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I just bought this machine a few days ago. It has a small oil leak. It sat for a few years. When I first started it, it had low oil pressure. I added oil and left the barn I found it in. When I was about 10 miles away the gauge went all the way low. I kept trucking. Made it home. Just started tinkering with it today. Started it again and the oil pressue was in the normal range. Took a trip around the farm and it went low again. I'm not a mechanic. Maybe should not have bought a toy but I'm sticking with it. What do ya think. The thermostat gauge is not working either. I do know that It is pumping water. Looks like the sensor is not there anymore. I think i found the oil sensor behind the carb and it looks like the wire is intact.
 

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you probably have 8 valve cover bolts.

and I bet thats a 351M

and I bet the rods and main bearings are worn

and I bed thats where your loosing oil pressure

and I bet 20w50 might help it a little unless the previous owner put that in there already.
 

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Put a real gauge in. I had a factory gauge on a 300-6 that read fine until the engine warmed up- then it would plummet. Real gauge showed I had good pressure all across.

It was the factory sender going bad once IT warmed up.

Probably don't even have to buy the gauge right off, i can practically guarantee you have a buddy with one hanging around that he'd let you put in your block for a 15 minute test.
 

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I had the same problem in my '73 Ranchero with a 351c, I replaced the oil sending unit at the back of the block {same set-up on the 351m and 400 motors} all was fixed.... I'm not one to let well enough alone, I used a tail pipe cutter to open up the sender and it had a white corrosive lookin' powder on the mechanism that was keeping it from making contact internally. And NO it is NOT an on/off switch !
 

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you probably have 8 valve cover bolts.

and I bet thats a 351M

and I bet the rods and main bearings are worn

and I bed thats where your loosing oil pressure

and I bet 20w50 might help it a little unless the previous owner put that in there already.
I'm with you. That was the story on my 94 when i bought it. Probably why i got it so cheap too...
 

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another vote for mechanical gauge, and funky sensor

AND another vote for this engine has been running since Reagan was president and its plum wore out
 

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added or changed oil

first thing if it has been sittin for a while, CHANGE the oil, or drop the pan and check strainer on the pump. i've seen um run with good pressure and all the sudden pressure drops, ...:smilie_slapSTRAINER
 

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x2. The strainer can easily get clogged with crap that has entered the motor one way or the other. Last year I pulled 2 351M apart from different sources and both of them had clogged strainers from somebody using too much RTV. Also change out the sending unit for a good one, it's a possibility.
 
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