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Old 07-17-2012, 06:20 PM
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350 chevy, oil pressure drops to zero when warm

i just rebuilt a 350 chevy its bored 30 over everything is new,oil pressure drops to 0 at idle when warm,any ideas .

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Old 07-17-2012, 09:09 PM
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Did you pin or tack weld the oil pickup to the pump? What's the idle pressure at startup? At cruise?
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Old 07-17-2012, 09:25 PM
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Main bearing clearance? Cam bearing clearance?




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Old 07-18-2012, 09:12 PM
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hotrodthug

yes i welded pick up,i checked main bearing clearence,i thought i did everything right.i have built a lot of small blocks,thanks.
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Old 07-18-2012, 09:18 PM
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Is the pipe plug under the rear main in place?
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Old 07-19-2012, 09:39 AM
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Quote:
Originally Posted by hotrodthug View Post
i just rebuilt a 350 chevy its bored 30 over everything is new,oil pressure drops to 0 at idle when warm,any ideas .
What does the pressure do at other RPMs and temps?

What weight oil are you using?

Have you used a known "GOOD" gauge to back up the data you're seeing? A corollary to that is; if it's an electric gauge have you verified the sender?

Bogie
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Old 07-19-2012, 10:33 AM
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Seems to be a common problem today- no oil pressure at idle
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Old 07-19-2012, 02:00 PM
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It's common when they say 'I checked bearing clearance, the oil pump is new, the pickup is at a proper height, replace the oil filter, etc etc etc." The last thing people forget to check is the distributor for proper oil sealing.
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Old 07-19-2012, 04:04 PM
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try pulling dist. and check end play on dist shaft or if there is any way for dist to "wobble" (lack of vocab.on my part). its happened to me once, ruined a good motor because of a bad dist.
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Old 07-19-2012, 04:28 PM
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The last motor I had that had that happen to it needed a new crank, all new bearings, and a new cam, which went flat, sending metal through everything ruining it.

Not saying that happened to you, hopefully it didn't, cause I was a dummy when I built that one. But, I would pull the pan and recheck clearances of your main bearings. And, what weight oil are you running?
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Old 07-19-2012, 05:08 PM
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oil pressure 40 pnds cold idle,hot 0 when rpms go up oil pressure goes up,35pnds 3000 rpms.using 10w40valvolen,ac filter.
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Old 07-19-2012, 05:39 PM
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Quote:
Originally Posted by hotrodthug View Post
oil pressure 40 pnds cold idle,hot 0 when rpms go up oil pressure goes up,35pnds 3000 rpms.using 10w40valvolen,ac filter.
What is the gauge electric or mechanical? Have you checked against another gauge preferably a mechanical but if electric also with a different sender?

The pressure is about 20 pounds under where I'd expect it to be in both cases of cold idle at 40, where I'd expect 60-65 and at hot cruise at 3000 holding 35 where I'd expect 55-60 with the 10w-40 in there.

My first blush at these numbers makes me suspicious of the gauge or its sender.

When you brazed the pickup on to the pump casting did you take the by-pass spring out? That's my number 2 pick that the spring has weakened or the shuttle valve is caught in a partially open position.

Going from easy to hard, I'd check out the gauge/sender first.

As an aside since it keeps popping up and is in your set of respondentís replies let me say that about a billion years ago one of the hot rod magazines published an article that said "if the oil passage restriction under the rear main was omitted... that the pressure in the engine would be reduced". This has developed a life of its own for decades now and pops up from people who probably actually know better. The restriction is either a ball or small diameter "freeze" plug that stops oil from continuing up the rear drilling to the main galley without passing through the filter. The original SBC did not have a filter (go figure) and this passage connected the pump's output to the main galley (the big object down the center of the valley floor for folks that need orientaion on this). When a filter was added to the side of the block; a lower side passage was provided to move oil to the filter while an upper return passage from the filter to the rising passage was made to return cleaned oil to the riser drilling. Between these two side drillings that intersect the riser drilling was placed a plug to force oil to the filter and return the clean oil. If the plug is left out, the filter will pressurize but not circulate so there would be no filtered oil but pressure and volume would be unaffected except for whatever flow resistance the filter provides. I suspect the original article intended to say that if the plug was driven too deeply into the passage it could cut off some or all the flow from the filter to the riser passage thus reducing oil pressure in the engine. This is not on my suspects list, just mumbling out loud.

Bogie
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Old 07-19-2012, 05:49 PM
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The diagram of the plug and measurements and its location is from HERE. By examining the diagram, you can see that the oil pressure shouldn't be lost if the plug is left out.

If the plug is driven in too far (blue circle), that's a different story.

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Old 07-19-2012, 06:02 PM
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The diagram of the plug and measurements and its location is from HERE. By examining the diagram, you can see that the oil pressure shouldn't be lost if the plug is left out.

If the plug is driven in too far (blue circle), that's a different story.

Great picture worth at least a thousand words maybe more, thanks.

Bogie
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Old 07-20-2012, 07:22 AM
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You have make sure the bottom part distributor housing itself as a proper oil seal, not just the shaft.
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