Oil pressure drop

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Jan 13, 2017 | 10:31 AM

bulldogcountry1

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Tuned

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From: Vicksburg, MS

Oil pressure drop

When I first fire the truck up cold, the oil pressure goes up to about 80% of the upper end of the normal range and stays there at idle. After the engine heats up, it slowly falls at idle at traffic lights to about 20% into the normal range. It goes back up with some RPMs.

Is this anything to be concerned with? It's been that way for years. It hasn't gotten worse, and I'm never really in a position where I have to sit at hot idle for long, anyway. Just seeing it fall makes me a bit uncomfortable. I didn't know if the pump was on it's way out or something else.

Jan 13, 2017 | 10:52 AM

meangreen92

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Joined: Jul 2005

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Sounds like mine. I think it's OK. But the only way to know for sure would be to try a mechanical gauge.

Jan 13, 2017 | 10:53 AM

good to doo

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Joined: Feb 2014

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From: british columbia, canada

If you have the stock gauge in the dash, completely normal. What engine?.

Jan 13, 2017 | 10:58 AM

bulldogcountry1

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From: Vicksburg, MS

400. Stock gauge.

Jan 13, 2017 | 11:02 AM

good to doo

Elder User

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From: british columbia, canada

The 351m/400 have low oil pressure at idol.

Jan 13, 2017 | 11:22 AM

79F150400M

Fleet Mechanic

Joined: Feb 2004

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From: Arlington,Wa

From my experiences, the pressure drop when hot is normal for these engines. My 400 did it and so does the tired 460 I have now. Just as long as there's no rattling from the bottom end or any lifter/rocker arm noise Id say your good. Another thing to do is run a little bit thicker oil like Shell Rotella 15W-40. It will give your oil pressure a little boost. It did for me.

Jan 13, 2017 | 12:28 PM

gittinwidit

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Joined: Mar 2003

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From: Durham, NC

ALL motors that are mechanically lubed by oil pressured by the crank journals do this by design. The oil pressure is high at first start when the bearings are cold. It's this way to oil the dry system where a lot of the oil probably drained back to the pan. Then as the engine warms, the bearings swell just a tad allowing oil to weep off the sides to oil the crank.
But!!!
A mechanical gauge is a must if the oil pressure is ever questionable. That way you get accurate information. You see exactly how much pressure you have at cold start and at warm idle. Anything above 80 pounds cold is not good for the fact that an oil filter is only rated at 100 pounds. So if you ever have to hop in and go fast, you risk blowing the oil filter off. If the pressure falls below 10-12 pounds at warm idle, you're not getting enough oil to the top end to keep it lubed during a long idle. Most healthy motors get anywhere around 50-70 at cold start and 15-20 at hot idle if not more. And no 2 engine families are the same. And there's a lot of variations to take into play after a rebuild / break in period / age and wear.