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Old 08-15-2020, 03:42 PM   #1
mcgarrett
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Default Oil pressure drop

At engine startup on my '40 Ford with stock engine, my oil pressure gauge reads 45+ lbs of pressure on the factory gauge and has done this for quite a few years. After driving for 20-30 minutes and the oil is up to temperature the gauge reads around 10 lbs going down the road and maybe 5 lbs. at idle. My oil pressure sending unit is an old one; maybe the original.

My questions are:
- Is the sending unit malfunctioning perhaps due to age?
- Is it possibly a weak pressure relief spring playing tricks when the oil gets hot?
- Is it a faulty gauge? (it's old too)
...or a combination of the above?
I want to make sure I am diagnosing my problem before I order and replace a part unnecessarily.
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Old 08-15-2020, 03:56 PM   #2
JSeery
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Default Re: Oil pressure drop

Many would say that these numbers aren't bad for an engine with a lot of miles on it. If you want to verify your oil pressure install a mechanical oil pressure gauged (temporary or permanently) to double check what the pressure is.

The pressure relief spring should not change as the engine warms up. The oil viscosity thins out as it gets hotter and with increased clearance in the bearings the pressure decreases.

What viscosity oil are you running? How many miles are on the engine?

If you want to read one (of numerus) thread on this topic, this is one currently up:https://www.fordbarn.com/forum/showthread.php?t=283042

Last edited by JSeery; 08-15-2020 at 07:55 PM.
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Old 08-15-2020, 08:20 PM   #3
GB SISSON
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Default Re: Oil pressure drop

I have 'improved' my oil pressure by removing the sender and squirting carb or brake cleaner into the sending unit more than once. . Give that a try and report back.
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Old 08-15-2020, 08:38 PM   #4
tubman
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Default Re: Oil pressure drop

The first thing you have to do is determine whether you have an engine problem or a gauge problem. Mechanical gauges are relatively easy to install (temporarily), cheap enough, and readily available.
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Old 08-15-2020, 09:04 PM   #5
mcgarrett
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Default Re: Oil pressure drop

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Engine only has about 6,000 miles on it since rebuild 20 years ago. I'll work on all these suggestions and get back with more details.
Thanks to all!
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