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Discussion Starter · #1 ·
Hi folks, I have a ‘rebuilt’ 4bt that I just completed swapping into my bronco. It’s always had 60+ pounds of oil psi cold, but now after driving it and getting the engine warmed up, the oil psi at idle is dropping very low, around 5 psi unless I bump the throttle up a bit. My idle might be slightly low, but I haven’t checked yet. I pulled the bypass plunger and it seems to move freely. I haven’t pulled the oil cooler to check that bypass. I found out from the guy who built it that the oil pump was not changed.
What do you guys think? Can improper oil cooler gaskets cause this? Should I just change the pump out since they didn’t already? Can’t believe they didn’t do that on a rebuild.
 

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Check the oil pressure with a quality gauge, grade of oil, oil temp, heat breakdown of oil (dino vs synthetic)... the phrase rebuilt can mean just about anything from all new parts to used parts that should have been replaced too.
 

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The oil pressure relief valve which is located on the back of the oil filter housing is factory set at 65 PSI. A cold start showing 60 PSI sounds fine but dropping to 5 PSI indicates a problem. Could be that relief valve has failed and needs replacing. They aren't expensive but you'll need the gasket as well. If during the rebuild the wrong gasket was used that could be a problem. Below are diagrams showing the 2 types. In the first diagram, part #8 is what is typical for most 4bt's. In the second diagram, part #13 is the one found on the STORM type blocks which started around 1998. What does the oil pressure show at normal driving speed? If it's in the normal range at road speed and only low at idle you could have a defective pump. Those pumps come with various flow ratings.
 

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Discussion Starter · #4 ·
It responds directly with throttle. At normal temp it would come up to 40-45 psi during normal acceleration.
 

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It responds directly with throttle. At normal temp it would come up to 40-45 psi during normal acceleration.
Verify the pressure readings with another known good pressure gauge.
 
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Your pressure at normal operating speed sounds fine. Sort of strange that the pressure is only low at idle. Could be a slightly worn pump. Kind of hard to figure these things some times. What is you idle speed? Maybe that is a tad too low. That can cause a low reading. There are oil pumps for these engines with various flow rates but they all pretty much have the same pressure.
 

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Discussion Starter · #8 ·
I have a tach but I haven’t installed it yet. I will do that tonight hopefully. But it seems like I have to bump up the rpms considerably to see a jump in psi.

I’m tempted to put a new oil pump in just because it apparently wasn’t changed already. Who knows what its history is.
I dont have a engine tag, the guy I bought it from says it’s a cpl 767. Could anyone direct me to a good pump to buy? Seems like most that I’m finding are $100 China pumps
 

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I’m running Mobile 15-40 and when engine is cold runs over 50psi at idle (900 to1000 rpms). After getting up to operating temp (180 deg) it will drop to about 10 to 15 psi at idle. Comes right up to 45 to 50 psi under acceleration. Brand new pump with about 13000 miles on it as is the rebuild
 

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The oil pump for a CPL 0767 is listed as Cummins part 3914005 which has been replaced by 4939585 which is the 8th number to replace the original. That one should have a flow rate of 14.6 GPM @ 4200 RPM. Most of those parts probably are coming out of China. Have to remember that the B series Cummins hasn't been built in this country in a long time.
 

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What he
The oil pressure relief valve which is located on the back of the oil filter housing is factory set at 65 PSI. A cold start showing 60 PSI sounds fine but dropping to 5 PSI indicates a problem. Could be that relief valve has failed and needs replacing. They aren't expensive but you'll need the gasket as well. If during the rebuild the wrong gasket was used that could be a problem. Below are diagrams showing the 2 types. In the first diagram, part #8 is what is typical for most 4bt's. In the second diagram, part #13 is the one found on the STORM type blocks which started around 1998. What does the oil pressure show at normal driving speed? If it's in the normal range at road speed and only low at idle you could have a defective pump. Those pumps come with various flow ratings.
What he said. Pull the oil cooler and go thru it thoroughly. I know a guy that went through 4 turbos on account of this. But he wasn't a very knowledgeable guy on Cummins setups. In fact it wouldn't hurt much to shim the regulator either to see a little more from the pump. Since these use a spur gear in a housing that is part of the block this can be a real issue. The block wears along with the pump gears and also there is only the front cam bearing and no others, if there is wear in the block on the other journals, you have a problem only major machining will fix. In any engine cam bearing clearance is more critical than about any of the other clearances.
 

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I’m running Mobile 15-40 and when engine is cold runs over 50psi at idle (900 to1000 rpms). After getting up to operating temp (180 deg) it will drop to about 10 to 15 psi at idle. Comes right up to 45 to 50 psi under acceleration. Brand new pump with about 13000 miles on it as is the rebuild
If your oil pressure is running in that range you have little to worry about.
 

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I have a tach but I haven’t installed it yet. I will do that tonight hopefully. But it seems like I have to bump up the rpms considerably to see a jump in psi.

I’m tempted to put a new oil pump in just because it apparently wasn’t changed already. Who knows what its history is.
I dont have a engine tag, the guy I bought it from says it’s a cpl 767. Could anyone direct me to a good pump to buy? Seems like most that I’m finding are $100 China pumps
CPL has more to do with the actual build and hp rating, what pistons were installed, injectors, what duty rating stuff like that, what it was originally slated to be installed in. Shouldn't have much to with the oil pump or pressure. It;s more for a data base to know who, what and where.
 

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Discussion Starter · #14 ·
Great info everybody! I installed my tiny tach tonight, and increased my idle speed from around 630 up to 880. Warmed it up and now at idle it seems to settle at about 10. Bare minimum, but seems less scary at least haha. I guess I’ll leave it there and watch it. Possibly increase my idle slightly more
 

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Oil pump may be going bad. Atleast your getting a warning. Mine went bad, and took out a low mile (14,400) engine. I would change the oil, cut open the filter, and have an analysis done. Of course a new oil pump may be needed as well.
 

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10 PSI is what Cummins lists as an acceptable oil pressure at idle. 630 RPM was a bit low for idle speed. Engine should have less shake at the higher number.
 

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Have not seen mentioned what oil you have in it.
Cheers Steve
 

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Discussion Starter · #18 ·
10 PSI is what Cummins lists as an acceptable oil pressure at idle. 630 RPM was a bit low for idle speed. Engine should have less shake at the higher number.
Way smoother! I was planning on turning up the idle but I had no idea where it was at.
 

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Discussion Starter · #19 ·
Have not seen mentioned what oil you have in it.
Cheers Steve
Great question. It still has whatever oil the previous owner filed it with to test fire it. Virtually no run time on it but I’ll be changing it with rotella 15-40 shortly
 

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850-900 RPM seems to be a pretty good idle on these engines to help smooth out the shake. You can smooth it out a bit more by installing a fluid type harmonic balancer. Cummins used those on engines mainly in marine applications. Not a lot of people use it because it's a bit pricey. Fluidampr makes one that is identical and it's not cheap either. The shake is just the nature of the beast due to its firing order. The 4bt really doesn't have harmonics like a 6bt. The balancer for a 4bt helps by adding mass to the crankshaft. Engines with heavier flywheels tend to be a bit smoother too.
 
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