Oil Pressure Changes - Unusual?

CWizard · #1 (**permalink**) 04-10-2006, 08:14 AM

Can anyone please explain the following:
When the engine (351W with a remote filter, Mobil 1 15W50) is cold, oil pressure at 900 rpm is about 65 psi. Increasing rpm also, as I think would be expected, increases oil pressure. This situation holds as the engine warms up, that is, increasing rpm increases oil pressure. The situation changes once the oil is hot (100-105 C). Max pressure drops to about 45 psi, which I understand, but an increase in rpm causes a small drop in pressure, which I don't understand. Conversely, backing off the throttle results in a small increase in pressure. Why the reversal in response to throttle changes between cold and hot oil? Anyone? Thanks.

Rick Parker · #2 (**permalink**) 04-10-2006, 10:30 AM

Start with the easy things: Which filter are you using. If the filter you have is not of a very high quality the volume of oil going into the filter (as RPM's rise)is retricted by the size of the holes around the perimeter and the filtering media itself. Pressure drop as the engine warms is due to the oil thinning and to some extent the clearances increasing. Secondly: If the pump is questionable it can easily be changed out, if you are running a remote cooler a hi volume pump is a good idea.

ItBites · #3 (**permalink**) 04-10-2006, 12:07 PM

About a year ago I had a problem with some similarity, but not exactly your issue.

All of a sudden, my oil pressure would drop when RPMs increased. This happened cold or hot. At idle, the pressure was within the historical range. I had put a new filter in some months before and had not too much time on it. I changed the filter and the problem went away.

What was the source? Some of the new filters (generally sold as Heavy Duty) have an anti-reversion flapper valve in the top end. This is supposed to be there to reduce oil-drainback and keep the oil galleries filled and ready for the next start. Well, this flapper had come "loose" somehow and was working against me. When the pressure tried to increase with increasing RPM, the flow grabbed the flapper and pushed it to close the flow stream...

The next filter I got, I made sure was just a regular filter without thid "feature".

Maybe just a filter change will help.

strictlypersonl · #4 (**permalink**) 04-10-2006, 12:42 PM

I've seen this happen with loose main bearings...

Also check the oil pump pickup feed tube.

Less likely - a problem with an electric oil pressure gage sender ground.

CWizard · #5 (**permalink**) 04-10-2006, 12:44 PM

Rick - My filter is a Bosch 3500. I've used other filters and still had the same phenomenon.
Also, I need to clarify that at hot idle (100 C, 900 rpm), oil pressure is lower than at 2500 rpm (25 psi vs 45 psi). When hot, from idle up to about 2500 rpm, pressure goes up as engine speed increases, just like when it's cold. Then, when crusing at say 2500 rpm, a throttle increase causes a slight (2-3 psi) decrease in pressure, and a throttle decrease causes a slight increase in pressure. It's this that I'm trying to understand.
Thanks!

SCOBRAC · #6 (**permalink**) 04-10-2006, 12:46 PM

It sounds like you have an internal leak. It wouldn't happen with the 351 but my 427 had a oil passage cap come partially off creating irratic oil pressure readings... (un?)fortunately the cap was finally forced loose and I had a huge drop in oil pressure 55-60 psi to 30 psi at street cruising speed.

I never lost pressure and drove the car home (probably a bad idea) I pulled the pan to find a 7/8" freeze plug (oil plug) and clip had come out of the crank (427 steel cranks are hollow).

I decided I would at that point install a HP kit in my new oil oil pump, a blueprinted HV unit. The result should have been (I thought) about 85-90 psi cold and 65-70 hot.

What I got was 160 cold and 100 hot... Not kidding... It was obviously way too high but it's interesting. I had really good oil pressure before changing the pump. Rather than changing the pump I drained the 20/50 and put in 5/30 (both Redline synthetic). The pressure went from 160 cold / 100 hot to 100 cold / 60 hot. It will still spike if I don't warm it up properly but once warmed up it will not generally go over 100 psi and "criuses" at 75-80 psi. The internal leak it seems had been present since initial start up.

This is still a lot of pressure and may (probably will) wear my cam gear / distributor over the long run. I've spoken to a good number of sources and other engineers about my "problem" and the consensus is I don't need the pressure, although the engineer from Melling told me their HP pump was designed to run at 120 psi +. The guy who originally built my engine was big on high pressure (back in the 70's).

It has allowed me to go to the 427 S/C 0-160 oil pressure gauge...

Carnut427 · #7 (**permalink**) 04-10-2006, 11:18 PM

Quote:

Originally Posted by SCOBRAC

...I decided I would at that point install a HP kit in my new oil oil pump, a blueprinted HV unit. The result should have been (I thought) about 85-90 psi cold and 65-70 hot.

What I got was 160 cold and 100 hot... Not kidding... It was obviously way too high but it's interesting.

Just an informational tidbit...max oil pressure in side oiler blocks can be adjusted (on the engine stand or with the trans and clutch removed) by adding (to increase pressure) or removing (to decrease pressure) thin washers between the side oiler pressure spring and the little valve in the back of the block. If yours doesn't have any washers, the spring is possibly too strong, causing your ultra high pressure.
In your case, it sounds like you've got the situation well under control.

rdorman · #8 (**permalink**) 04-12-2006, 07:31 AM

Hopefully Rob Frink will find this one. He was showing me a product that is being developed for this sort of issue for SPF. I will forward the link and see if this is what he was talking about.

REDSC400 · #9 (**permalink**) 04-12-2006, 07:40 AM

My oil pressure also starts out in the 65 to 70 pound range when the engine is cold and then as the engine warms up it goes down to about 30-35 at idle (750~) but it does increase to a normal 60~ at 2000 RPM.

Perhaps, is your wet sump starved or dry (due to all the pan oil being topside) at high RPM thus causing a low oil pressure reading? Do you have enough oil in the oil pan?

How many quarts of oil do you install at a typical oil change (including the filter) My engine takes 9 quarts with the remote oil cooler and a Milodon pan. Just a thought...

SCOBRAC · #10 (**permalink**) 04-12-2006, 07:43 AM

Indeed Carnut is right... There is an axillary bypass system in the left rear corner of the side oiler block. It works just like the bypass system in the pump with a plunger and a spring but unlike the pump it is held in place by a threaded plug. It would have been quite handy if the designers had oriented the plug so it could be accessed without removing the engine or transmission.

It is a simple matter to shim the bypass spring, increasing the effort required to open the valve or replace the spring with a higher pressure spring.