Help! Oil pressure dropping under acceleration

[ActionGrant]

Hi Guys, I have a 1951 DeSoto Custom with a 252 flathead 6 and the three speed fluid drive transmission. When I first start the car, the oil pressure is around 40 PSI, but once it warms up and I step on the gas, the pressure drops to near zero. What could be causing this? If it is just an easy fix, what do I do?

Thanks!

[ActionGrant]

Shel Bizzy, I read that post, but that really seems to be the opposite issue, stepping on the gas makes my problem get worse, and the engine will start to make some... distressing noises, as if it were running without oil. As soon as I let off the gas and let it coast it will go back up to around 5 or 10 PSI and not sound so rough. I did check my oil level and it was alright.

How do I get at the screen on the oil pick up assembly? Do I have to drop the whole pan or is there a way for me to get at it from the outside?

[aero3113]

The only way to get to the screen is to drop the pan. Try and do some more trouble shooting before you remove it. Pull the oil pressure relief valve and check the condition.

[Niel Hoback]

It sounds like its really low on oil.

[Joe Flanagan]

Aside from checking the oil, follow Aero's advice and pull the oil pressure relief valve. Mine was stuck in its bore and it raised heck with my oil pressure. I cleaned it up and reinstalled it and everything has been fine since.

[49powerwagon]

When you say it comes back up to 5-10 psi, That is not that encouraging. My 230 runs 35psi at idle, and it is worn out and needs a rebuild. Pressure dropping from 40 cold to 5-10 warm could very well indicate an engine that is not going to live much longer. DO NOT drive it with such low pressure, find your problem first. If it is a supply problem, fix it. If it is worn bearings, you are better to park it and fix it, if you keep driving it with a worn bottom end, the failure will be spectacular and expensive:eek:

[Young Ed]

My p15 when I got it was supposed to run. Well it ran. As soon as the oil warmed up a little the oil pressure dropped to almost 0 and you could hear knocking. It wasn't pretty.....

[aero3113]

My p15 when I got it was supposed to run. Well it ran. As soon as the oil warmed up a little the oil pressure dropped to almost 0 and you could hear knocking. It wasn't pretty.....

Hey Ed,you say when you engine warmed up a little it dropped to zero. I drove 30 miles the other day at highway speeds and the last 5 or so miles my oil pressure dropped to zero. I let it cool and had 40 psi again, headed home and again the last 5 miles it dropped to near zero. What do you think my problem is?

[aero3113]

The thing that gets me is that about 2 weeks ago I did the same drive and had no problems. Then drove around town between then and now with no issues. Would bearing clearance be that sudden?

[aero3113]

I really do not like the way my relief valve plunger looks. Also the spring has some weird wear marks in spots (the spring is not perfectly straight). I think I am going to replace them and see what happens. I looked on Vintage Power Wagons site and they want $198 for new crank bearings:eek:

Edited June 15, 2010 by aero3113

[james curl]

That looks like an oil pressure relief piston from an early engine, mine in my 41 Dodge engine looks like that. Jim Terrell at Terrell Machine in De Leon, Texas had or may still have the new improved after market replacement pistons and springs for the early engines. The bore diameter is smaller than the 42 and later engines.

[Young Ed]

Aero your problem sounds different then mine. My oil pressure hit rock bottom after a few minutes of idleing in the driveway. I agree with your plan to find a new spring and plunger for the relieve valve. When I rebuilt my engine after a run it would drop way down into the 10-20s psi range. Unacceptable for a fresh rebuild. We swapped in a used plunger and spring from another engine and now it barely drops at all. I still think the plunger might be sticking a little though.

[Joe Flanagan]

Aero, I believe I have a spare plunger with its spring that you can have if you want them. It's out of a 49 block.

[james curl]

Measure the diameter of the plunger, the later models have the spring going up inside the bore in the plunger. The early models are a smaller diameter and the spring rides on the bottom edge of the plunger. I tried the spring out of my 48 P-15 engine in my Dodge 41 engine and had way too much oil pressure because the spring in that configuration is too long.

[james curl]

Aero 3113, looking at your picture it appears that you have a circular relief in the upper edge of the plunger. There has been a lot of information about these plungers in previous posts and the Chrysler tips showing the bevel on the upper edge of the plunger. I cannot remember if it was Shel that has access to the old service tips, maybe someone here will remember and produce the tip on oil pressure relief plungers. You might try a search for oil pressure relief pistons or plungers.

[james curl]

Is this the same engine that you posted back on Oct.2006 about needing a new oil pressure piston? If it is you are a very patient man to put up with it that long without being able to fix the problem.

[aero3113]

Is this the same engine that you posted back on Oct.2006 about needing a new oil pressure piston? If it is you are a very patient man to put up with it that long without being able to fix the problem.

LoL,yes it is. I have never seen the pressure drop below 10-15 psi at idle before though. In my manual it says any pressure above 0 psi at idle is normal.

[aero3113]

Joe, I sent you a PM

[aero3113]

Measure the diameter of the plunger, the later models have the spring going up inside the bore in the plunger. The early models are a smaller diameter and the spring rides on the bottom edge of the plunger. I tried the spring out of my 48 P-15 engine in my Dodge 41 engine and had way too much oil pressure because the spring in that configuration is too long.

Yes,the spring does go in the plunger.

[james curl]

Aero3113 My manual states under oil pressure relief valve. "Inspect the relief valve plunger and the spring after removing the valve cap and the gasket. See Figure 39. If the plunger is scratched, remove the scratches by polishing, or install a new plunger. If the old plunger is to be reinstalled, clean it and flush out the bore with clean engine oil."

The plunger in mine has a small champher on the upper edge to help it seat in the end of the bore. In the picture of yours as posted I commented that it appears to have a relief etched into the upper edge, which would allow some oil pressure to leak past the edge of the plunger.

[BobT-47P15]

As I recall from some previous posts, there are different colored springs

for the relief valve - with each being a different strength. I don't know

which color/strength is correct for the Plym or Dodge engine. Could

any of these engines beng discussed possibly have the wrong spring?

Maybe someone would want to comment on or check on that.

Just a thought.

Edited June 15, 2010 by BobT-47P15

[aero3113]

Hey James, the plunger does not have relief etched into the upper edge. The area that I am concerned about is raised and definately looks like it should not be there.

[aero3113]

As I recall from some previous posts, there are different colored springs

for the relief valve - with each being a different strength. I don't know

which color/strength is correct for the Plym or Dodge engine. Could

any of these engines beng discussed possibly have the wrong sring?

Maybe someone would want to comment on or check on that.

Just a thought.

Bob, the spring that I have is unpainted which is supposed to be a medium spring. I am not sure if it is the correct one for my engine but I assume it is. I forget what the other 2 colors are. I remember reading that the springs do get weak after a while.

[BobT-47P15]

Seems like maybe other colors were orange or red, and green.

Don't recall what that signifies, though. More stout, less stout???

Have you tried pulling up old posts under the heading of oil pressure

valve, or spring, or whatever might relate?

[Joe Flanagan]

The colors of the springs and their ratings are in the manual. I don't happen to have it handy at the moment. I want to say green is heavy duty. That's what I seem to recall.