Good oil pressure cold (60 psi). When hot 15 psi at idle, 35 psi at 2000 rpm's. Problem starts when I go over 3500 rpm's it momentarily goes up and then comes down to 30-35 and wavers there. Sounds like the low mileage (less than 100) oil pump has a problem. Anyone care to comment?

 

I would check the sender and the gauge. however after saying that it would depend on whether it was a early (direct pressure to gauge) oil line to gauge or electrical hook up to gauge. if i remember right you should have at least 10 lbs per 1000 rpm.

 

What oil are you running? Sounds like it could be losing oil internally. If it is a fresh rebuild it could be missing a cam galley plug under the timing cover. Could be the bypass in the new pump is defective-bad spring, trash, not sealing completely.

 

could the weight of the oil have anything to do with what the gauge is reading? my gauge too is acting funny.

 

The pump has <100 miles on it - is the motor freshly rebuilt , or did you just replace the pump for whatever reason , and now have a problem that didn't exist prior to the new pump?

 

Yes, oil thins as it heats up. You can have good pressure cold, but insufficient pressure hot if you have an internal leak. Internal leaks really only occur after a rebuild if the machine shop doesn't put certain plugs back in. I have not heard of a 100k motor suddenly spit out an oil galley plug.

 

Engine and oil pump both <100 miles..but some track time Oil pressure gauge (mechanical) was gunked up (would only go to 35 psi max), so I wasn't aware of this problem, although it was probably there all the while. I'm thinking the pick-up may be bent (if not welded), and sucking air.

 

I don't really have a guess, but you are usually answering questions like this rather than asking.

 

As long as I have been doing this (too long ), I still see problems pop up that are new to me. I am not a noobie at this, but I am always learning....

 

Pressure relief valve sticking?

 

It could also be that the relief valve is set for 30 psi when hot. The fluttering could be due to an instability at that 3500 rpm. If it goes to a steady 35 psi at higher revs, I'd bet it is the way the pump is set up.

 

Pulled the pan....many surprises. I always thought my 7 quart pan was really an 8 quart pan, it took at least 8 to the full level. Look at the photos. The dipstick tube BENT when the pan went on. Why, you ask?...the friggen trap door was welded in crooked, blocking the route for the oil dipstick! The dipstick hits the top of the trap door. So with no windage tray, I suspect the extra quart or more of oil was beign whipped into foam by the crankshaft, hence the low oil pressure above 3500 rpm's. The "FULL" mark is right at the bottom of the crank throw. At this point, I'll re-position the trap-door in the pan and leave the pump alone. The last pic. shows the oil pan with 7 quatrs of oil. I feel lucky...

 

If you didn't change oil and filter immediately after your initial break-in and adjustment run times , that's the first thing you should do. On a fresh rebuild , you will have some residual lint , dirt , and a bunch of assembly lube that will have the filter pretty clogged up. The bypass system is there to ensure that the engine gets unfiltered oil , rather than no oil at all. Regardless of oil viscosity , when the engine is cold started , the oil is at it's "thickest" state, and if it encounters too much flow resistance in the filter , it will open up the bypass to get the oil to the engine. Hence , the high cold reading.

Once engine and oil are @ normal operating temps , 15 psi idle is ok, 35 @ 2k is ok , but once rpm's go up, oil pressure doesn't climb upward with increases in rpm. This would indicate an obstruction to flow , since the gauge shoots up , then falls back and wavers. Oil can't freely flow through the filter , but the filter is flowing enough to keep the bypass closed. As an additional note here , it isn't oil pressure that lubricates and cools metal , it's the presence of a steady flow of oil that accomplishes that. As long as pressure is sufficient to keep a steady non-aerated flow of oil moving , it will carry the heat away. If it were pressure that did this , machine tools would use a fire hose to lubricate the metal being machined on them. ( My father was a tool & die maker , I got to sweep up the chips in my tender years. Guess what kinds of conversations we had , aside from my occasional griping about being a janitor, with the expected parental response to those remarks )

Since you didn't state that the gauge was giving you these kind of readings prior to the rebuild , and since the gauge was all gunked up , this would support the premise that the cold oil was bypassing the filter and reaching the gauge. And , gunked up or not , cold oil was giving you a 60 lb. reading , so that shows that the gauge was ( is ) capable of reading higher than 35 , assuming that oil flow could reach it. More support for the hypothesis.

Oil pressure is primarily a function of bearing clearance , and rod side clearance on the rod journals. Hot oil differences in viscosity will have small practical effect on oil pressure. The reverse corollary to this is building a motor with large clearances to keep the motor " loose & free-revving " , and using a 60 wgt. oil to " float the rods and crank on a thick film of oil ". Dream on , and keep those flashbacks comin'. The idea that a film of oil will keep will keep a rod end from wanting to continue in the direction it was travelling @ high velocity when the crank stops moving in the same direction and starts to reverse direction to push the rod in the next stroke direction is absurd.

For all the above reasons , that's why my first suspect is the filter being clogged. However , having said all this , if you have already changed the oil & filter , or if the problem remains after doing so , then I would move on to the following checks. ( It's also a good idea to use a magnetic drain plug , if you aren't already using one , btw. )

Verify the pressure readings using another gauge ( owned , begged , borrowed , but not stolen ) .

Check oil pickup distance to the bottom of the pan , and check it for parallel axis to the bottom of the pan. Also check the pickup screen for clogged debris in the mesh. I use a 1/4" pickup to pan clearance distance.

Also check , as you mentioned , that the pickup tube has been brazed to the pump body , once it's correct alignment position has been verified. Remember to remove the relief spring before brazing , or the spring will be junk. If the pickup tube has been brazed , and you aren't absolutely certain that the spring was removed during the brazing process , replace the spring as a prudent precaution. A spring that has been overheated during brazing will also cause intermittent erratic pressures before it fails completely.

Check that the oil pump body has been correctly located on the dowels it aligns to , and that there aren't any gaps between the pump and journal mating surfaces. Also check that the pump bolt has been torqued to spec ( 60 lbs. , IIRC ).

Last practical culprit could be excessive end play between the pump gears and pump cover, but I think this very unlikely. End play should be @ .002 to .0025.

Hope this will be of benefit to you. Sounds like you've got a strong powerplant that'll put a smile on your lips and a song in your heart.



David

 

Glad you found it , and it's quick & simple. Was that a brand new pan with a defect?

 

Thanks, David....this is about the 3rd oil / filter service. The oil gauge would never go above 35psi (hot or cold) before I rebuilt it, so I can't say if this problem has been around for a while....about 2 dozen 1/4 mile passes and about 100 miles of street use. The pump was brazed by my machine shop (Melling 55A). The pick-up is about 3/4 inch from the bottom of the pan. Any response to my theory of oil foaming causing low oil pressure at high rpm's?

 

Yep, thank you China....

 

( note my remarks in middle of paragraph 2 of my original post ). I'd be very surprised if your findings didn't solve the problem. I wish that I could say in these times that I was surprised that it was a defective new product. ( You would think that to be fair the price would be the same as it would sell for in China. )

This is exactly the kind of thing that prompted me to construct a new personal consumer mantra. Buy American , if you can find anything American that you can actually afford to buy. $$ =

 

I had low oil pressure at 3500 rpm's and above (35psi max). Pulled the pan, fixed the trap door which caused the dip-stick to bend, causing an over-fill condition (possible oil foaming). Decided to check the oil pump, along with the engine has <100 miles. Lookie here....Damage to pump plate. Needless to say, new pump installed. Next photo is warm idle oil pressure, next is 5,000 rpm oil pressure, last photo is a healty 383 again.

 

Glad you didn't hurt it! Man, don't any C3 clocks work? Mine was rebuilt 3 times in 5 years and I finally went to quartz. Went to the Bloomington Normal All Chevy show. There were 9 C3 cars including mine and none had working clocks except me.

 

Is that the reason they're referred to as being " timeless classics " ?