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Old 12-30-2002, 06:01 PM
maynard2 maynard2 is offline
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Post Good oil pressure when cool, drops to zero when hot?

[updated:LAST EDITED ON 30-Dec-02 AT 07:02 PM (EST)]I've just recently rebuilt my 351M, the main crank journals, clearances are .002, connecting rod journals .0175, new cam and cam bearings, lifters. New high volume oil pump. Any thoughts as to why I can't maintain my oil pressure when the engine is hot? I don't think the problem is in the bottom end, thanks for any suggestions.
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Old 12-30-2002, 06:10 PM
fordlowboy fordlowboy is offline
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Good oil pressure when cool, drops to zero when hot?

hi
when i first read you title of your question i was automaticly thinking lower end but you rebuilt it. what weight oil are you using and wwhat temp is the engine at when its (hot)? it really shouldn't matter the weight of oil when it has a new lower end, but just curious anyway. the only thing i can think of is your oil pressure gauge, some times when the stock gauge gets old it tends to not read acuratly a.i. (sending unit). And does it knock at high rpms or at start-up? if awnser these questions i might be able to help you more, thanks
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Old 12-30-2002, 06:11 PM
rustynuts rustynuts is offline
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Good oil pressure when cool, drops to zero when hot?

how low does it get? i just put new bearings and oil pump in my 60,000 mile motor and it used to drop to 7 lbs at hot idle, but now its at 25 lbs at hot idle.
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Old 12-30-2002, 06:48 PM
maynard2 maynard2 is offline
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Post Good oil pressure when cool, drops to zero when hot?

Wow, thanks for a speedy reply. To answer your questions...

At first 10/30, then I tried 20/50. I bought a brand new auto-meter mechanical oil pressure gauge (second one, but that's another story). It reads the same as a cheap one I switched it for. THe temperature is about 200 when hot. There are no knocks, noises, nothing she purrs like a kitten. Could there be a plug out somewhere in the oiling system that was taken out by the shop when they hot tanked the block, that maybe didn't get put back in? There is no oil leaking out anywhere, nothing on the ground.
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Old 12-30-2002, 07:11 PM
fordlowboy fordlowboy is offline
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Good oil pressure when cool, drops to zero when hot?

[updated:LAST EDITED ON 30-Dec-02 AT 08:13 PM (EST)]hi
How low does the oil pressure get when hot? with 20/50 in there it shouldn't get lower than 15 psi, and since it is new it shouldn't get lower than 25 psi. my motor is pretty old and it drops to 3psi, yes 3!...(74' 390). if it's actually droping to zero than your motor would freeze up. try taking off your valve covers while it's running and see how the oil flows on valve train. Maybe your lower end oil pressure is good and the oil isn't getting to the oil sender unit or gauge, why i don't know. Well have to wait for toruqe king, he's better than me at this stuff. send me your info on this post and maybe i can think of some more things to help you. good luck
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Old 12-30-2002, 08:15 PM
f100dw f100dw is offline
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Good oil pressure when cool, drops to zero when hot?

I have a 390 that has bad oil pressure. When I first built it, it had no oil pressure. I took off the front cover and found two plugs that had been removed by the machine shop. They were behind the cam sprocket by the distributor. After I fixed it, it had about 50psi at highway speeds and 2-10 psi at idle depending on how hot it was. I put on a melling hi-volume pump and it went to 80-90 psi cold at 2000 rpm and 60 at highway speeds, but it still drops to 10-15 at idle in cool weather and 5-10 in hot weather. I run Valvoline racing 50 wt oil in it. That is the main thing a don't like about my 390. I hear that is common with 390s. I don't know the oiling system on a 351m, but I would look for a plug left out somewhere. You might pull off the front cover and distibutor and use a drill to turn the oil pump and visually look for leaks. That's how I found the missing plugs on my 390. Hope this helps. Dave
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Old 12-30-2002, 08:53 PM
maynard2 maynard2 is offline
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Post Good oil pressure when cool, drops to zero when hot?

Thanks Dave, appreciate the info. I'll give it a whirl and we'll see what happens. Would still like to hear from others who may have ideas.

Thanks everyone.

Chris.
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Old 12-30-2002, 09:19 PM
fordlowboy fordlowboy is offline
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Good oil pressure when cool, drops to zero when hot?

hi
i agree with f100dw. Now that i remeber on my 460 project there are two oil ports behind the timming gears. And two behind the flywheel but i think that would be more obvious.
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Old 12-30-2002, 09:53 PM
jcs17 jcs17 is offline
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Good oil pressure when cool, drops to zero when hot?

i had a friend that had a cracked pickup on a 302. It had fine oil pressure cold and when it heated up, the crack spread and sucked air and dropped pressure. Replaced the pickup and it was fine..... just a thought


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Old 12-31-2002, 01:12 AM
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Good oil pressure when cool, drops to zero when hot?

[updated:LAST EDITED ON 31-Dec-02 AT 02:48 AM (EST)]maynard2, To be losing oil pressure after warm up, it would seem that bearing clearances are increasing with heat.

Did you assemble ther motor yourself?

Did you get your clearance measurements on the crank with plasti-gauge?

The lifter bores were not wallowed out, or scored?

Maybe the cam bearings were beat up while installing the cam. I have also heard of people scraping the cam bearings if the cam won't go in. ( cam bearings not lined up properly from hole to hole, where the cam will pretty much slide right in ) Could have scraped to much. But I would hope this did not happen. You need to have tight tolerances on the cam bearings also, just like the bottom end.

I don't think you have an oil plug left out or you would not have any oil pressure like the above post mentioned.
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Old 01-01-2003, 11:21 PM
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Good oil pressure when cool, drops to zero when hot?

Pressure does not matter (that much) at idle, however flow does.

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Old 01-03-2003, 12:08 AM
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Good oil pressure when cool, drops to zero when hot?

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Old 01-03-2003, 08:01 PM
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Good oil pressure when cool, drops to zero when hot?

Wow I have exactly the same problem!!! I had the same problem on two 390's then I cleaned out the valve train and replaced the seals and springs, i will eventually get around to it on the 351 but i work to much! try the seals that might help.
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Old 01-03-2003, 08:01 PM
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