Oil Pressure Drops After High RPM

[JamesSam]

I will lay this out as simply as possible:

-Truck cranks and runs 30-35 PSI then drops to ~5-9 PSI while driving, usually after leaving the highway(going from high RPM to low).
-After initial drive, parked then cranked again, OP will be back up to 30-35 PSI.
-Makes a new low volume knocking sound after driving a while at low speed or at idle. Maybe an exhaust leek? Hopefully not a rod knock...? I have been told the knocking could be from low oil pressure.
-No loss of power.
-Engine not running hot.
-Sending unit is in place and wire is plugged in. I replaced this about a year ago and it helped.
-Maybe a bad oil pump?
-I run 5W30. Should I run 10W30 now?
Feedback please. Thanks All!

 

[legopnuematic]

5-30 is too thin in my opinion for a run of the mill small block or big block.

10-30, 10-40, 15-40, 20-50 for these old mills.

Personally 15-40 in everything I own. The 76 has 20-50 VR1 in it, but was 15-40 before the last oil change.

 

[CountKrunk]

I run rotella t4 which is 15-40

 

[ChuckN]

I’m running 20w50 in mine. If I was in a colder climate, I’d run 15w40.

Is the pressure drop coinciding with oil temp? Like after it’s warmed up or does it do that even when cold?

Perhaps the oil pick up is a bit clogged, maybe drop the pan and see. It may be able to pull enough when at low rpm but then the restriction can’t keep up when the revs come up.

I’d be hesitant to drive it again until you figure something out- if you’re willing to run it long enough you could check it with another gauge to verify.

 

[JamesSam]

I’m running 20w50 in mine. If I was in a colder climate, I’d run 15w40.

Is the pressure drop coinciding with oil temp? Like after it’s warmed up or does it do that even when cold?

Perhaps the oil pick up is a bit clogged, maybe drop the pan and see. It may be able to pull enough when at low rpm but then the restriction can’t keep up when the revs come up.

I’d be hesitant to drive it again until you figure something out- if you’re willing to run it long enough you could check it with another gauge to verify.

The drop definitely only happens when the engine is warm to hot. Most times I crank it up and OP is above 30 PSI but will start to dip slightly while warming up and as stated, after a good 20 minute or more drive, it drops down further still. I will say, even when the guage shows low pressure it still fluctuates with RPM'S.

 

[JamesSam]

Here is a pic of cold oil level on the dip stick before swipe with a towel and after:

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[JamesSam]

Before ignition:

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[JamesSam]

Cranked up:

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[JamesSam]

After warming up for 4 to 5 minutes

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[fast 99]

Change to a thicker oil 10-30 or 10-40. If it continues need to confirm with mechanical gauge. If indeed pressure is that low how old is the engine? Could have a restriction, possible pickup plugged with crud or RTV? Could also be holding the oil on top if drain back holes are restricted. Other possibilities are worn bearings and or oil pump, pickup at wrong angle.

 

[ChuckN]

Can’t tell from the pic- is that residual on the dipstick and it might actually be dry?

If I’m wrong and it’s where it should be, this is what I would do if it were my truck.

If I were fairly certain that it wasn’t a knock (you’d mentioned you’d heard something) I’d hook up an actual gauge to check apples to apples for accuracy before going to a thicker oil.

If you’re worried about a knock, I’d pull the pan and make sure the oil pick up fit tightly or wasn’t clogged.

A wise man here once said that these are just indicators and having an actual mechanical gauge to verify is critical. If the gauge checks out, and your oil level isn’t low, the oiling system is dead simple. You have a pump with a pickup, a filter with a bypass mechanism in the block, oil passages and bearing clearance. That’s just about it. Well, then you have the oil pump pick up that can fall off or not fit tightly. But you get my point.

What does your spidey-sense tell you?

 

[ChuckN]

More or less, @fast 99 beat me to it.

 

[Ricko1966]

Low knock at idle,with low oil pressure,is usually main bearings. Put a good mechanical oil pressure gauge on it to confirm readings I don't like the way your gauge reads with engine off. It is possible that there is trash in the oil pan ,as you drive a little gets sucked into the screen,then more,then more until the pickup is blocked. You turn off the engine the suction goes away and the debris falls off the screen. A low end knock though, listen hard,eliminate everything you can but,usually a main.

 

[fast 99]

More or less, @fast 99 beat me to it.

Were on the same page. Actually, most 5-30 oils shouldn't be used with flat tappet cams, additive package is wrong.

 

[ChuckN]

Were on the same page. Actually, most 5-30 oils shouldn't be used with flat tappet cams, additive package is wrong.

Man, good call- I don’t think I’ve ever seen an oil that low in viscosity with high zinc content (unless added afterward with Lucas ZDDP, etc.)