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Old 09-29-2003, 11:09 AM
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Oil pressure drop under load

Guys:
My son and I did a fresh rebuild to a Chevy 350 for his '67 Chevy 2. We used a Moroso 6 qt pan with matching pickup, and a stock type Chevy hi pressure oil pump. Under normal conditions, it idled at 18 psi and ran at 45-50 psi. Under heavy load, the oil pressure dropped from 50 to around 30 psi. So we took the shortblock to a reputable engine machine shop for analysis. They told (and showed us) that the thrust bearing surfaces were essentially eaten up, and said that was the source of the problem. They told us that the transmission was likely the cause of the problem, exerting forward pressure on the thrust surfaces of the crank. So we changed from a powerglide to a turbo 350 just in case. We had THEM do a rebuild for us this time. They bored it .030 (It was a brand new block) put in a new forged crank, checked align bore, new rods, pistons, and rings. We got it running Saturday, took it out on the road, only to find out that IT HAS THE SAME EXACT PROBLEM!!! Only this time, the idle pressure is better, (27 psi) and it runs at about 58 psi. But under full load, the pressure drops to near 40 psi at approx 4500 rpm.
Has anybody on this board actually had this problem? I've posted before, and got many well-meaning responses and suggestions (many I tried, none worked). I need a response from someone who has actually experienced and fixed this problem!
Please help. We now have nearly $10,000 invested in this engine, are going broke, and it STILL has problems.
Thanks
Tom

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Old 09-29-2003, 01:52 PM
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Pump cavitation or restriction in the oil return control. You didn't specify wether the oil return holes were enlarged or if the machine shop even bothered to tell you not to use a high pressure pump. With out the proper oil returns in the heads and block the pump can suck the pan dry and you will lose pressure. You can swap pumps yourself and save money. If the machine shop won't help you through this matter than it isn't a reputable one. What heads do this engine have?
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Old 09-29-2003, 02:08 PM
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Johnson:
Thanks for your reply. The pump we are currently using is a new stock GM pump for a 69 Z28 Camaro. It's a standard volume pump with a bypass spring set at approx 60 psi. We're also using a Moroso 6 qt. pan with matching pickup, Dart Iron Eagle heads. The machine shop inspected the oil return galleys and told us they were large enough, so I don't think we're sucking the pan dry. We're not using a windage tray, and the pan's not baffled. But there's a lot of guys out there that don't use windage trays or baffled oil pans that don't have this problem. That's why I'm confused. We've ordered a windage tray from Summit, but I don't think that's gonna help. It may be that the pickup is too close to the pan, but we measured nearly 3/8" clearance. I thought that would be enough. I'm open to any ideas.
Thanks
Tom
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Old 09-29-2003, 02:16 PM
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Have you tryed a diffrent pump?
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Old 09-29-2003, 02:21 PM
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Yes. We've tried 2 new pumps. Both with the same result.
Tom
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Old 09-29-2003, 03:22 PM
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Hmmmmmmm
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Old 09-29-2003, 03:34 PM
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Hi.

If the pressure drops when the engine is under load, I would suspect excessive main bearing clearance. I always test for this by driving the vehicle at highway speed (just light throttle) just cruising. Then accelerate hard for a few seconds. For example, if the speed is about 55 at 2800 rpm with little load and the oil pressure is 40 lbs, hard load and the pressure drops significantly at the same relative rpm I would suspect main brg clearance being somewhat high.

My theory for this is that on most chevy engines the upper half of the main brg is groved, the lower half is not. When the load is applied it attemts to push the crankshaft away from the upper brg and creates a major oil pressure leak. When the load is removed the crank returns and the leak is reduced.

Just some thoughts
Jeff
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Old 01-11-2004, 06:25 AM
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oil pres.

Exactly how do you measure pick up clearence considering its take with the oil pan and pump all installed. ( i imagine)

Im having a similar prob. no dropping under load but just all around low... idle about 20 maybe get 45 if im lucky throught the range. 85 fullsize blazer 350/700R4

any reasons why I should or should not be alarmed about htis? thank

R4X4R
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Old 01-11-2004, 11:43 AM
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Your crank may be stirring up the oil in the pan and airating the oil. If this happens it will cause oil pressure drop esp at higher rpm. Try running the engine down 1 qt of oil and see what happens. This wont hurt your engine as your still running with 5 qt. You are using a mechanical guage I hope.

Steve

Last edited by Steve karch; 01-11-2004 at 11:53 AM.
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Old 01-13-2004, 02:26 PM
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hey steve

what do you mean mechanical guage?

its an autometer someone before me put on there. with a plastic line to a hole behind the intake.
mechanical as in that or the plug that goes in the block and turns the dummy light on? i assume the one like I have.

im going to blow the line out this weekend when I got time to turn on the air, and hopefully put the copper line on that pos.

just noticed the front of the oil pan did not seal right, has a slo leak under pressure.

thanks for thought. r4r

Last edited by rally4x4racer; 01-13-2004 at 02:37 PM.
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