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454 losing oil pressure

skunked

skunked

phat block everything
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Location
Placer County CA
79 BBC with a mild cam and EFI.

We hit the trail Friday and while I was dropping off a 4’+ ledge the truck started making a noise that I thought was lifter noise. I looked over at my oil pressure gauge and it was damn close to zero so I shut the engine off. Talked it over with my passenger and we decided to fire it back up so we could get off the ledge and check the oil level. Once the truck was off the ledge I checked the level and found it halfway between the “add” and “full” line. I went ahead and added the quart of oil I carry and the truck was fine the rest of the weekend. Keep in mind the truck never left double low and the rpms were kept below 2k for the most part.

This afternoon we unlocked the hubs, put the boxes back in 2 high and aired up for the hour drive home. Less than a minute into our freeway drive (55mph @2400rpms) I hear a faint tapping noise again so I lift the throttle and put the transmission into neutral (downhill), noise went away and psi jumped back up to 25sh. We repeated this process all the way down highway 80 until I was able to exit on some backroads were the truck drove fine at 40mph (1800 rpms) the rest of the way home.

my gut tells me the oil pump is failing, maybe the pickup has some crap on the screen or the tube is loose? There’s always a chance the filter is restricting flow as well. My plan at this point is to drain the oil through a screen and check the oil filter for material. If I don’t find anything I’ll probably need to drop the pan for a deeper inspection of the pump. Is there anything else that I’m missing here?
 
Every once in a blue moon a gauge will fail. Mechanical or electric gauge? Make sure to rule that out as a possible fault.
Otherwise I think you have a reasonable plan.
 
Curious if you remember when you had it apart, is the pickup fixed to the pump?
 
beags86 said:
Every once in a blue moon a gauge will fail. Mechanical or electric gauge? Make sure to rule that out as a possible fault.
Otherwise I think you have a reasonable plan.
Click to expand...

It’s an electric autometer setup with the sensor above the filter. I installed it a year or so ago. The fact the tapping starts when the psi drops leads me to believe the gauge is accurate (within reason).


Bent77 said:
Curious if you remember when you had it apart, is the pickup fixed to the pump?
Click to expand...

I was trying to remember what setup I had but I can only remember the pickup was clean so I decided not to mess with it. I might have some pics on my computer.
 
What kind of pressure did you have cruising at 40 mph (~1800 rpm)?

How steep was the ledge? Could the motor have run dry (oil in the front of the pan) for a short time?
 
It’s possible with all that light duty use that you knocked the pickup off
 
nvrenuf said:
What kind of pressure did you have cruising at 40 mph (~1800 rpm)?

How steep was the ledge? Could the motor have run dry (oil in the front of the pan) for a short time?
Click to expand...

25-30 psi at 1800rpms

ledge was a vertical 4’ drop so the front bumper was in the ground and ass in the air but that’s nothing new for this truck. I also though the oil could be low so it was starving the pump but it had plenty of oil.
 
You might have killed a bearing, but the oil will tattle on that
 
Since the oil pressure came up when you let off of the throttle, it makes me wonder if the crankshaft moving is making the difference in the resistance to oil flow, or is it just moving enough to make the pressure drop, and there is another problem that it can't keep up when the crankshaft moves.
Is the crankshaft centering better in the bearings better until you make the engine work which pushes the crank down?

I don't have any help, really. But I will tell you that my '75 454 in my '70 has had the problem of the oil pressure going up and down depending on if I am on or off the throttle. I have my mechanical gauge plumbed into the same place. My engine has done this for 45K miles, and 2 different builds. It ate cam lobes after 25K. It never loses oil pressure, but I haven't stood it on the nose terribly. Just once that I can remember.
 
6872xtc said:
Since the oil pressure came up when you let off of the throttle, it makes me wonder if the crankshaft moving is making the difference in the resistance to oil flow, or is it just moving enough to make the pressure drop, and there is another problem that it can't keep up when the crankshaft moves.
Is the crankshaft centering better in the bearings better until you make the engine work which pushes the crank down?

I don't have any help, really. But I will tell you that my '75 454 in my '70 has had the problem of the oil pressure going up and down depending on if I am on or off the throttle. I have my mechanical gauge plumbed into the same place. My engine has done this for 45K miles, and 2 different builds. It ate cam lobes after 25K. It never loses oil pressure, but I haven't stood it on the nose terribly. Just once that I can remember.
Click to expand...

I was thinking the pump cavitation could be rpm dependent. I read something about the crank causing foaming of the oil when it’s overfilled but I ran the rubicon before this trip without any of these problems.

I should clarify that the psi does rise as I accelerate however it slowly drops off as I hold the rpm steady above 2k rpms
 
You saying it does it going down hill makes me think it's the oil pump pickup. Had one fall off on my 400 a number of years back. Did just what your saying.
 
Can you replicate the RPM pressure loss in the drive way? Might tell you if it's purely RPM related or engine load plays a part.
 
mrk5 said:
Can you replicate the RPM pressure loss in the drive way? Might tell you if it's purely RPM related or engine load plays a part.
Click to expand...

On the side of the road yesterday I was able to replicate the condition in park.

Most gear heads I’ve spoken with over the last 24 hours think it’s the pump, most specifically say the pickup is loose or clogged. After I drain the oil I’m going to see if my camera will get a decent view of the pickup tube.
 
About the only other time you hear about this is a high volume oil pump with a pan that's too small.
 
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