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Old 06-09-2006, 10:17 PM
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Chevy 235-6 cyl Oil Presure Fluctuations

Hi Guys, Any help here will be appreciated! Last month I installed a rebuilt head onto my '55 model 235-6 cyl (in my '48 Chevy Pick Up) to get hardened seats, new valves, springs, etc., It runs great with more power and no more lead additives to put into my gas-Yeah! But last week I noticed my oil pressure gauge, which normally runs full up at 30 psi when under power and dropped to 15 psi when at idle at operating temp ( with no pulsing)-NOW it will drop from 30 down to about 20, go back up in about 4-5 seconds. At full stop, it will drop to almost 0 and then move up to about 15 and pulse down to 10psi and back up to 15. I am use to no change except when I idle but the needle keeps dancing and it drives me nuts! I hate it when something is not right-so any ideas on what or where to check-throw them my way! Thanks!!

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Old 06-09-2006, 10:47 PM
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Did you remove the oil pressure line at any time? Sounds like an air bubble in the line. It's nearly impossible to get all the air out of the line, but you can bleed it by loosening the fitting behind the gauge just after shutting the engine off. You can do it with engine idling, but will get more oil out that way... and all over everything. Use a rag to catch the oil. Air could have been trapped in the block oil passages when you took the head off and made it's way to the line. If it's an electric gauge, suspect the sending unit.

If that's not the problem, you need to double check the gauge. A portable air tank with about 20-30 psi in it will work, or try another gauge. I'd give it 200 miles or so to work any air out before I started suspecting oil pump issues. Then I'd change the oil and use an engine flush (or replace a quart of oil with trans fluid or diesel oil, just don't operate the car with the mixture -- fast idle is enough -- diesel is the main ingredient in Gunk Engine Flush, by the way) and change the oil before suspecting the pump or bypass valve.
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Old 06-09-2006, 11:35 PM
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Thanks for the info. I did take the oil line loose. It is the original mechanical guage so it may well be an air bubble. I will try the bleeding first and see what happens. I had thought about the flush also as I had to scrape gaskets and stuff so I may have some trash in the pickup screen, but I drained the pan and put new filter on after the head change, so I hoped that would take care of any trash. Thanks for the info!! Dan
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Did you reuse your old rocker arm shaft? Those things get pretty worn over the years on 235s (mine was worn almost 1/16" when I tore down my engine for a rebuild!) and can be a major source of pressure leakage.
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