Oil Pump: To change or not to change?

[RedBarchetta]

Short intro: Started noticing gradual loss in oil pressure. Wasn't getting better and more noticable when warm. Dropped the pan and have started to inspect the crank bearings. Looks like minimal wear (a little copper showing so far...no debris in pan), but am planning to change all bearings. Will also inspect and R&R any rod bearings that exhibit similar wear. All this assumes the crank looks good (which, again, so far it is!).

Motor is stroked 351W (392CI) from RDI, 10,000 miles, built in year 2000.

Given the declining oil pressure condition, and the fact that I'm "in there" (i.e. dropped pan and all the mess that goes with that), and inspecting/replacing bearings and replacing those that look suspect, should I ignore the oil pump or replace with a new one?

It's a Melling adjustable unit (PN10832) and I'm told that oil pumps either work really well or not at all. The replacement cost is around $75 (about as much as I'll have in all the bearings...LOL!). Trying to put a price on peace of mind and the odds that the pump is part of the problem.

Thanks for the opinions.

-Dean

[Silverback51]

What is the difference in oil pressure that you have observed?

[RedBarchetta]

At an oil temp of 212 degrees and water temp of 196 degrees

Cold Start (idle) = 45-47 PSI
800 rpm (hot idle) = 20 PSI
1000 rpm = 26 PSI
2000 rpm = 32 PSI
3000 rpm = 35 PSI
4000 rpm = 35 PSI (no change above 3K)

I bypassed the remote oil filter, Canton oil thermostat and front mounted oil cooler just to eliminate a bad filter or psi drop from all of the plumbing. These are the results:

(+/-) values represent the net change from the first test above:

Cold Start (idle) = 46 PSI
800 rpm (hot idle) = 19-20 PSI
1000 rpm = 24 (-2)
2000 rpm = 35 (+3)
3000 rpm = 36 (+1)
4000 rpm = 36 (+1)

It's either the pump or bearings. Pulled the #3 main cap late last night and there was a little copper starting to show through in the middle. The saddle side of the bearing looked awesome. The crank was shiny and no scoring or marks. Will have to pull the rest of the caps this weekend to get a better picture.

Gauge is near brand new and I tested another gauge just to make sure. Same readings.

-Dean

[Silverback51]

Truthfully, I do not see anything that is really out of line with the pressures. But then, I don't know if it's a high volume, low pressure pump, and what your original clearances were set at.

$75 is cheap peace of mind as long as you have it apart. I would just replace the pump and know that you have eliminated that variable from the equation.

[RedBarchetta]

Originally Posted by Silverback51

Truthfully, I do not see anything that is really out of line with the pressures.

Maxxing out at 36? That's the problem! If the general rule of thumb is 10psi for every 1,000rpm, then revving above 4,000 is asking for problems. This would be especially true if maintaining that level or above. I just don't want to chance it...the psi should be climbing with the rpms and it's not.

-Dean

[Excaliber]

I wonder if the oil pressure relief valve is not fully seating? On a 351 WHERE is that valve?

[Rick Parker]

Dean:
Replacing the pump at 10000 miles for the type of cars we have and as anal and particular as most of us are would not be considered out of the norm. Given that you have it opened up and it is readily accesable I would really consider it. If the pump you have is the type with the bottom plate supporting the pump shaft, then you have the best type available. Internnally it is quite common to find scoring and grooves on the working surfaces, but this is also what reduces the output and thus reduces pressure. Here is a source for an upgraded version of that very pump. They are up in a suburb of Fresno. Very good to deal with.
http://www.precisionoilpumps.com/ima..._Race_Pump.jpg

http://data.melling.com/TECHtips.php

This is the "heart " of the engine.........a transplant when necessary is not out of the question. It will be the best $75-$100 you spend.

I also noticed that Melling also offers a line of internally Phosphate coated series of pumps that have a 25% increase in volume. These are in their Performance line. Part # 10833


10832
Stock volume gerotor set assembled to an extended drive shaft that allows for additional support in the
cover. Supporting the drive shaft in both the housing and cover eliminates dynamic shaft deflection at
increased RPM levels. The cover is doweled to the pump housing to assure alignment of shaft bores. The
housing and cover are CNC machined and maganese phosphate coated for wear and rust prevention. The
relief valve has a pressure adjustable screw-in plug instead of a cup plug. Works with the original bolt-on
pickup screen assemblies"

10833 The same as 10832 except gear rotor displaces 25% more volume than the 10832.

[64 Daytona]

On a 351W, the pressure relief valve is in the pump. I would get a new pump, preferably a mieling HV unit. HV is the series of pump that is high volume and you can install a pressure relief spring of your choice at that time. I would also blueprint the pump or have it done if you do not know how. I know that this sounds like a lot to do but it is cheap insurance. I have done this on EVERY motor that I have ever built, (3000 plus motors) from the start and never had a problem. Good luck, Scott

[bobcowan]

It's only $75. That's pretty cheap for piece of mind. I can't think of a good reason not to change the pump.