Oil Pressure
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Tuned
Joined: Feb 2013
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From: Peterstown
Oil Pressure
I am a Disabled Veteran and have been out of the Auto Mechanic profession for 14 years. Needless to say I am a little rusty on a lot of things. My question today is.....I have a 400 in my 1978 Ford F150, it has good oil pressure when cold, when it reaches normal operating temperature, it drops to zero. What is the most likely cause?
Hotshot
Joined: Apr 2009
Posts: 14,333
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From: Stanley, VA
Wrong grade of oil could be a possibility but the most likely cause is a worn bottom end. Excess clearance between the bearings and the crank journals.
Pressure is a measure of resistance to flow, when an oil is cold it is thicker and takes more pressure to "push" it through the engine. When it warms up and thins out it takes less pressure to force it through the engine.
As clearances open up from wear, it takes less force to push the oil through the engine.
Pressure is a measure of resistance to flow, when an oil is cold it is thicker and takes more pressure to "push" it through the engine. When it warms up and thins out it takes less pressure to force it through the engine.
As clearances open up from wear, it takes less force to push the oil through the engine.
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
If you aren't able to rebuild the engine right now, there are a few things you can do to buy some time.
First, switch to a heavier grade of oil 20W50 or straight 50 weight, you can also add a viscosity improver (STP or something similar). A little more involved is the installation of a high volume (volume, not pressure) oil pump.
As a rule of thumb, at operating temp you'd like to see 10psi pressure for every 1000 RPM the engine is operating at.
First, switch to a heavier grade of oil 20W50 or straight 50 weight, you can also add a viscosity improver (STP or something similar). A little more involved is the installation of a high volume (volume, not pressure) oil pump.
As a rule of thumb, at operating temp you'd like to see 10psi pressure for every 1000 RPM the engine is operating at.
Postmaster
Joined: Nov 2011
Posts: 2,553
Likes: 10
Another fix, short of a rebuild, is to replace the main and rod bearings in chassis. Pull the pan and replace from below. The main bearings are "rolled in". Not guaranteed but can help a lot. You would need to find the right mechanic for this.
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Tuned
Joined: Feb 2013
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From: Peterstown
Thanks fellas. I am a mechanic by trade, just been out of it for a few years, since I got my disability. You know, one of those things where, if you don't use it, you lose it. I believe I can handle the rebuild, I just wanted to get another opinion second to my own.
Fleet Owner
Joined: Dec 2009
Posts: 20,678
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From: NorCal
Thanks fellas. I am a mechanic by trade, just been out of it for a few years, since I got my disability. You know, one of those things where, if you don't use it, you lose it. I believe I can handle the rebuild, I just wanted to get another opinion second to my own.
Elder User
Joined: May 2004
Posts: 836
Likes: 2
From: Shreveport, LA
Before you go tearing in and trying to replace bearings and change the oil from what it's supposed to be, have you verified the problem?
Are you seeing this pressure drop on the stock gauge or a quality aftermarket unit?
If you dont have an oil and temp gauge set up that tells you the truth, that's the first step. I happen to like Equus gauges; If you get the mechanical gauges, the oil units are good quality bourdon tube construction and will give you years of service.
They also make electronic gauges...which I recommend.
Are you seeing this pressure drop on the stock gauge or a quality aftermarket unit?
If you dont have an oil and temp gauge set up that tells you the truth, that's the first step. I happen to like Equus gauges; If you get the mechanical gauges, the oil units are good quality bourdon tube construction and will give you years of service.
They also make electronic gauges...which I recommend.
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Tuned
Joined: Feb 2013
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[quote=psychlopath;13412097]Before you go tearing in and trying to replace bearings and change the oil from what it's supposed to be, have you verified the problem?
Are you seeing this pressure drop on the stock gauge or a quality aftermarket unit?
X2,
Every ford truck i have owned does this to a degree, with my current truck the needle is almost pegged at start up and when warm reads on the low side. sure the oil warming causes this but i think the sender or the gauge is overly temperamental.
Chris
Are you seeing this pressure drop on the stock gauge or a quality aftermarket unit?
X2,
Every ford truck i have owned does this to a degree, with my current truck the needle is almost pegged at start up and when warm reads on the low side. sure the oil warming causes this but i think the sender or the gauge is overly temperamental.
Chris
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Laughing Gas
Joined: Nov 2013
Posts: 1,207
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From: S.E. Louisiana;so far USA
The 1978 Ford truck shop manual says the oil pressure for the 400 engine when hot at 2000 RPM should be 50-75 PSI. Try the aftermarket gauge and 20W-50 engine oil before tearing into it.
Junior User
Joined: May 2019
Posts: 52
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Unfortunately the issue you are describing is typically caused by too much main bearing clearance. For whatever reason the 400 was very hard on mains. The 351M was not however. My experience with these is I would usually find a tremendous amount of wear on the main journals as well as completely worn our bearings. I have seen them get so bad that the rear main seal would begin to leak also. I hope I'm wrong but that is what it sounds like to me.
Laughing Gas
Joined: Jul 2017
Posts: 1,203
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First thing I noticed when I opened it, but I have the same issue so nice to know it's not only me.
The 400 in my '79 has 111k miles on it and the oil pressure does what the OPs did. Holds middle to slightly high on the gauge when cold, but falls off when hot. Changed to 15-40 instead of 10-30 and it's a little better but not great. I believe my rear main is also leaking. Didn't put two and two together until I read this. Might be closer to a rebuild or engine swap that I thought...
Unfortunately the issue you are describing is typically caused by too much main bearing clearance. For whatever reason the 400 was very hard on mains. The 351M was not however. My experience with these is I would usually find a tremendous amount of wear on the main journals as well as completely worn our bearings. I have seen them get so bad that the rear main seal would begin to leak also. I hope I'm wrong but that is what it sounds like to me.