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Talk me off the ledge - Oil Pressure on new rebuild


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After 6 months I was finally able to crank and fire my newly rebuilt (by me) motor. Unfortunately I am only seeing about 30-40psi cold idle (which takes some cranking to build). I didn't let it warm up as this seems way to low to me. Before the rebuild on a tired motor I'd still see 90 cold. Some thoughts:


1.) Oil pump has assembly lube in it, preventing it from flowing fully. Once the oil warms up and removes the assembly lube, the problem might resolve.

2.) I put a new backing plate on the oil pump. From everything I've read, this should have increased psi, not decreased.

3.) Main bearings were at upper end of limits per plastigage. (approx .060mm)

4.) Rod bearings were spot in the middle.

5.) I double checked the o-ring in the tube-oil pump connection. However, this was a different block than my original. Is it possible the feed tube is a slightly different length causing leak by? 

6.) I did not check the feed tube-block connection as it was already installed and "marked" by a reputable builder.


What would you do? Is it worth warming it up to see if the pressure holds? Or do I cut my losses now and drop the engine again?


"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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We will assume you know your gauge is accurate. 30-40 psi is more than enough pressure to run the engine long enough to get it up to full temperature. With no load on it anything over 15 is more than enough to be safe. What oil are you using? The possible answers to low pressure are: excessively loose clearances ( this could be at the main bearings, rod bearings, cam bearings in the head). A oil plug in one of the rocker arm shafts could be loose or missing.  The oil pump relief valve could be stuck part way open.  The oil pump could be worn. The oil pressure tube leading to the pressure relief valve could be leaking at the block or at the pump.  


You want 55-60 psi at 3000 rpm and above with warm oil and engine at full running temprature. 

1970 1602 (purchased 12/1974)

1974 2002 Turbo

1988 M5

1986 Euro 325iC

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I'm running 20w50 VR1 Dino with a break-in additive. I'm with you on the overall #'s, I've just never seen an 02 start up that low. I'm guessing (hoping) that it's just the Orings. The head was done by Terry Tinney, so I'd highly doubt a misstep there. I'll warm it up tomorrow under no load and see what happens.

"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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Since I rebuilt my 73's engine back in 2004, I've been using synthetic oil (except for the first 1000 or so break-in miles), either 20w-50 or 15w-50, and I've noticed that my oil pressure has been consistently about 10 psi lower at highway speeds with the synthetic oil than it was with the old worn engine (204k miles) using dino oil.  And this with a new oil pump, pressure relief valve and practically everything internal in the engine new.  I just attributed the lower pressure to the synthetic oil.  Am I wrong?  I still have 25-30 psi at hot idle and 45-60 psi at 3k rpm, so I'm not too worried...or should I be?



'69 Nevada sunroof-Wolfgang-bought new
'73 Sahara sunroof-Ludwig-since '78
'91 Brillantrot 318is sunroof-Georg Friederich 
Fiat Topolini (Benito & Luigi), Renault 4CVs (Anatole, Lucky Pierre, Brigette) & Kermit, the Bugeye Sprite

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+1 on calibrate your gauge.


To add to what Byron says, pull the valve cover and make sure you have oil on top and that it's oiling

the cam.

30 psi, cold, if you're getting flow everywhere,  is, I agree, plenty for an unloaded motor.  It's not 

RIGHT, but it's fine.


What you may find is that the spring in the bypass is a bit short, and that pressure jumps up to 60 at 2k, cold.

Or any of a number of things.  But if it's oiling the top (and yes, you can run it with the valve cover off, it's not

all THAT messy at idle) then I'd warm it up (at 2500 rpm if it's a new/reground cam) by varying a high idle speed,

on and off throttle to help the rings clear/seat, and see what it holds at hot idle.  Burp the coolant, etc, while

you're doing it.


Then see how things are.


As long as it's flowing oil at 30 psi, nothing bad will happen.  It can idle at 10 psi, too.  Don't freak yet.







"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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You might double-check the bolt at the center of the spray bar; there should be an aluminum crush washer between the head and spray bar, and another between the spray bar and bolt head). For folks in Southern California (generally), I offer an M10 test / heat cycle service.


Also: on a fresh build, I pull the spark plugs and turn the engine over with the starter until it pumps pressure (nothing secret there, just a note for those going forward). Even without a gauge, you can usually hear the oil start squirting through the spray bar with the oil filler cap removed. -KB


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While i have much bigger problems, the oil pressure stabilized at 15psi hot, raising to 45psi at 3000rpm. The pressure also bled off very rapidly once the engine shut off, so I think I've got an issue with the pressure relief or o-rings.  My head had to come off due to other issues, so I'll be able to dig into it further.

"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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