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Discussion Starter #1
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?
 

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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?
 

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Discussion Starter #3
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.
 

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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.

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.
 

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Discussion Starter #5
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.
 

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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?
 

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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?
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. 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.

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.
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.

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.

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.

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



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Discussion Starter #8
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 :happy:
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.
 

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Discussion Starter #9
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.
 

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Discussion Starter #11
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?
 

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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
 

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20-50 now, going to change to 25-60. I am trying for 50psi hot; engine turns 6000.
I doubt the oil viscosity is going to help much. Bet you ran 20/50 before as well. The clearances aren't that big either. I still think if it was a sloopy pump it wouldn't make that much pressure at an idle warm. Anti cav grooves and sloopy gear and cap clearances tend to wreck havoc on low rpm pressure, but they recover the second you hit the gas.



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GN/Bob is the MAN with his feedback...but I'll add, when I did my last rebuild and installed a NEW Melling HV performance oil pumps > I now have lower overall psi. I did take it apart to check gear to cover clearances prior to installation and was all good however, I do believe the colored spring installed in the pump was in fact was wrong...because psi is lower.
 

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When you put the new Melling pump in did you weld the pickup tube in place? If so, did you take the spring out before welding?
 

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When you put the new Melling pump in did you weld the pickup tube in place? If so, did you take the spring out before welding?
The pickup on a BBC is in the body of the pump. The spring is in the cap. You don't need to remove the spring, you simply take off the cap, which you should do anyway. That said, it never hurts to take the relief apart anyway. I have seen my share of metal in the relief port to jam the piston.



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Discussion Starter #18
The wierd part is that it makes 80lbs of pressure until the engine warms up; maybe the relief is stuck partly open?
 

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Sticking??

The wierd part is that it makes 80lbs of pressure until the engine warms up; maybe the relief is stuck partly open?
Usually when they stick, they are stuck closed, the "at rest" position.... It is not un common to have them rust in the closed position after an engine has seen some water in the oil, but this is a new pump. If he is seeing 80lbs and not blowing the filter O ring out, the relief isn't stuck.....
Ray
 

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Ray, you have a hard time keeping the o ring in the filter at 80 PSI?

If the relief is frozen at some mid point, its usually caused by some debris from the manufacturing. Its no uncommon for crap to be in the relief port.
Not sure thats your problem. It would be difficult to make 30 psi at an idle with the relief partially open. But, just as a for instance, say it had a spring for a stock sized small block pump, and the fact that you a remote filter and lines, (and cooler), then the spring could be weak enough to open with only 40 psi block pressure.

The fact that it makes 80 psi cold is not usual. I have seen many pumps make 80 psi with the relief full open when cold. With 20/50 in the pan, virtually every engine I have ever built will make 80 psi, and I know the relief is wide open. Chances are yours is wide open when cold, and still making 80 psi block pressure.



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