oil pressure fluctuation

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TimDart

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hi guys, concerned about my pressure guage showing some wild fluctuation.

did a long hot run (250 miles) last weekend on the highway. Pressure showed usual 55psi and after about 45 minutes started to fluctuate mildy between 45-55. Onthe return journey i had the same experience, but eventually it dropped to 40 if i ran over 3500rpm

Last night it got strange. The guage read 55 at 1200prm (never had that before) and basically showed anywhere between 20-60 psi at diffeent engine speeds.

Engine is 12000 miles new with all bearings replaced, regular changes. Tried Fram and wix filters without much difference.

the pressue guage is mechanical with a capillary tube

I'm also having overheating issues, but cant see how that could be directly relevant.
any help much appreciated!
 
The heat thins the oil. Extra heat thins it further.
Fix the overheating issue and continue on to a oil cooler for better oil temp control and longer oil life if so desired.

Sorry I can't be much help past that.
 
I agree with Rumble to get the overheating fixed first but the description you gave sounds like a sticking pressure relief valve in the oil pump.
 
Ive been offered a spare guage so will fit that first, just in case. hope it isnt the oil pump, really dont want to pull this motor again.

What concerns me is that at the start of two long journeys the oil pressure sat at 55psi for about 45 mins then fell away. Guess the heat would have something to do with that. It was 80+ air temp plus reflected heat of the highway.

That said, on a cooler evening last night, it still fluctuated.
gonna replace head gaskets this weekend. The've only been on two months:angry7:
 
It is possible it's a gauge problem. Hope so for your sake cause I know how much work an oil pump is to change on an A-body. After it heats up the oil pressure will drop some but it usually happens much faster than 45 minutes. Mine levels off in about 15 minutes. What head gaskets did you use that need replacing this soon? Did you check the deck and head surfaces to make sure their flat?
 
did a long hot run (250 miles) last weekend on the highway. Pressure showed usual 55psi and after about 45 minutes started to fluctuate mildy between 45-55. Onthe return journey i had the same experience, but eventually it dropped to 40 if i ran over 3500rpm
Click to expand...

that seems like a pretty big drop. does it fluctuate that much while at a steady rpm? or is it just at different engine speeds?

what do you mean it dropped to 40 when over 3500 rpm? was it higher at say 3000 rpm? if so get a new guage in asap and proove weather or not its a guage problem. if thats not it get that pump out of the motor asap.

Last night it got strange. The guage read 55 at 1200prm (never had that before) and basically showed anywhere between 20-60 psi at diffeent engine speeds.
Click to expand...

maybe the relief valve in the pump is bad? mine started acting all funny a few months before i spun a bearing. wanted to change the oil pump just never got around to it. would it have saved my motor? we will never know. get to the bottom if this before you put too many more miles on it,
 
Thanks for the updates, guys. The worse aspect was driving hard past 3.000rpm and seeing the guage dip below 40 psi last weekend. Backed off and maintained at least 40psi.

Anyway, after the guage replacement it does look like the oil pan comes down. Might as well do the bearings as well. Pretty sobering on a 12000 mile motor that was built correctly.

Still, I gues a relief valve can go at any time.
 
If a piece of trash gets into the relief valve it'll sure make it act squirlley. Yrs. ago I worked for Hyster forklifts and I ran into that allot. Debris would get into the hydraulic relief valve and sometimes the hydraulics would work and sometimes not. Then sometimes it'd work but be low on power. On them you could usually clean and reseal the valve and everything was ok. Its possible you can clean and debur your pump but even if you decide to get a new pump I suggest disassembleing it and checking for debris. The last 2 new Mellings pumps I got had rust inside of them I had to clean out.
 
fishy68 said:
If a piece of trash gets into the relief valve it'll sure make it act squirlley. Yrs. ago I worked for Hyster forklifts and I ran into that allot. Debris would get into the hydraulic relief valve and sometimes the hydraulics would work and sometimes not. Then sometimes it'd work but be low on power. On them you could usually clean and reseal the valve and everything was ok. Its possible you can clean and debur your pump but even if you decide to get a new pump I suggest disassembleing it and checking for debris. The last 2 new Mellings pumps I got had rust inside of them I had to clean out.
Click to expand...

I think this may be more on the right track. A oil pump is a constant displacment pump, the speed of the pump can lower or raise the output but it can not flucutate on its own accord at a fixed turning speed. I've heard oil pumps get the blame for more problems only to find out it was bearing problems. The bypass valve is there to prevent overpreasure conditions which can cause alot of problems especially on cold start up. If it doesn't bypass, it will blow the oil filter apart at the seam. I've seen this happen. I would advise checking with another gauge, if your problem is still the same, drop the pan and check ALL of your bearings! Cut your oil filter open and check for metal. Take your pump apart check for debree, remove the bypass spring and valve check for trash and valve not sticking, check oil pump cover for excessive wear at the gear contact area. I've seen mains with so much wear that would hold 35 PSI hot at idle and then go all the way to 5 PSI when cruising at 50. (The crank is forced away from the block under power resulting in more crank to oil hole clearance on the top main bearing.)
 
im having a bit of the same issue with my 383... one day was cruising took a corner and hopped on her and the press dropped to 0 psi, being paranoid i threw in a meling high volume pump ran consistent 65-75 psi hot or cold any rpm then one day after passing some one the press droped to 25 psi at roughly 1000 rpm and would rise to about 65 psi hot when the revs are brought up to about 1500+ cold shes 75 psi, and at an idle its roughly 10-15 psi at 700 rpm... and have no idea what the cause is, i cant imagine the mains would wear that fast punching it to pass a car?? starting out with 75 psi to drop down to 25???
 
SGBARRACUDA said:
I think this may be more on the right track. A oil pump is a constant displacment pump, the speed of the pump can lower or raise the output but it can not flucutate on its own accord at a fixed turning speed. I've heard oil pumps get the blame for more problems only to find out it was bearing problems. The bypass valve is there to prevent overpreasure conditions which can cause alot of problems especially on cold start up. If it doesn't bypass, it will blow the oil filter apart at the seam. I've seen this happen. I would advise checking with another gauge, if your problem is still the same, drop the pan and check ALL of your bearings! Cut your oil filter open and check for metal. Take your pump apart check for debree, remove the bypass spring and valve check for trash and valve not sticking, check oil pump cover for excessive wear at the gear contact area. I've seen mains with so much wear that would hold 35 PSI hot at idle and then go all the way to 5 PSI when cruising at 50. (The crank is forced away from the block under power resulting in more crank to oil hole clearance on the top main bearing.)
Click to expand...

HMMM. You quoted my post and said you have an idea that is more on the right track. I guess that means I was wrong. Funny though you later mentioned disassembling the pump to check for a sticking bypass valve. Coulda swore I mentioned that. Oh well:dontknow:
 
fishy68 said:
HMMM. You quoted my post and said you have an idea that is more on the right track. I guess that means I was wrong. Funny though you later mentioned disassembling the pump to check for a sticking bypass valve. Coulda swore I mentioned that. Oh well:dontknow:
Click to expand...

No I agreed with you on the bypass valve being the problem more so than a "bad" pump. I meant no disrespect.
 

The pump/relief valve does sound likely from what you guys are suggesting.
I have 20/50 oil, less than 1,000 miles back. i'll wait for the spare guage to arrive, check and get back. Thanks to all.
 
Did you put in the HP spring kit or shim the existing relief? I agree that it indicates something is amiss in there. The oil pump is constant displacement. So if it moves around, something is not working right. I would be very careful about running the engine like that. You may have bearing damage as a result and I would inspect all of them to be sure while the pan's off.
 
moper said:
Did you put in the HP spring kit or shim the existing relief? I agree that it indicates something is amiss in there. The oil pump is constant displacement. So if it moves around, something is not working right. I would be very careful about running the engine like that. You may have bearing damage as a result and I would inspect all of them to be sure while the pan's off.
Click to expand...

I have a sneaky suspicion that he is going to find allot of main bearing wear. I hope I'm wrong.
 
guys, i'm puling the motor this week. Once I have it apart i'l feed back and we can share the results.
 
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