Cummins 6CTA Low Oil Pressure

LMBoat

Well-Known Member
Oct 4, 2006
748
Ft Lauderdale
Boat Info
1999 450 EB
Engines
Cummins 6CTA's
I have two 6CTA’s in my 450EB (2400 hrs). I just changed the oil 5 hrs ago (Rotella 15w-40, same oil I’ve been using in these engines for 11 years). Oil pressure is always approx. 50 PSI for both engines (helm gauges). Yesterday, strb engine went down to 20 PSI (verified with mechanical OP gauge on the engine). Engine runs as normal, no leaks, etc. Low oil pressure alarm did not go off.

Since the oil change, the oil in this engine has been “clearer” (more honey color, vs black as in the past, and other engine). My first thought is something leaking into oil (seawater, coolant, fuel, etc.) and diluting oil. I’m not losing any coolant, can’t judge the others. And the oil level has not been rising. I did an oil & filter change this morning, but the oil pressure is still low. I have pulled oil samples for testing, but won’t know the results for a few days .

My aftercoolers were pulled and done 6 months ago. All was good.

Any ideas on what might be going on? Is there some type of pressure adjuster, spring, etc., that might cause this issue?

Thanks for any help you can provide.

Larry
 
Oh, and one more thing. When I rev-up the engine, oil pressure stays same. It used to go up a bit as throttle was advanced.

Thanks
 
You may have a bonefied issue if both the dash electrical gauge and the engine's mechanical gauge are showing the same. The alarm is from a separate OP switch which probably has a lower set point.
Regardless, not good. You may have debris plugging the pickup screen in the oil pan. You may have the pressure regulator / relief valve stuck. I'm not sure on your engine but it is not a big deal to pull the oil pump or pressure relief valve on the QSM engines. First things first is to get the oil analyzed.
As an edit - while you are taking the oil sample get a separate test gauge on the engine to measure the oil pressure while warming the oil.
 
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Have done lots of work on these engines in off highway applications. With the exception of the marine bolt-ons, the base engines are the same though.
An aftercooler is for charge air and would not have any influence on the oil pressure.
Oil temp and viscosity will though. When you say the pressure dropped to 20, are referring to cruise or idle rpm? Is it both start up and operating temp? 20 psi would be acceptable at idle but getting dangerously low with the engine under a heavy load. The alarms on most mechanical engines could be as low as 7 psi. They do not compensate for rpm like a lot of the newer electronic engines can.
The oil pressure regulator (spool and spring) is a large plug screwed into the bottom of the oil cooler cover and is serviceable on the engine. If the spool hangs up, it would likely effect high or low pressures and sensitive to oil temp (viscosity). If the spring were to break (which I have never seen) it would likely effect low pressure across the rpm and temp range. The large plug in the top of the oil cooler cover is for oil cooler bypass when oil temp is low.
There is another valve in that cover that is seldom mentioned. It is a bypass for the oil filter itself. With the oil filter off, it is located up inside the filter head from the bottom. Have seen it fail twice now and drop pressure across the rpm range.
I'm heading off to the bush today but will bring my manuals home with me tonight if you have more questions.
 
Thanks for the comments. I'm waiting on the oil sample analysis. 20 PSI was at 2200 on plane. A little lower than idle.
 
Update: Oil analysis came back. Sodium was high at 12.3 (last one a few years ago was 6.0). All else was fine, no metals, etc. Swapped all oil senders, mechanical pressure qauges, and pressure regulator. Problem remains at the same engine. Looking at the oil filter housing next.

The only other things I can think about are a clogged intake screen on the oil pump or a failing oil pump, but I've never heard that. They usually work or don't

Other ideas?

Thank for the help.
 
Update: Oil analysis came back. Sodium was high at 12.3 (last one a few years ago was 6.0). All else was fine, no metals, etc. Swapped all oil senders, mechanical pressure qauges, and pressure regulator. Problem remains at the same engine. Looking at the oil filter housing next.

The only other things I can think about are a clogged intake screen on the oil pump or a failing oil pump, but I've never heard that. They usually work or don't

Other ideas?

Thank for the help.
Yes the pressure regulator/relief valve on the oil cooler.
 
I vote regulator. Inspect ball, spring and seat. I have seen oil pressure regulators with different debris stuck between the ball and seat lowering the OP. The last one I saw had a tail of a cotter pin jammed in. It was on my brother's O540 Lycoming on a Pawnee and we traced the tail to a cotter pin in the accessory case that someone had clearly reused!
 

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