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78 Custom 351M NP435 NP205 Sniper EFI Hedman Headers Magnaflow Muffler 4.56 Gears Grizzly lockers
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Discussion Starter #1
Figured I'd post this on here for the heck of it. I'm not worried about it, but I thought it was interesting and was wondering what would be the underlying cause.

This is occurring on my 1977 F-100 w/302. I don't have a mechanical oil pressure gauge on it, so I'm just going off the factory gauge which may not be accurate, but it seems consistent with it's behavior to say the least.

Here's a google pic of what my gauge looks like for reference.




When the truck is unloaded (not hauling anything) the cold oil pressure is about 2/3 the way up on my gauge. Once it warms up it probably sits about 1/2 the way up or slightly under 1/2 and never really changes from there.

Right now I have (guessing here) maybe 1000-1200 lbs in the bed and under light acceleration you can watch the oil pressure gauge dip down to the bottom of the "normal range". It does it when it's cold and when it's warmed up.

Out of curiosity I tried some harder acceleration (hard for that slow truck anyway) and it would dip even lower to just above the L. When I let off the throttle the oil pressure creeps back up or once I reach speed and let off the throttle to cruise it goes back up, but not quite to where it would be with no load in the bed. That goes for any kind of acceleration.

Now, none of this is under and significant rpm except for when I tried some harder accel which I had to stick the C4 into first and hold it there to see what would happen with the oil pressure. The truck has 29" tires and a 2.75 rear end, so under normal driving it's not spinning a lot of rpm. It probably never sees more than 2500 rpm under my normal driving. Approximately 1200 rpm is 35 mph and 1500 rpm is 45 mph and I'm leaving a guess of 2500 max rpm for my acceleration as I drive the thing like a grandpa.

Anyway! What would cause this behavior if the gauge is accurately reflecting what's going on? Worn bearings? Weak oil pump? I'm assuming the engine is the original engine for this truck, but I have no idea if it's been gone through. It runs pretty well for what it is, even if it's grossly under-powered, and this is the only "major" issue it currently has. Again, it's nothing I'm worried about, because if it dies any time soon I'll just retire the 302 and work on swapping the 460 I have for it sooner rather than later. It's due for an oil change soon, so I'll probably try a slightly heavier oil now that the weather is warming up. It currently has 10w30, so I'll probably try a 15w40. I know it complains on cold start with a lot of lifter ticking when I use that oil in the winter, but it should be ok with warmer weather.

Thank you for coming to my TED talk.
 

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The Tennessee Warden
96 XL, 5.8L, E4OD, BW1356, 4.56 gears
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Hey man, one of the things I love about old vehicles is the simplicity. That oil gauge you have there may not be mechanical (with an oil line to the gauge) - it should be electrical with a bell-shaped sending unit on the block and one wire to the gauge. While it may or may not be accurate (what IS accurate anyway with no numbers on the gauge? Lol), it does operate in real time, unlike the “oil pressure switch” on my 96. So the loss of pressure you’re seeing very well could be indicative of a true pressure loss under load. On an engine of that age, my first inclination would be what you already said - bottom-end bearings and oil pump wearing out. You’re PROBABLY ok for a while. When you start seeing noticeable power losses though, this is usually due to the lowered oil pressure not being sufficient to fully “pump up” the hydraulic lifters, thereby decreasing your valve lift and power.

I’d start getting that 460 and new transmission ready though! Oh, and you’re gonna want to upgrade to heavy duty the other drivetrain components - driveshaft and rear end specifically. What’s that rear axle anyway, an 8”? You’ll want new front springs and shocks too, for that heavy engine!
 

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Premium Member
84 Bronco, 351w, c6, custom doubler, np208, 5.13’s, TTB44, 9”, locked f/r
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842 Posts
I’ve had 3 or 4 oil pressure scares in different vehicles over the years, all but one of them has just been a crap sending unit.
But worn bearings and loose clearance could be the issue too, when working the engine the oil gets hotter and thinner, which flows better and reduces pressure. What weight oil you running in it?
Also, how does the truck sit when you have it loaded down, and what is the oil level at? Depending on where the pickup is in the pan, if it’s low and the truck is tilted back under load it’s letting the pickup suck air. Or it could even be too full and getting into some windage problem and foaming up the oil
 

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78 Custom 351M NP435 NP205 Sniper EFI Hedman Headers Magnaflow Muffler 4.56 Gears Grizzly lockers
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Discussion Starter #5
While it may or may not be accurate (what IS accurate anyway with no numbers on the gauge? Lol), it does operate in real time, unlike the “oil pressure switch” on my 96.
Accurate as in the "normal" range actually being a normal range. lol I suppose with a bad sender that the normal range on the gauge could be displaying something too high or too low.

I'll probably check out the sender at some point, but I'm pretty sure it's in a pita location. It's not on top at the back of the block which is fairly easy to get to, but it's right next to the oil filter, which is conveniently crammed in there with the fuel pump, power steering pump, and lower radiator hose. Fun times.


I’d start getting that 460 and new transmission ready though! Oh, and you’re gonna want to upgrade to heavy duty the other drivetrain components - driveshaft and rear end specifically. What’s that rear axle anyway, an 8”? You’ll want new front springs and shocks too, for that heavy engine!
I have a pretty beefy C6 on standby for it though it's on about a 15-ish year old rebuild. It probably only has 40-50k miles on it. 460 is a different story.

This truck has a 9". I'm not too concerned on the drive shaft or rear end falling apart, honestly. The driveshaft on this thing is stupid big in diameter, it's gotta be at least an inch, if not more, bigger in diameter than either of my Bronco's shafts. Not sure why Ford did that, but they did!

I don't have any good 1 to 1 comparison pictures, but here's a shot of my Bronco's front shaft when I was rebuilding it and a shot of my F100's rear shaft near the diff. They both use the same 1310 u-joints, so you can kind of get an idea on the shaft diameters that way. Just compare the width of the yoke ears to the shaft widths.







What weight oil you running in it?
Also, how does the truck sit when you have it loaded down, and what is the oil level at? Depending on where the pickup is in the pan, if it’s low and the truck is tilted back under load it’s letting the pickup suck air. Or it could even be too full and getting into some windage problem and foaming up the oil
These are very good questions!

Right now the oil is in the "safe" area on the dipstick, but it's a little low, maybe needs a half quart.

It's a front sump pan.

The back of the truck is sitting maybe an inch lower than the front at the moment due to all the weight.

I'm not sure that all of that is causing the pressure issue, because it's got a good amount of oil and it's not spinning a lot of rpm to toss all the oil into the heads and away from the pickup even though it's tilted back a little more than usual, but I guess you never know.

Right now it has 10w30 in it as during the winter anything thicker than that and the lifters tick a ton until the engine warms up. With 10w30 in the winter it ticks for a few seconds while they pump up then it stops. Now that it's warmer I'll try something thicker.

The first variable that is going to be changed the soonest is the fully dressed 351M in the back on the bed will be removed in the next couple of days. After that I'll try an oil change and then last I'll check the sender.
 

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House of Windsor 4ever!
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If it's on the block side, the pressure loss could just be the pump loose enough from the block to allow unwanted pressure loss under load; I would think that loose bearing clearances would cause a pressure loss all the time, let alone low pressures. Even though the sender is in that location, I'd go buy or rent a pressure tester, and test what happens. Basically, even under load, as long as the pressure doesn't dip below 15 PSI hot, the engine's protected enough.
 

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Figured I'd post this on here for the heck of it. I'm not worried about it, but I thought it was interesting and was wondering what would be the underlying cause.

This is occurring on my 1977 F-100 w/302. I don't have a mechanical oil pressure gauge on it, so I'm just going off the factory gauge which may not be accurate, but it seems consistent with it's behavior to say the least.

Here's a google pic of what my gauge looks like for reference.




When the truck is unloaded (not hauling anything) the cold oil pressure is about 2/3 the way up on my gauge. Once it warms up it probably sits about 1/2 the way up or slightly under 1/2 and never really changes from there.

Right now I have (guessing here) maybe 1000-1200 lbs in the bed and under light acceleration you can watch the oil pressure gauge dip down to the bottom of the "normal range". It does it when it's cold and when it's warmed up.

Out of curiosity I tried some harder acceleration (hard for that slow truck anyway) and it would dip even lower to just above the L. When I let off the throttle the oil pressure creeps back up or once I reach speed and let off the throttle to cruise it goes back up, but not quite to where it would be with no load in the bed. That goes for any kind of acceleration.

Now, none of this is under and significant rpm except for when I tried some harder accel which I had to stick the C4 into first and hold it there to see what would happen with the oil pressure. The truck has 29" tires and a 2.75 rear end, so under normal driving it's not spinning a lot of rpm. It probably never sees more than 2500 rpm under my normal driving. Approximately 1200 rpm is 35 mph and 1500 rpm is 45 mph and I'm leaving a guess of 2500 max rpm for my acceleration as I drive the thing like a grandpa.

Anyway! What would cause this behavior if the gauge is accurately reflecting what's going on? Worn bearings? Weak oil pump? I'm assuming the engine is the original engine for this truck, but I have no idea if it's been gone through. It runs pretty well for what it is, even if it's grossly under-powered, and this is the only "major" issue it currently has. Again, it's nothing I'm worried about, because if it dies any time soon I'll just retire the 302 and work on swapping the 460 I have for it sooner rather than later. It's due for an oil change soon, so I'll probably try a slightly heavier oil now that the weather is warming up. It currently has 10w30, so I'll probably try a 15w40. I know it complains on cold start with a lot of lifter ticking when I use that oil in the winter, but it should be ok with warmer weather.

Thank you for coming to my TED talk.
Did you ever consider that the reason for the drop in oil pressure is that the oil pickup is sucking up air instead of oil? You said that this happens on acceleration when you have 1200 lbs of load in the back of the truck. Maybe the acceleration, in combination of the reverse rake of the truck, and the load, is causing the oil pickup to cavitate and not suck up pure oil, but a mixture of air and oil. Once you stop the front end rise when you stop the acceleration, the oil pressure returns to normal.
Just for a test, put an additional half quart of oil in it and see if that helps.
 

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Discussion Starter #8
Did you ever consider that the reason for the drop in oil pressure is that the oil pickup is sucking up air instead of oil? You said that this happens on acceleration when you have 1200 lbs of load in the back of the truck. Maybe the acceleration, in combination of the reverse rake of the truck, and the load, is causing the oil pickup to cavitate and not suck up pure oil, but a mixture of air and oil. Once you stop the front end rise when you stop the acceleration, the oil pressure returns to normal.
Just for a test, put an additional half quart of oil in it and see if that helps.

Yeah, I believe that's what @NickOille was getting at with his line of questioning. My thinking on that was that I don't think the engine spins enough rpm to send enough oil to the heads and starve the pickup, even with it tilted back a little.

I don't have a tach, but entering my info over at grimmjeeper.com/gears it says my 302 is spinning 1650 rpm at my 1st to 2nd shift (20mph), and 1700 rpm at my 2nd to 3rd shift (35 mph). Sounds ridiculous, but that's what it is and what it feels like, believe me. I cruise 45 mph at 1500 rpm. I really don't have to guess why my trucks gets such better gas mileage than my Bronco.

My first change has been made, I've unloaded most of the weight that was in my bed (fully dressed 351M, two 460 heads, radiator, and NP435). The rear sits higher than the front now and my oil pressure is better. It's not amazing, but it is better. If the engine loads, it still drops the pressure on the gauge. When I say loads, I mean if the engine doesn't down shift from third at lower speeds and I try to accelerate in third from like 20 mph, the engine is under a higher load and it'll drop on the gauge to the bottom of the "normal" range.

I picked up some 10w40 yesterday as I'll be changing the oil in this truck and another one I have, so I'll see what difference a heavier weight and full level on the dipstick makes. After that I'll try a new sender and see what happens there. I don't know if 302's have a spot for the pressure gauge on the back of the block, but if it does I could try both locations to see what each spot says.

Again, I'm not really worried about having lower pressure, I'm just curious why it does what it does. It's been doing this for quite a while, the gauge fluctuating when the engine is working a lot harder than it does when the truck is unloaded, so if it was going to give out I'd imagine it would have done it already. I first noticed it when I bought a donor 460 a couple years ago and was hauling it back on the freeway. With the added weight in the bed and my 302 trying to keep up to speed with it's long gearing, the needle was dropping.
 

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95 Bronco, 351W, E4OD, 4.56 gears, 35x12.50x15 Patagonia MTs.
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I'd reccomend trying some diesel oil, like 5W40 Rotella T6, or even just 15W40 once it's warm outside. It'll help flush out any contaminates, and the T6 is very stable at high temperatures. I ran in it oil cooled motorcycles for years, it's good stuff.
 

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78 Custom 351M NP435 NP205 Sniper EFI Hedman Headers Magnaflow Muffler 4.56 Gears Grizzly lockers
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Discussion Starter #10
I put some 10w40 into it on Saturday. I want to say that improved it slightly, but that's probably just wishful thinking. I think the 302 is just tiring out when it comes to clearances. I'll probably give the sender a shot, just to see what it does. I'm curious if Ford used the same sender on most of their engines of this era like they did with their oil filters. I think I have a couple oil senders from other engines lying around.

Here's the location of my sender. Pretty tight. It's actually on an extension and I have no idea if that's factory or not. This truck was pretty unmolested when I got it, even had factory jade green paint on the drums that matches the truck's paint before I had them replaced.



 
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