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Discussion Starter · #1 ·
I just had a new 350 built for my 69,it's running but not on the street yet.It's only been run 15-20 times for tuning purposes.Why would the oil pressure suddenly drop after the last couple of times? When the motor was first broken in the oil press. was around 40-45.Just recently it dropped to 10 psi and now every time I run it ,it starts out at around 55-60 psi and gradually drops as the car sits and idles until it gets to around 15-18 psi.It has a standard pan and a high flow pump but why would it suddenly drop after being much higher after those first 10 or so start-ups?I've checked the gauge,changed the oil and not sure what to try next.I really need some help here.Everything on this motor is brand new except the crank and was built by a very experienced and knowledgeable engine builder so I know it was assembled the right way.
 
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There is not any single answer to your question. It could be any one of five or six things, so are easy to correct others a a major pain.

The check valve could be sticking in the oil pump, not to hard to deal with, but on a chevelle the oil pan is a problem.

The high volume pumps tend to pump the pan dry, putting lots of oil top side in the motor, could cause low pressure or pump cavatating. Not likely at low rpm.

It goes on. There is a popular myth that excessive bearing clearence causes low oil pressure. Maybe a pound or two due to bearing bleed but not any radical numbers.

I would look at the pump, and if you go in the pan, change it to a standard Mellings pump for a small block, not the high volume or high pressure unit, you don't need it.

I don't think you need due anything as long as the pressure does not change any more.

Put a good mechanical gauge on it and check it for pressure.
 

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When you get the rpm up above 2000 or so, does the press go back up to 40-45? Cold oil pressure will be hi and as the oil heats up, it will drop. But once warmed up it should be constant, depending on pump, 35-45 for stock pump, 60-70 for hi pressure pump above 2000rpm. After warmed up, at idle, it will be low, that's normal.

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The only other thing I can think of that may cause a drastic change might be the oil pump pickup screen vibrating off. Did you tack weld the pickup?

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Steve Strasemeier (70SS)
 

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In Wallys reply he stated that their is a myth about bearing clearance causeing low oil pressure. I have about the same problem with my '66 396. I have no idea how many miles are on the engine but when I start it up I have good oil pressure, about 60psi. But when it gets to operating temp. it will literally drop to zero. I plumbed in a new mech. gauge and got the same result. I thought it was the engine was just worn out but maybe I have just a pump problem? Any one have any ideas? Oh yeah, when I rev the motor up oil pressure comes right up to about 40 or so.
 

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In Wallys reply he stated that their is a myth about bearing clearance causeing low oil pressure. I have about the same problem with my '66 396. I have no idea how many miles are on the engine but when I start it up I have good oil pressure, about 60psi. But when it gets to operating temp. it will literally drop to zero. I plumbed in a new mech. gauge and got the same result. I thought it was the engine was just worn out but maybe I have just a pump problem? Any one have any ideas? Oh yeah, when I rev the motor up oil pressure comes right up to about 40 or so.
 

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In Wallys reply he stated that their is a myth about bearing clearance causeing low oil pressure. I have about the same problem with my '66 396. I have no idea how many miles are on the engine but when I start it up I have good oil pressure, about 60psi. But when it gets to operating temp. it will literally drop to zero. I plumbed in a new mech. gauge and got the same result. I thought it was the engine was just worn out but maybe I have just a pump problem? Any one have any ideas? Oh yeah, when I rev the motor up oil pressure comes right up to about 40 or so.
 

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In Wallys reply he stated that their is a myth about bearing clearance causeing low oil pressure. I have about the same problem with my '66 396. I have no idea how many miles are on the engine but when I start it up I have good oil pressure, about 60psi. But when it gets to operating temp. it will literally drop to zero. I plumbed in a new mech. gauge and got the same result. I thought it was the engine was just worn out but maybe I have just a pump problem? Any one have any ideas? Oh yeah, when I rev the motor up oil pressure comes right up to about 40 or so.
 

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First I would change the oil. If while tuning you fluded the motor to allow gas in the oil it will dilute the oil "thin it" and cause low pressure. What wieght oil are you using? Is the assembly lube still in the oil?
 

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Discussion Starter · #10 ·
I already changed the oil after break-in and then I had problems with the carb .After the carb was fixed I did change the oil again because it was full of gas.Ever since the car was first broke-in I never had the car run for too long because the carb was messed up (blown power valve) Once the carb was fixed and tuned I could let It idle for a longer period of time and notice the oil pressure drop. I finally got ahold of the guy who built my motor and he said as long as the pressure goes up after giving it a little gas and it does'nt drop then internally it's ok, but just go to a heavier weight oil(right now I'm using castrol straight 30 and I'm going to change to straight 40.I let the car idle today for about 20 min. At start-up I had 55-60 psi and after it warmed up It steadily dropped to about15 psi and stayed so I think It'll be ok.
 

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Ask your machinist what the mains and rods are..030 ect. that sounds like a loose motor with a stock pump. Ive had target motors from GM with that pressure.Possibly silicone or something got into the pump.
 

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Hi guys,
Wally is right on the money about oil pressure not being related to bearing clearence. If the bearings had worn enough to cause an oil pressure problem, they would be into the copper and causing a whole lot of more serious problems. More than likely if the oil pressure is going down to near 0 at idle, something else is wrong. A stuck relief valve in the pump will do it, also on a Small Block like rancon's one of the front oil galley plugs could have come out and fallen into the pan. This will cause the same problem. The size of the bearings (10 - 20 -30 )has absolutely no effect on the oil pressure unless you have the wrong size bearings in the motor. I had a friend build a 302 Ford once and put .010 bearings on a crank that was ground .020 and beleive it or not, it had great oil pressure when cold but dropped to about 10 pounds hot, so this is another possibility.
By the way rancon, was your crankshaft reground ? If so ask your engine builder what the clearences were when he assembled the motor. It would be interesting to hear the answer. I just finished a 350 motor for a 85 MonteCarlo SS and it has 70lbs cold and 40 hot at idle, this is with a stock pump. My 85 Blazer still had 30 lbs hot idle oil pressure after 135,000 miles. Our shop S-10 with a 2.8 has 170K miles on it and has never been touched, it runs 30 lbs hot at idle with 5w30 Mobile 1. I would not accept anything less for your motor. You should not have to run heavier oil to get correct oil pressure. 10w30 should give you at least 30lbs hot idle oil pressure with a stock pump, even more with a high voloume or high pressure pump. I would insist on it being fixed now rather than have a problem later !

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Bill Koustenis
Advanced Automotive Machine
Waldorf Md
 

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I have a fresh rebuild 396 in a 67 elky, less than
5k miles. There is constant 40=50psi of OIL hot or cold.
I noticed that their is no oil getting up to the rocker arms. I recall when installed, tried setting valves running, it turned into a mess.
Now there is no oil @ rockers, I need some posts as to what the proper fix is short of a tear down. Is there gallies pluged if so whats a fix?

Team Techs,
Give some answers, PLEASE!
 
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