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Old 01-04-2004, 08:37 PM
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Low oil pressure at idle on rebuilt 350

I have a rebuilt 350 with a mild cam, Edelbrock 650 cfm carb, Powerhouse intake and a noisey gear drive. I am using Dakota Digital Gauges. I also have a high volume oil pump. Here is my problem, at idle the oil pressure drops between 12-17 psi and a rattling noise occurs. As soon as I begin to drive the pressure will go back up to 27-39 psi. It will remain constant along with the constant speed. I have followed the proper break-in procedures. The motor has approximately 42 miles on it now. As long as the rpm remains above 900 the pressure doesn't drop and the noise does not occur. Can someone tell me what is going on? Can someone also tell me what the idle rpm should be? I would also like to know what the oil pressure should be at? This is my first project and I am having a darn time figuring this thing out. The motor runs and sounds smooth, doesn't smoke, burn oil or do anything out if the norm with the exception of the low oil pressure if the rpm falls too low. Thanx guys.

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Old 01-04-2004, 08:50 PM
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Did you check the pickup depth? Was it 1/4" off of the pan? What kind of oil are you using?
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Old 01-04-2004, 09:00 PM
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Stock type high volume oil pumps are only rated at 25 PSI hot idle and 55 or so off idle.

750 in gear sounds like a good idle speed for a very mild cam.


Please elaborate on the noise you hear. Does it come from high, low, side to side?
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Old 01-04-2004, 09:41 PM
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most gauges aren`t accurate below 10 psi, some as much as 15. I run a high volume oil pump in my small block, at idle when it`s hot, the gauge reads zero, but at 650 rpm idle speed, that`s not turning very fast, and the cam is going half the speed of the crank, so that puts it to 325 rpm, so it`s going to be low at idle, as long as there isn`t any death knocks or loud rattles, I don`t worry about it.
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Old 01-05-2004, 08:33 PM
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First I want to thank you guys for your responses. I am using Castrol 10w30. I believe the pick-up tube is within the correct distance. The noise I hear sounds like it's coming from the bottom half. The noise almost sounds like a deep pinging noise, like going up a steep hill. As long as I keep the idle adjusted the noise will not occur and the oil pressure will stay above 12psi. I was wondering if a regular stock oil pump would make a difference or maybe a high pressure one. I had even thought about using 20w50 oil. What do you guys think. Or should I not worry about it? Thanks again.
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Old 01-05-2004, 08:53 PM
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Let it break in some more. Just baby it around for a couple hundred miles more. Maybe that noise will quiet down some. I don't know why your oil pressure is that low using the info you have given. As long as you have 10 psi. for every 1,000 RPM you are okay. You could use thicker oil and the pressure will go up some. Try some 10W 40 next oil change.

Oh, speaking of which, did you change your oil after intial break in. That could be your problem. The assembly lube etc. could have clogged the oil filter enough for that to happen.
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Old 01-05-2004, 09:02 PM
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Wow Palmtree.... I was just there and the fellas can 2nd that!

My high volume oil pump scared the hell out of me too, but like double said AND my machinist...as long as the pressure goes up when you step on the gas, your good. Nuff said.

Your noise. Make sure your timing is right. I had a good rattle and decent knock. My problem was a too small carb with my initial timing too far advanced and the rocker arms needed adjusted.

I hope that helps.

Using 20-50 oil helps a little...not much but it helps with the oil pressure.
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Old 01-05-2004, 09:35 PM
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12 lbs at idle? spinning it with a speed handle should have more than that .All the new syle engines only use 5w30 and prostocks use 0w oil. In all my old style engines i keep the mains at .002 to .0025 and the rods at .0015 to .002 max. What were the bearing clearances? A volume pump should still have a normal bypass spring in it so it still should have 30 to 45 lbs idle pressure. U might check the bypass in the oil pump and see if it is hung up with a burr or not. I still like to see at least 15w40 or 20w50 in an old style engine. I would pull the pan before u have a bearing go away and at least confirm the cond of the mains and rods. if u say u have a noise down their,even more reason. what brand of pump is it? Hope u find ur trouble.
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Old 01-06-2004, 01:22 AM
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Al tho its never happened to me personally,its possible to blow out a lifter galley plug fully or partially if the penning doesn't hold behind the timing chain. I always tap em out and use thread in ones like big block Chev's when bumping my pressure up,just a thought. Ive also seen sending units go buzerk from the heightened pressure quite often.
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Old 01-06-2004, 05:10 AM
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Let it Be

You should probably not worry too much about the low oil pressure, GM V8s in general have a tendency to run a little on the low side at idle. 10 lbs for every 1000 RPM is safe. Your noise probably isn't due to the oil pressure. 20-50 oil might help, might not.
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Old 01-06-2004, 09:30 AM
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Mine run about 35 depending on how hot I run the thermostat at 750 with 15/40 oil,sounds low to me,coupled with noise Id be nosing around. Bypass valve,bearings,and galley plugs would be my first stops,easiest first. Maybe put a different gauge on to be sure,maybe pour an extra quart of oil in temporarily to check if its a pickup issue.
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Old 01-06-2004, 09:31 AM
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I would be leery about using a high pressure pump. We used one on a 350 rebuild. During cam break-in, at 2000 rpm, we had about 80# of pressure!! It caused more problems than it was worth, in terms of leaks, etc, and I finally put the standard spring in.
I would say 15#-20# pounds at idle is ok. As stated above a lot depends on the quality of the gauge.
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Old 01-06-2004, 03:38 PM
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MY Dad has the same problem right now. Driving pressure is fine but at idle there is little to none. So we dropped the pan to find the mains going bad, making the noise. Come to find out it was a bad pump. I suggest pulling the pan before anything else.
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Old 01-06-2004, 03:57 PM
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Could be due to high main bearing clearances. Even if higher than normal clearance is causing the lower oil pressure, hard to say that it would be causing the noise w/o hearing it. Another possibility that I would check on (briefly mentioned in a previous reply) would be valve adjustment. If valves seem to be in adjustment, you could have a "leaky" lifter that is loosing it's prime with the low oil pressure at idle and starts ticking, the pumps up fine when oil pressure increases. Pull the valve covers off (careful, this could get messy) and see if the noise gets louder and/or if you can isolate a particular noisy rocker arm. And, to verify oil pressure, I would pop a live guage on it temporarily and see what it reads. Electrical sending units can be... finicky... at times.

n2o_racer
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Old 01-06-2004, 09:12 PM
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I want to thank all you guys for such great information. I am a bit new to this stuff but you guys have shed a lot of light on me, again I thank each and every one of you. I now have some things that I will look at this coming weekend. Thanks guys, this type of support is what makes it all worth it.

Oh by the way, the motor is installed in a 57 Chevy pick-up which has been mounted on a 73 Chevy truck chassis. It also has a TH350 and four wheel disc brakes. The truck has been lowered 5 inches in the front and 7 inches in the rear. The doors and the front bumper have been shaved. The building of my truck has been fun up until the engine problems. Thanks to all you guys I hope to get back to the fun stuff again soon.
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