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Oil pressure does not change with RPM

4K views 54 replies 11 participants last post by  Robinls5 
#1 ·
So here is the situation.... I have a 72 Chevelle with a 396 and a 4 speed. About 6 or so months ago I had to pull the motor to fix the oil pickup tube since it fell off. Welded the pickup tube and put the engine back in and the oil pressure was great 10-20 PSI per 1000 RPM. Drove the car around town a little and no issues. I take the car out for a decent drive on the freeway and I start having problems. I put about 60-70 miles on it round trip on the way out there I had no issues 55 MPH on the freeway good oil pressure. I get almost home and stop at a light and the oil pressure drops to 0 but then bounces back to a decent number. I baby the car home making sure the oil pressure stays alive. This was back in May fast forward to today when I get the chance to play with the car again and I start it up and the oil pressure is good 18 PSI at idle (600-800 RPM) and 9 PSI at idle when I was on a down slope. I brought the RPMS up to like 2200 and there was no change in pressure sometimes when on the hill it actually dropped. I'm lost here my pump is obviously working cause it's got pressure but revving the engine does nothing to change it.

When I first started the car I had oil pressure in the 40's at like 1600 RPM.

Any ideas or suggestions I really don't want to pull this motor again.
 
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#6 ·
So are you thinking it held for that one long drive but maybe the engine heat caused something to change along with the welding? There was 2 or 3 tack welds
 
#5 ·
Maybe the pickup fell off again.
Its hard to weld to cast to get it to stick.
I usually grind a groove in the housing,and fill that with weld onto the steel pipe.
Havent had any issues yet with any of my motors.(No jinx!)
Although,on the last one I remember making a bracket,welding it to the pipe,and using the existing bolt down on it.
 
#7 ·
Could be the weld didn't penetrate deep enough and it fell off again. Drain the oil and try to see in the pan or stick a skinny magnet in the hole and feel around. Or, buy a bore scope and stick it in there. Costco..,$59.
 
#8 · (Edited)
I'm 99% sure it didn't fall off cause these symptoms are different than when it did fall off. When it fell off I had great oil pressure until i tried to maintain an rpm.

Sounds like either way the engine is coming out.
 
#9 ·
I found 1 tack is all i ever needed with a snug fitting tube.
I did have symptoms about like yours.. Oil pressure during stopping would go to 0 or close and during hard acceleration it would get very low or NO oil pressure.

My tube was still attached but the lower section fell off the tube... so all that was left was the tube.

I always buy a new one now.. That one was from 1969 300HP 350 that failed on me.
 
#11 ·
New pickup is what I buy each time... sometimes I get a new pump .. but If I have an old pump that checks out ok I will use it.

There are exceptions though.. If the pickup tube is a performance deal that bolts the end of the pickup to one of the pump cover bolts I will inspect it really well and use it.

But the old original press on factory tubes with 100,000 miles on them will get replaced every time.

The one that failed me was on the car for 100,000 miles and I reused it .. at 70,000 more it failed.

Just saying .. Yes that is a long life.. Only one I had fail me.. but the replacement is pennies compared to what must happen to fix it.

Either way... Looks like your oil pan needs to be removed.
 
#13 ·
I can go buy a cheap mechanical gauge but this port and gauge has worked great for the 125 miles I have put on the car (plus all the non miles from tuning) but I get your point a cheap gauge could save me hours of pulling a motor.

If I were to replace the pump I was thinking about getting the moroso ones with the tube already welded. Which one would fit the car? 67 chevelle 396 motor. Also should I put a high volume pump on it (not sure whats on it right now). Is my other option to buy a new spring for the current pump and just replace that?
 
#15 ·
Flat level surface has same issue. Minor pressure changes on a slope. Worked just find in all situations until the issue
 
#16 ·
So I'm leaning towards putting in a high volume oil pump since I have to pull the motor I am not sure which pump is in it but I'm trying to get all the pieces together for a 1 day out and back in repair. I'm leaning towards a moroso setup since the pickup is already welded but I'm not sure which P/N fits the car with the stock pan.

Is this the one I'd want: Oil Pump and Pickup Package

The engine was rebuilt about 8 years ago but I don't know what they did for an oil pump (wasn't on the build sheet). I'm assuming for a $150 bucks it's probably best to just get a new pump instead of a spring since I don't know if the pump that is in there is high volume or not. I'm open to suggestions.
 
#18 ·
I was just reading a post about that and I did have good pressures before this happened. I should just put a new spring in the pump and run it would be the smart thing to do. I assume the spring replacement is easy. Can anyone point me to which spring I should buy? I also know some people have the opinion of changing to a slightly higher pressure spring.

Does anyone have a write up of how to replace this spring?
 
#23 ·
So I guess I have the following questions:
1) Is there a good chance my existing pump is good and there is just a bad pressure relief spring?
2) Were the Mellings pumps standard for these motor or an after market swap?
3) I suppose the best path would be to replace the spring (if I go that route) and then prime the pump with the motor out of the car and see what the oil pressure is.
 
#24 ·
Let me ask another question just because I'm trying to understand what is happening....if it were the pressure relief spring wouldn't you expect the oil pressure to remain constant in any rpm and not drop when increased?
 
#25 ·
depending on clearances and the volume of the pump.
If you do not have enough pump volume to fill all the leaks the pressure will not be there.
As you spin the pump faster the volume increases and the pressure will follow until it hits the relief pressure.

You can actually check the pressure of the pump spring when you have oil pump in hand.. If you have an air compressor.
You will need an adjustable regulator on the compressor.. most have those.
 
#26 ·
I can check it once I pull the motor but i'm trying to have all the parts in hand before pulling the motor so i can do everything in 1 day. I'm just lost as to why the pump would be working just fine good pressures and everything and then at the end of a long drive starts to have pressure problems. being able to hold pressure and stuff. For example now I start the car and the oil pressure goes to 30 but then drops. I'm trying to pick the component that is most likely going to fix my problem and order that. Right now I'm thinking about getting the moroso pump and being done with it. My question is if it isn't the oil pump nor the drive for the oil pump what inside the motor could cause this issue and can I eliminate it as a possibility now.
 
#27 ·
Dustin:

1st. buy or rent a mechanical pressure gauge and verify that you actually have a pressure problem, and not a gauge or sending unit problem.
2nd Has oil been changed recently or is it contaminated with fuel causing it to be EXTRA thin?
3rd Its hot out... maybe try the little heavier oil and see if it changes. not sure what weight you are currently running...
4th Since it hasn't been mentioned, excessive bearing clearance can cause pressure drop. If you end up pulling the engine, I would also take a look at the rod & main brgs.
 
#29 ·
Dustin:

1st. buy or rent a mechanical pressure gauge and verify that you actually have a pressure problem, and not a gauge or sending unit problem.
I already bought one and plan to do this check soon.

2nd Has oil been changed recently or is it contaminated with fuel causing it to be EXTRA thin?
I didn't smell any fuel on the dipstick when I have checked the levels.

3rd Its hot out... maybe try the little heavier oil and see if it changes. not sure what weight you are currently running...
I am running the AMSOIL ZROD stuff (I forget the weight right now). This occurred on a cold motor.

4th Since it hasn't been mentioned, excessive bearing clearance can cause pressure drop. If you end up pulling the engine, I would also take a look at the rod & main brgs.
The engine is coming out I'm just trying to plan ahead.

There are no leaks so the oil is staying in the motor and I'm using the front bearing port (I think that's what it's called on the drivers side) as my oil pressure pickup (same place it's been forever)
 
#28 ·
Nice to be able to do it in 1 day. sometimes plans fall through.

There are many things that can make oil pressure drop.. bad lifter.. cam bearing clearances spun bearing.

You basically have either a problem getting oil to the pump or from the pump into the engine.. OR your engine had developed a big leak somewhere. basically a big hole in the water hose.

One thing for sure it will need to come out. Or the oil pan needs removed in car,,, that is a pain.

You will know more than the rest of us once the pan is off and you have the pump in hand.
 
#30 ·
So I did a little testing today and I think I have 1 of 3 problems.... 1) Oil pickup fell off 2) Oil pump is bad 3) Oil level is low even though dipstick shows overfull. Here are a few videos the ones with the mechanical gauges are initial ones before I added 1 QT of oil (then the line broke and I lost a bunch of oil). The startup was on a level surface the other mechanical one was on a nose low hill (driveway).





Thoughts?
 
#32 ·
Gonna have to run it at least 1 more time to turn it around. It seems like oil pressure is good as long as I don't hold a high rpm
 
#33 ·
That is fine but I would quit testing things after it is in place.
Aint fun pulling them I know.

I broke a cam in on a fresh rering deal for my 47 Dodge.
Everything went really perfect.
Pulled it into the street to drive it and it began missing.. back up the driveway and did a compression test.. one hole was 60 PSI the rest over 200. Did a wet test . no go.

Pulled the hot head right there.. Pulled the valves on that hole and under an intake seat I found what looked like a gnat wing.

Yes the insect.. it was a tiny tiny thin piece.. I removed it .. lapped the valve.. it was running again less than 2 hours.

That is easy to work on though.
I drove it 8 miles and the NEW B&M converter gave up .
Spun something in it that is press fit on the lower priced converters.
Only 1.25 mile walk home.

Crap happens!
 
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