

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Hey Brian, Thanks for that plug. wink.gif

Every rebuild that I do always requires a teardown of the oil pump, or replacement of a pump. I've redone engines that were so called "Rebuilt" and put together with an old oil pump, and failed because the rebuilder just through it back together without going through the pump. I also have saved several motors for folks that realized that there was a problem early on, and did not continue to run until things were taken down and corrected.(Yes on several so called "rebuilt" engines.) One thing needs to be kept in mind about an old oil pump, especially in a motor that has failed, or is worn, is that filings and chips from the failure have gone though it. Never assemble a motor with an old pump without a thorough cleaning and inspection. The oil goes though the pump and engine before it gets to the oil filter. And many of these early auto engines do not even have an oil filter.

smile.gif Dandy Dave!

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Thanks to everyone for the info.My next course of action is going to be a completely rebuilt oil pump.I have a complete parts car and I will take the oil pump from it have rebuilt and try that.The pump that is in the car now does have new gears,relief spring ball check.I am not positive of the clearance of the cover plate. Thanks again and I will let you know how I make out.

George from Western Pa.

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Hey George,

Please do keep us posted. I have a 38-41 with the same issue (high PSI at start, 10-15PSI as it warms up, then 5-10 at hot idle). Mine was rebuilt back around 1975, has about 12,000 miles since then.

I'm running straight 30W, non-detergent oil. No oil filter, so I change it every 800-1000 miles. I'm reluctant to pull the head or do anything major unless I'm sure I'll see an improvement, so your results would be most valuable.

Thanks,

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Just a question and a couple of comments. I am not familiar with the oiling specifics of Buicks although I owned a 29 and 3 53's. However does your oil presssure guage line come off the engine first or last (ie by the pump or at the end of the line perhaps at a cam bearing)? Are you sure you pressure guage is reading correctly. Perhaps you only need to clean the seat, ball and spring on the pressure regulator.

My Pontiac (certainly not in the same category as a Buick) comes off right at the pump. When cold with straight 20 oil it reads 30 lbs at an idle, when hot after three or four hours at highway speeds (50-60 mph) it is 35 lbs at speed and 20 lbs at idle. My manual however says that 10 lbs at an idle is sufficient.

One other note I was told and have read (but have never seen)that too high oil pressure can actually cut groves in babbitt. Of course the question might be what is too high????

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Brian Here with a '38 special and haveing the engine rebuilt at CAMS in New Bern, NC. Along with needed internals I also purchased a new oil pumb from Bob's Autombilia. I'm lookng at these posts with all the oil pressure problems and wondering if there is anything specifically to look at while the engine is apart. New parts include 20 over pistons and rings, cam bearings as far as the internals are concerned and the new oil pump. Otherwise the engine was in great shape inside with little sludge and probably only 38,000 miles which showed on the speedo. Problem with the pistons seemed to have been detonation and we pulled it apart before any damage was done. Engine probably hadn't been run seriously in 25 years or so so the damage occurred sometime in the past according to what the rebuilder found, which was good for me. Any ideas that would help before reassembly would be well appreciated now. Wouldn't want to put it back together to find low oil pressure.

Brian

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  • 3 weeks later...

Good Morning All: I have only seen this posting this morning and am not in a position to look at my Buick library of source material. I am away from home. However, I have owned three 1937 Buicks and about 15 or more straight eights. The owners manual and related material will tell the reader what to expect in terms of oil pressure, but the pressures mentioned should be adequate, especially in hot conditions. Many of my straight eights have been diven thousands of miles in hot July weather with oil pressure between 25 and 30 pounds of pressure. I do not see anything wrong with this and the 10 pounds of pressure at idle certainly is sufficent to lubricate the engine for a brief period of time. We should not expect the engine oil pressure to remain at 35 to 45 pounds under hot conditions at all times. I am not willing to assume that anything is wrong with the oil pressure as stated. Thanks, Brooker

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  • 1 month later...

Not the cover if it is a gear pump. You use emery cloth on glass and take a few thousands off the thickness of the body by rubbing it back and forth. Then there is less play on each side of the gears.

Are you sure you have low pressure or is the pressure relief valve set too loose????

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