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Low oil pressure questions 1951 Plymouth, 218 with 3sp. manual


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Late last summer I bought a 1951 Plymouth, 218 with 3sp. manual. According to the paperwork that came with the car the engine was rebuilt by a reputable machine shop (now closed due to the owner's retirement) about 7 years ago with anywhere from 150 to 400 miles put on each year and an oil change at the end of the season. A couple of 5 qt. jugs of full synthetic 5W-30 and filters were included with the car so I assume this was the oil used and what I used last fall before putting it away. I will admit I never paid attention to the oil pressure while driving last year, just at start up.

 

As I have been driving it this summer noticed a worrying concern, no (or little) oil pressure after the engine is warmed up, even if driving at 55 mph the gauge looks to be at or close to zero. If I come to a stop and go through the gears with higher RMPs the needle will move up a little. At start up with a cold engine the gauge will pop right up to 40 until the engine warms up then seems to never get that high again while driving and the longer I drive the less the pressure and no pressure, according to the gauge, at idle.

 

A few questions. When a shop does an standard engine rebuild do they check or do anything to the oil pump? Looks like an oil line running to the mechanical gauge, could the gauge read wrong as the oil warms up? Is there any sort of relief valve that is staying open so no pressure can be built up when warm? Is synthetic 5W-30 too thin, even with a rebuilt engine? With no smoke or other issues with the engine, is it okay to drive as long as I had pressure at start up? I thought about adding some STP Oil Treatment to thicken the oil up but I would think with a relatively fresh rebuilt engine I shouldn't need to.

 

In case the pump is bad I checked price for a new pump, yikes! Even a rebuild kit isn't cheap. However, a salvage yard near my cousin has a 1952 Dodge that was T-Boned early in it's life (low miles) and although it is pretty well picked over, as of last fall the oil pump was still with the engine, any reason I couldn't take that one out and put it in my car?

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Posted (edited)

Try getting a manual gauge and tap it into your oil line to make sure your gauge is correct.  There is a oil pressure relief valve near the filter but I cannot remember exactly where without getting the book out. I assume your engine isn’t making a lot of bad engine noises either?  Since you didn’t mention that is why I suggested verifying with a known good guage. 

Edited by plymouthcranbrook (see edit history)
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Question Number One. How sure are you about your gauge? I find it very hard to believe that you could be driving 55 miles per hour and have no oil pressure.

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The oil pressure ģauge flex hose on my 50 Chrysler swelled shut so the oil pressure was way low.

Replaced the hose..pressure back up to 45lbs...normal.

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  • Peter Gariepy changed the title to Low oil pressure questions 1951 Plymouth, 218 with 3sp. manual
8 hours ago, c49er said:

The oil pressure ģauge flex hose on my 50 Chrysler swelled shut so the oil pressure was way low.

Replaced the hose..pressure back up to 45lbs...normal.

Metal (copper?) line running to the gauge.

 

Looks like the next step is to get a aftermarket gauge to verify if mine is reading correct.

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The Plymouths have a 5" rubber flex hose right in front of the firewall.... placed between the 3/16" steel oil line from the engine and the steel line going to the oil gauge.

The Chryslers are the same except the flex hose is 20" long and goes all the way to the oil gauge.

These hoses do fail...plug up or start leaking.

The original brass fitting that the oil line connects to down on the block has a 1/32" diameter hole in it. That hole is small to dampen oil pump pulsations. That hole also could be plugged.

As mentioned first install a oil pressure test gauge to the msin galley on the drivers side off the block. There are four 1/8" plugs....take your pick.

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I have never seen a original factory oil pump fail in over 50 years of working on these MoPar flatheads....new replacement pumps...yes there are known failures...poor machine work and drive gear failures.

A Pressure relief valve sticking open will cause very little pressue if any at idle but pressure should rise noticably at high engine rpm's.

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I had a '50 Plymouth with the 218. I was concerned about oil pressure with my car, but that was about getting low readings at idle on a warmed up engine. Then I read the shop manual for my car, which said that minimal oil pressure readings at idle were common, as I recall. Having said that, at speed you should be getting at least 30 pounds with a warm engine, and maybe a little better at highway speeds ( or 10 pounds per 10 mph, though that's a rule of thumb, not a formula.)

 

I agree with what others about suspecting the gauge. I NEVER trust original oil gauges in old cars to give accurate readings (some people do, but I don't...ever.) In your case, though, I'd be concerned because you're getting decent readings at cold startup, i.e., the gauge may be working okay. An aftermarket gauge may be the only way to tell what the oil pressure really is. Even if you don't want to alter the originality of the car, it's worth spending 80 bucks on a temporary oil gauge install. 

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27 minutes ago, JamesR said:

I had a '50 Plymouth with the 218. I was concerned about oil pressure with my car, but that was about getting low readings at idle on a warmed up engine. Then I read the shop manual for my car, which said that minimal oil pressure readings at idle were common,

That must have been the mentality of the engineers who designed the Ford Mustangs like mine. The oil pressure switch doesn't trigger a low pressure warning until the pressure is below 6 psi. When I found hat out I tested the pressure with a manual gauge. At hot idle the lowest the pressure goes is about 30 psi with 5w30 synthetic oil.

 

17 hours ago, Tripps51 said:

A couple of 5 qt. jugs of full synthetic 5W-30 and filters were included with the car so I assume this was the oil used and what I used last fall before putting it away.

If I had a '50 Plymouth I think I would run a heavier weight oil than 5w30 synthetic.

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9 minutes ago, Ronnie said:

If I had a '50 Plymouth I think I would run a heavier weight oil than 5w30 synthetic.

X2!

 

Glad you posted that, I was thinking the same thing but figured some folks wouldn't see a problem with that range..

 

Unless one is driving in extreme cold weather, there is zero need to run this thin of oil.

 

Modern vehicles are often spec'd 5W-20 or even 0W-20, they do so in order to maximize the vehicles fuel mileage..

 

Myself, I run 10W-30 in all of my modern vehicles in spite of the OEMs 0W-20 suggestion year round as I feel 5W and 0W in summer time is a pretty thin place to start..

 

For that old of car, 10W-30 would be better or even 15W-40 should be fine..

 

Although, double checking pressure with a known good gauge will tell you if your existing gauge is correct.

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Posted (edited)
3 hours ago, Ronnie said:

If I had a '50 Plymouth I think I would run a heavier weight oil than 5w30 synthetic.

I also agree with that. I probably used 10w-40 or 15w-40 in my tired old Plymouth flathead engine. I'm not saying that weight of oil will cure sub-5 pound oil pressure at speed (with an accurate gauge) but if you have an engine with 5000-10000 miles left on it and you put 500 miles per year on it, that translates to a lot of years of use...try using whatever tricks you can. As you said, the place the OP should start is verifying oil pressure.

Edited by JamesR (see edit history)
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4 hours ago, Ronnie said:

That must have been the mentality of the engineers who designed the Ford Mustangs like mine. The oil pressure switch doesn't trigger a low pressure warning until the pressure is below 6 psi. When I found hat out I tested the pressure with a manual gauge. At hot idle the lowest the pressure goes is about 30 psi with 5w30 synthetic oil.

 

If I had a '50 Plymouth I think I would run a heavier weight oil than 5w30 synthetic.

I run non synthetic 10-40 in mine and have excellent pressure readings(40-50 hot at above idle)

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