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Oil pressure sending unit question

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Rivercrest View Drop Down
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    Posted: 15-October-2020 at 4:19AM
On my 1972 Cleveland engine, is there any relationship between the rear engine oil pressure port ( and the sending unit ) and manifold intake vacuum. I am chasing down a small intake manifold vacuum leak and just wondering about any possible relationship between the two. Currently I have a mechanical gauge connected with a clear plastic tube. the tube has both oil and air pockets in it and the oil pressure reading is good. Any advice would be appreciated, Thanks.
Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72 RS 351 Quote  Post ReplyReply Direct Link To This Post Posted: 15-October-2020 at 4:36AM
Hi Bob, there's no relationship between the oiling system and the intake vacuum. That rear oil port does connect to the oil gallery. It's a convenient port that was done away with in later Ford engines. Old racers used to route an oil line from the front port to the rear, to more directly get oil to the mains at the rear on Clevelands.

Check your vacuum hose for age, how hard or brittle they are. They can shrink over time from the heat, and end up with a loose connection at the PCV for example. Check the rubber grommet on the booster for the large feed hose, those can get a bit loose also.
Don
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rivercrest Quote  Post ReplyReply Direct Link To This Post Posted: 15-October-2020 at 5:26AM
I've pretty well narrowed the leak down to the very back of the intake manifold, spraying that area with starter fluid results in a leveling out/increase of the RPM's. Gotta be sure though as when the spray get into the carb horn area this also causes the RPM's to go up. Just trying to cover all bases first before I pull off the intake manifold. Thus the previous oil question. Engine is older and runs great but I realize gaskets, like me, can get old and brittle. LOL BTW, I have replaced every vac line, PCV valve and valve cover grommet. I have swapped out the carb and I have also disconnected and plugged the main vacuum connection at the carb base thus isolating the engine only as a source for the vacuum leak. Thanks for your input.

Edited by Rivercrest - 15-October-2020 at 5:35AM
Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72 RS 351 Quote  Post ReplyReply Direct Link To This Post Posted: 15-October-2020 at 8:32AM
If you think it might be the back edge of the intake, the strip of cork section, that you could seal with Ultra Black RTV. I have repaired or sealed many leaks along places like that and the oil pan etc.

I had the whole piece missing there in my old 85 Crown Vic, 351W. It was blown out from blow by, serious old police car wear, oil in the air cleaner through the breather/PCV system. I cleaned that area really well, lots of brake cleaner on paper towels, and a rag. I sealed it with Ultra Black and it held. I also had to help the blow by, with a higher flowing PCV, and rerouted the PCV to pull intake vacuum on the crankcase. It stopped the oil in the air filter, and the intake didn't leak again.

So if you can figure out where the vacuum leak is, it may not require pulling the intake. In the corners to the ports would be tougher to fix, the gasket may be leaking to the crankcase. There could be more than one leak too. Clean it up and hopefully you can fix it easily.
Don
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Post Options Post Options   Thanks (0) Thanks(0)   Quote californiajohnny Quote  Post ReplyReply Direct Link To This Post Posted: 15-October-2020 at 4:38PM
air pockets in the clear gauge line are normal! you can bleed it at the gauge initially but they come back as the engine oil drains back after shutdown, and doesn't effect anything!Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rivercrest Quote  Post ReplyReply Direct Link To This Post Posted: 15-October-2020 at 4:58PM
Thanks Don for the RTV advice, as I approach this leak problem today I made note that the leak "appears" to become less as the engine warms up. I am going to spend some time trying to narrow down the leak as of now I am still not sure where along the rear manifold the culprit area is. I'll keep at it......
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72 RS 351 Quote  Post ReplyReply Direct Link To This Post Posted: 16-October-2020 at 12:35AM
Also, how old is that plastic oil sensor line? John is right to not worry about the air in the line. But ideally you don't want that to be plastic and going into the interior or dash.

The bad from having that line crack or blow is very bad, the mess alone makes people swear to never do that again. I'd see about replacing it with a copper line made for the oil pressure gauge. That's a rare subject because not many mechanical oil gauges are sold now compared to 40 years ago. That used to be a common subject, but most people have gone to electrical gauges for the oil.
Don
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73 Ranchero Sport "72 front end", floor shift/console, planning EFI 351-3V &4R70W
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Big Bird Quote  Post ReplyReply Direct Link To This Post Posted: 16-October-2020 at 12:15PM
copper tubing was the old standard, with a loop/coil between the engine and the firewall to keep the line from flexing/hardening/breaking. If putting in a mechanical gauge, consider adding a "T" and keeping the idiot light as well.
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