Thread:

Oil pressure problem

Engine has 80 lbs of pressure when oil is cool; as the temp goes up, the pressure drops. By the time the oil is 200 degrees, the pressure is down to 40. The odd part is that it has 30psi at idle; rev it up and all it will make is 40psi. There are two gauges on the engine; one just after the remote oil filter and one in the main gallery of the engine, of course after the thermostat and coolers. They both read very nearly the same at all times; one is mechanical the other electrical.
Any guesses as to the problem?

Originally Posted by SFOcean

Engine has 80 lbs of pressure when oil is cool; as the temp goes up, the pressure drops. By the time the oil is 200 degrees, the pressure is down to 40. The odd part is that it has 30psi at idle; rev it up and all it will make is 40psi. There are two gauges on the engine; one just after the remote oil filter and one in the main gallery of the engine, of course after the thermostat and coolers. They both read very nearly the same at all times; one is mechanical the other electrical.
Any guesses as to the problem?

What block is it. What's the history of the engine and how has the oil pressure been in the past? What oil are you running,, how long since it was changed?

Originally Posted by obnoxious001

What block is it. What's the history of the engine and how has the oil pressure been in the past? What oil are you running,, how long since it was changed?

Chevy 555
Just rebuilt, no changes to bearing clearances; does have T&D rockers now and a new Melling oil pump. Same cam followers and everything else.
Had good pressures prior to rebuild.
Had Comp Cams break in oil in it, now changed to 20-50 (change only removes 10 of the 14 or so quarts in the system).
Pressures did get better after the change, it was only 20 at idle and 35 at 2,000RPM and up.

Originally Posted by obnoxious001

You didn't answer my question about the block,, doesn't happen to be a Merlin II, does it?


Based on what you are saying, something changed during the rebuild, do you know the bearing clearances?

Which Melling pump,, std or high volume?

What oil pan and pickup? Same that were previously used?

I believe that the standard volume Melling pump may possibly come with a lighter spring in it, I normally use the high volume pump with bearing clearances that work well with it. I know of one shortblock that I built, and someone put a standard volume pump on it, with a Milodon oil pan, and ran a lighter weight oil with very similar results.

Block is a Mk IV Chevy, forgot that, sorry. It is not a Merlin. The block was not replaced. The oil pan and pickup are also the same.
The bearing clearances are 2.8 to 3.0 mains; 2.6 to 2.8 rods. The oil pump is a Melling high volume with 80 lb spring; this part was changed.

Originally Posted by SFOcean

Block is a Mk IV Chevy, forgot that, sorry. It is not a Merlin. The block was not replaced. The oil pan and pickup are also the same.
The bearing clearances are 2.8 to 3.0 mains; 2.6 to 2.8 rods. The oil pump is a Melling high volume with 80 lb spring; this part was changed.

OK,, I specifically asked about the Merlin II block because quite a number of them had defects that would cause low oil pressure, and you had mentioned that yours is a 555 cubic inch engine, so I assumed it could be an aftermarket block.

If the rest of the oil cooler and filter plumbing has not been changed, then I would think it might be worth pulling the engine to inspect the oil pump. Is there any chance that the inlet and outlet lines to the block got swapped?

Originally Posted by gn7

If I am right, and I think I might be for the very first time in my life, let it serve as a lesson to everybody.

The fact that you are reading the pressure where you are is somewhere between useless, and almost useless.
gn, your just trying to make me feel bad
NOW, it you said you were reading it BEFORE the filter, and in tha main gallery, that whould mean something. Your current arrangement tells you almost nothing.

True enough, the mechanical gauge is on the Accusump, it needs filtered oil. I will get a pressure reading just out of the pump, good point.

Its not the pump efficiency because it has decent pressure at idle. Leaking sloopy pumps tend to have low pressure at low Rs. Same with hi po pumps with anti cavitation grooves. Its simply a indication of internal pump leakage.

It is a hi po pump with grooves and clearances set.

Its not the block, and I doubt its the clearance, nor the oil viscosity for the same reason. But it does sound alot like a volume issue. Since this is not Sleeper CP's deal, I will guess the volume problem lies in the filter. Possibly the lines size to the remote, but am assuming thats the same as before also.

All are the same.

So, any chance this thing is equiped with some short ass 4" long POS Fram PH 30 with no bypass, and no bypass in the block because someone thought it was cool to remove it.

K&N HP-3002, also same as before. When cut apart, looks good inside.

think there is any possibility that you could be choking the engine for oil at higher RPM because the filter just flat ass can't move that much oil. Sure, at an idle it flow fine? But ask it to flow 3 times that much and it becomes a road block.

The odd part is that it flows (has good pressure anyway) with cold oil but not hot.

What filter are you running. Are you running a block mounted bypass?

K&N HP-3002. My MK IV Block has a bypass on the filter pad.

Originally Posted by obnoxious001

OK,, I specifically asked about the Merlin II block because quite a number of them had defects that would cause low oil pressure, and you had mentioned that yours is a 555 cubic inch engine, so I assumed it could be an aftermarket block.

If the rest of the oil cooler and filter plumbing has not been changed, then I would think it might be worth pulling the engine to inspect the oil pump. Is there any chance that the inlet and outlet lines to the block got swapped?

I will double check, but they really only fit one way so I doubt it. I am beginning to think oil pump also.

Originally Posted by gn7

I have to lean towards a relief opening pre maturely. Like 50 psi at the pump. Seriously doubt its a leakage deal between the gears and cap type thing. That will usually show up a horrible pressure at an idle.

Maybe I will get lucky and it is the block's relief valve opening early or stuck partially open which would allow cold oil pressure but not hot?

Its a clearance issue somewhere, the oil gets thinner as it gets hot. Could be rod- main-cam bearing clearance or in the pump itself.
as long as it has 10 lbs of pressure per 1000 rpms it will be fine, seen em before with 30ish at idle and never have a problem.
What weight oil are you running? a heavier oil will increase the pressure slightly

20-50 now, going to change to 25-60. I am trying for 50psi hot; engine turns 6000.