Inliners International Forums General Discussion Engines real low oil pressure

Howdy fellas,I put a rebuilt 292 in my truck and the oil pressure was showing low when I put it in but I was hoping my factory gauge had gone bad sitting up.Today i hooked up a quality snapon tool oil pressure gauge to the engine and I have only 7psi at 700 rpm idling in drive.At 2200 rpm I have only about 18psi and at 3500 rpm I only have 23 psi.My old 250 had 30 psi at idle and 45 when cruising.Anyway I was wondering had any of you experienced this with a rebuilt engine.It came with a new oil pump or maybe rebuilt one -I can't remember but I do remember it was a Mildon pump.This would be the easy way out for i sure hope it's not due to bearing clearances.Any responses will be appreciated.

Sincerely,Gerald

If you put in a new oil pump you have to fill it with oil first. Then use a big screwdriver shaft on a electric drill put it where the distributor usualy sits and spinn the pump until oil appears on the rockers.

hope it helps, Frank

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LIVING IN THE PAST TODAY
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I am experiencing the same problem. My oil pressure is not as low as your's but I have lost about 20 lb. I done the rebuild on my engine and even went to the volume pump. The pressure stayed up a while and then went down again. I also developed a bad leak at the rear of the engine. I haven't went into it yet, going to be my winter project. Keep us posted on what you find. Just wanted you to know you are not by yourself.

Gerald,

you might want to take off your oil filter and take a close look at the bypass valve. Some reman companies take the valve out when tanking the block, some don't. Regardless, if the fiber disc does not seat properly when it's all back together, then you will get symtoms like you are describing. I remember Hud1 saying that he put a plug in to block off the bypass, and his symptoms are still a mystery.

If the disc appears eroded or if it sets cocked and not sealing, the net result is that you will need to replace the valve with a new one (preferred) or block it off with a plug. Don't try to repair it, it's not worth the headache.

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David
newbie #4153

Thanks for the replies fellas.I have driven the truck over 500 miles and when I put the engine together I primed the oil pump and engine as you described so it's not that.Hey David now what you told me is a strong possibility.I changed the oil a few days ago and I think I recall seeing a hole there and for an instance wondered about it.Also back when I was putting the engine together I also think then I saw a threaded hole inside the oil boss and wondered then what it was.Well to solve this I'm dang sure going to remove the filter tomorrow and look.Thanks alot again for your insight.I will let you fellas know the outcome.Take care all......Sincerely,Gerald

Isn't the bypass valve in the block just to re route oil if the filter is clogged? If so, it won't really affect oil pressure. I believe the actual pressure relief valve is in the oil pump.When you say rebuilt, do you mean all the bearing clearances were checked and were within tolerance and or the crank was turned for new bearings? And the cam bearings are good?

I think Tony P has it right.

You guys are correct about the bypass valve at the filter NOT being the culprit. I have to apologize about that; I was picturing a different engine plumbing in my head when I was writing. Sorry.

Do consider the pressure relief valve in the pump itself. It has a metal-to-metal valve, that, if not seated correctly, will result in a steady stream of oil constantly being dumped back into the pan. The spring behind it controls the maximum pressure, which you did not say was low. So there is probably a constant leak on the pressure side considering the pressure goes up in relation to the rpm; it's just lower than it should be all the time.

The pump casting could be cracked or the sealing surface between the pump and the block isn't sealed. You should use a hard-paper gasket there for street applications. On some engines, the factory doesn't install a gasket there, but the L6 should have one. You can remove it only if the surfaces have been planed or ground flat. I've never seen a rebuild shop do that on a stock motor. Even a little nick in the metal can screw it up.

There is also an outside possibility that the top cover on the pump has blown it's gasket. That thing is very thin paper, and rarely goes south, but is still a possibility. All in all, you will likely have to drop your pan to find the reason for the low pressure.

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David
newbie #4153

I dug out an older Motors Truck Manual to look at their oiling diagram to remind me how these engines oil..The oil is fed thru the oil filter directly to the cam bearings and then to the crankshaft..The oil galley is above the cam. Ok the lifter galley.Clear to the end of the block..I looked at a 153 CID inline (just a short inline six) I have apart.It appears to have a plug on the front of the block directly in line with this galley..The cam gear is still on it so its hard to see...I wonder about an obstruction in the galley..But if there was one you would think the crankshaft bearings would be starving for oil...BUT...how about the small passage way between the rear cam bearing and rear crank bearing..That must be where the gauge outlet is...Maybe there is an obstruction there..In other words every thing is being fed enough oil except for the gauge....Al...

BRAIN FART!!!...If the oil galley above the cam (lifter galley) was blocked you should be getting some lifter complaint...there I go ..thinking SB V8's again...They have a third galley that feeds the cam bearings and then the cranks mains...I wonder how good the rebuilders clean out the oil passage ways anyhow !!...

Ok,I installed the oil pump on the reman engine.I also installed a paper gasket between the pump and the block.Now the pickup tube didn't have any kind of o-ring where it goes into the pump but it fit real tight and I adjusted the bracket so the pipe would be held tight in the pump.Well I guess I need to go to Autozone and talk to them about it because they told me if I had any trouble with it for 1.5 years to bring it to them and they would have it repaired by their mechanic who I know of and he has a reputable buisness.This is the second reman engine I've put in this truck in the last three months and I really don't want anyone else working on it but it would void my warranty if I did so without telling them and I also am fed up with the reman company I have been dealing with.The first engine had a sticking valve and no lobe on the cam for a mechanical fuel pump so I jerked it out and got my money back.About two months later I got a call from the parts house where I had bought the first engine and they said they had a check for me.I went there and they had a check for me for $450.00 for my labor trouble so that helped some........dang now it's something with engine number two........same reman co.........next time it'll be a small block chevy for I never had troubles with them lol.Thanks all for the input.........Sincerely,Gerald

Another question about this oil pressure problem.The oil filter screws on to the engine and there is a big hole in the middle of the filter.There are two other holes on the edge of the block inside of the oil filter gasket and one has the bypass valve in it and the other hole is part of the oil plumbing.Ok either the big hole is the inlet or outlet or viceversa the little hole that is left in the block is the inlet or outlet.Now think about the whole plumbing system and if there is 45psi at the main bearings....then there is 45psi inside the oil filter because it is a sealed system.If this is the case then if the relief valve on the block inside the oil filter were leaking then pressure would drop I think.What do y'all think about this?Also if the oil filter got stopped up pressure would rise to the point that relief valve should be forced open.Tonight I checked that valve out and it doesn't take much pressure with my pinky finger to depress it.Also I cranked the engine and while cold it was showing 30 to 40 psi.......at 3500 rpm it was showing 40 but at times it would jump between 30 and 40.At idle it was showing 20 psi.I let the engine warm up while I did my chores in the yard and when I checked again it was idling at 15psi oil pressure and when I revved it up to 2500 rpm it only showed 20 so it is also dropping as the engine warms........I guess this could be due to the variable weight oil.......I'm running 5w-30w in it.Anyway thanks guys for your inputs and help..........Sincerely Gerald

It makes sense that the oil pressure changes as the temperature does. Oil flows faster or slower depending on the temperature. So I'd let the engine warm up and then concern myself with the oil pressure once warmed. And I don't like how low yours seems to me.

There isn't full pressure in the oil filter just like there isn't full pressure at the rocker arms..It is all volume at these points...The system is not as you think (sealed)...The pressure is being read off the tolerance areas..Aka..crankshaft...I'm guessing the bypass at the filter is about 15LBS..Fords don't have it in the block.That is why the spin ons to fit them have it there instead and they are about 15LBS...All engines need some sort of bypass because the oil filter can not handle the volume at higher RPMS..So....Only if the oil filter was like completely clogged would the pressure be affected...

Second verse ..same as the first...I'm Henery the 8th I am I am...OK..second verse not the same as the first...I've been repairing marine engines for some time now..Mercruiser stern drives used the 250CID for their 165HP model..One of the most reliable drives ever..The blocks rotted thru from salt water long before they ever wore out..But I had a number of them that we had long or short blocked mostly because of age..Too many hours of slow speed operation and sitting thru off season..Getting to the point at hand........I just don't think the rebuilders have it when it comes to old style American iron..They are so busy with the off shore crap ..Sure nice of them to install a nice front cover Huh ?...To protect the fiber cam gear during shipping huh?..Yeah right..It was because the a hole used the old gears over!!!...The cam bearings new and line bored??...Crankshaft bore lined bored???..I give up...The guy on the other end of the line spoke in some foreign tongue that got me nowhere..Getting the point?? I don't trust the new modern rebuilders..I have the blocks sent out and have them bored ..what ever..new cam bearings etc...and assemble them myself..I can then see exactly what was done..I mark the blocks to..I tell em I have marked them..They will steal high nickel content blockes etc. I send over an American made inline and a Heco in Mececo comes back..Old used cam bearings??..A 10-10 crank with standard shells ??.who knows..OK OK I am ranting..I am sick and tired of getting screwed without getting kissed !!!..If it were my engine I would be hauling it back out and having someone you trust check it over and tell the reman guy or what ever their sex is that I want it right or I am contacting the Attorney General's office in my state...I have done that to get results..The excuse of the warranty is void because of someone other than them pulling it down for inspection is crap!!!...Poor workmanship and fraud are crap!!!..You deserve to get what you paid for...I can really rant about the fraud that exists in the automatic transmisson overhaul world..But not now...Al...

Al,

hydraulic pressure is exactly the same throughout any container that is being pressurized if the fluid is not moving. Think brake system. Doesn't matter if it's a round pot (oil filter) or a long system of pipes like the oil passages in a motor or a master cylinder and wheel cylinders. If it is moving, then friction along the pipe walls and at corners and such reduces the pressure the farther along it moves. Net effect is that once the pressurized oil leaves the pump, it can only decrease in pressure as it moves through the system. So if the guage reads 40 psi, and it's located at the back of the block near the end of the cam galley, then EVERYTHING between it and the pump has to be AT LEAST 40 psi. That includes inside the oil filter.

A good example to consider is the air-cooled VW engine. It has no spin-on filter (stock) and no bypass valve, only a pump and a pressure relief valve. You can purchase filter kits that mount where the stock oil cooler bolts to. BUT in order to use them, you MUST install a different relief valve spring, or you will explode the new filter. That's because the stock pump puts out about 100 psi or more. The filters will split open around 80 to 90 psi, so they use pressure relief springs set aywhere from 60 to 80 psi.

The same theory applies to our L6's. The only difference is that the L6 has the bypass valve which opens if the flow through the filter is restricted. That means nothing more than the oil will go through the valve (and bypass the filter) if the pressure drop through the filter exceeds the valve unseating threshold. The backside of the bypass valve is the same passageway as the filter outlet passage.

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David
newbie #4153

To add a little more to the thread. When I went with the volume pump, I checked one main and it was .0015. That's where it should have been. I have the bypass plugged and using a remote filter. I also use 20/50 Synthetic blend oil with Lucas. On initial fire(before it warms up) the pressure should be higher. Now it is less than 30 pounds. When you increase rpm's, the pressure comes up very slowly, but not much above 40. I can't say anything about other rebuilders, because I am the rebuilder. I am sure everything was right when it went together. (you know how that goes-sometimes right ain't right) I'll know more when I start the tear down. Got a couple of more projects before I get to that one.

They call it lifetime guarantee, because it's guaranteed you'll be replacing those parts for the rest of your life.

Just a six...The net effect of the hydraulic pressure between the forward cup in the master cylinder and the furthest wheel cylinder has got to be so minute that its gone to a scientific theory of some kind..If there is a certain pressure created at the master cylinder ..then there is the same pressure at the furthest wheel cylinder to us mechanics anyhow..Because we don't care about the minute difference..I guess I was thinking crankshaft bearing clearance pressure because of the small volume that exists in each and every journal compared to the large volume that exists in the oil filter..Sure there is probably 40 LBS in the oil filter all right but it is a different kind of pressure than exists at a journal..In your last thread you state that once the oil leaves the oil pump it can only decrease in pressure as it travels to the end of it's passage..(gauge)...And that means EVERYTHING has 40 LBS including the filter...If the pressure is decreased how could the system be the same through out?..I would say that volume is decreased at every machined surface that it encounters creating pressure. The pressure lost due to friction would be so minute it is not a factor here dealing with only 40LBS or so..The oil pump is made with a relief valve that is set for every engine ..true.. Another relief valve exists at the filter.Any oil that can not pass thru the dense pleats is simply bypassed..true..So if the gauge at the furthest end is reading too low the oil being bypassed at the filter would not effect the reading as long as the relief valve in the pump is working correctly..If the clearances at the journals is too large oil is bypassed there in too must quanity causing a low reading...true?..To bad you can't tap into the pressure line right after the oil pump..Compare it to the end of the block..I would sure be looking for an obstruction between the shafts and the gauge outlet pass the last lifter....

Al, I'll agree on the brake system "minute" difference, I was only using that as an example of a system where the fluid is no longer moving once pressure is achieved (but it moves a lot until the pads and shoes snug up to metal).

The oiling system is considerably different because it's always moving. The true "end of the passage" is everyplace that the oil squeezes through to get back to atmospheric pressure (zero guage). So at the last little bit of tube before the bearing is where the pressure goes from whatever it was (say 40) to zero on the edge of the bearing exposed to air. The details of when and where it goes to zero often depend on the individual bearing and how fast the crank/rod or camshaft is turning, as well as the clearance. There is a "pressure distribution" across the bearing face, highest at the centerline where the outlet hole is, and lowest (zero) at the edge. But it is not necessarily a straight, predictable amount of change per distance, and it changes with rpm.

Backing up the system into the block, you have to go through small diameter drilled tubes. If they are too small for the amount of oil flowing through, then the velocity of the oil is quite fast. The pressure loss from friction in the fluid stream depends on the square of the velocity. Going back upstream is the large-diameter feed tube that has the guage sender threaded in at the back. The velocity of the oil in this tube is quite a bit less than the small feeders to the crank, especially at the back of the motor. This is due to the fact that about 75% of the engine has already taken it's needs from that galley. So the oil is moving slow here, and won't lose hardly any more pressure in that part of the motor. You don't want to put a guage before the filter, because the important value is the pressure that the bearings are getting, which is close to that indicated in the galley.

Really, the idea of pressure and hydraulic losses can be visualized by thinking about the water system where you live. The users open valves that allow fluid out of the pressurized tubes back to atmospheric. Those valves are no different than the lifters and bearings in an engine, except the holes in the motor just never turn off. They can, however, open up more due to wear. If there are too many open valves (lawn sprinklers), or a huge valve is opened up too much (think fire hydrant), or a device breaks and makes a huge hole (a worn out or spun bearing), then the entire system pressure becomes lower. But the closer you get to the source of the pressure, the higher the pressure still is, because the fluid has not had to go through the long pipe system at high velocity and get past all the little open valves that take fluid out.

Bottom line is: pressure is pressure. It can only decrease as it travels through a system. The faster it moves, the more energy it loses due to friction. In order to have high pressure at the end of the line, you have to limit the fluid going out (keep bearing clearances tight), or limit the velocity (drill passages larger), or put a bigger pump in at the start (high volume). If the fluid is not moving at all, then it has no friction and loses no pressure.

I sincerely hope I haven't caused any confusion with this too-long-winded explanation.

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David
newbie #4153

Hi David,
you wrote: ". But the closer you get to the source of the pressure, the higher the pressure still is, "

The pressure in a hydraulic system is going to be controlled by the leaks. The pressure will be the same through out the system, no matter where you check it as long as the fluid is not compressable.

I don't think the friction losses are significant enough in the oiling system of an engine to drop the pressure as much as the pressure in Geralds engine has dropped.

Jim R

Had a rebuild 350 once that the rebuilder turned one main journal about .002" too small, and only had about 5 lbs. of pressure when warmed up (using stock pump). It takes very little to ruin oil pressure. All the mains and rods need to be checked. Sounds like a quality control problem with this reman company. Are these rebuilt in Mexico by any chance?

There is no vendor bashing on this site so I will tell you both my reman engines came from Recon......which is located I think in Pa.I have heard they do some real fine work.....LOL........My next qhestion is at 15 to 20 psi and at say 2300 rpm will this hurt the engine.I realize that probably 30 to 45 psi is usual but can I run it for a couple weeks untill I can get them to check it out or should I wire the throttle wide open,crank it and run like hell? Gerald

Rule of thumb is 10 lbs per 1000 rpms. It should survive the situation that you describe. Either way, they are going to replace it, right?

Where are you taking the pressure reading from?
Does it make any noises it isn't supposed to?
Is the top end getting enough oil?

Dear Gerald;

Another Idea.

If the 'top end' has plenty of oil (every rocker wet)then the loss is not there.

I would try another pump & if it has a relief valve like the early models stretch the spring a little. If this solves it, GREAT.

It may not however; Then pull it & get an independant evalustion etc. It's a big hassle but you will need that should you need to go to court over it.

It really should be much higher.

Good luck, John M., #3370

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Gerald,
Could viscosity have anything to do with your problem?
5W30 sounds a little lightweight to me. These engines were designed around straight 30 SAE. The only multi viscosity oil I'd use is 20W50. I live in the desert, which is only marginally hotter than your "swamps."

When I first got the engine running I was running 10w40w in it and the oil pressure was still the same and this was from the first startup.I had to adjust the valves with the engine running and they were all getting good oil as a matter of fact I made a big mess even with a diverter screen.I like most of you think it's in the oil pump I hope.This engine has a 1.5yr warranty and I can take my truck to the parts house where I bought the engine and they will take it to a mechanic and get it repaired so they say.I could drop the oil pan myself and my truck is jacked up so high I don't think I would even have to jack the motor up to get the pan out.As a matter of fact the truck is so tall I wouldn't have to jack the truck up either for i can sit under it and work on it.I'm thinking of doing this since the problem has been there since the first startup,just to make sure I installed the pump right........which I'm sure I did.......but go ahead and get a new pump and install it and a new pan gasket and see what happens.If this doesn't repair it then get them to take care of it.Thanks all............Gerald