Cummins stalls at low RPM

[DMR]

RT25 with 2010 Cummins merceriser 130 HP stalls occasionally at low RPM. Primary and secondary fuel filters just replaced. Once smartcraft alarm "Low Fuel Pressure". At 700 RPM fuel pressure 30-35 psi at higher RPM pressure up to 70-80. Never stalls at higher RPM. What is recommended fuel pressure? Does idle speed need to be higher? Does the fuel pump need replacing? Thanks for advice.

 

[NorthernFocus]

I've got the same engine w/Smartcraft panel and don't have fuel pressure reading. It sounds like you're reading oil pressure. The numbers you posted are almost exactly what my oil pressure runs at idle and high rpm. Fuel pressure runs upwards of 1500psi.

It certainly sounds like a fuel supply problem. But typically if there is a restriction in the fuel supply it gets worse at higher rpm, not better. Which leaves the fuel pump, fuel pressure control valve, or the rail fuel pressure(RFP) sensor. The sensor or valve seem more likely than the pump with the symptoms you've described. Replacing the sensor is the easier/less expensive next thing to try vs the pump or valve. If you can stand the boat being out of service for a few days you can pull the sensor and get the bosch part number off of it rather than ordering it through Mercruiser. The Merc p/n is 879172105. It is located on the end of the fuel rail.

The idle rpm is controlled by the ECM. To change it you would need the Mercruiser software to do so. But that would simply be masking the real problem anyway.

 

[DMR]

Thanks, NorthernFocus. You are right about the pressure readings - they were oil pressure. The engine started poorly today with "559 low fuel pressure". Not sure how the rail fuel pressure sensor is at fault, since the fuel pressure was identified as the problem. Doesn't that mean the pressure sensor is working or does the pressure sensor control also provide a feedback control of the pressure via at the pressure control valve? I will try your suggestion.

 

[NorthernFocus]

Thanks, NorthernFocus. You are right about the pressure readings - they were oil pressure. The engine started poorly today with "559 low fuel pressure". Not sure how the rail fuel pressure sensor is at fault, since the fuel pressure was identified as the problem. Doesn't that mean the pressure sensor is working or does the pressure sensor control also provide a feedback control of the pressure via at the pressure control valve? I will try your suggestion.

Pressure sensors can still function but give an erroneous reading. In other words pump could be working and pressure is actually fine but the sensor is reading low. The ECM controls the pressure control valve based on the reading from the sensor. So if the sensor reads low the PCV will actually close. The PCV failing open could also be the problem. It is most active at low RPM. But without diagnostics if troubleshooting by changing parts the sensor seems like the easiest. Also the manual warns that the PCV has to be matched to the pump by a Bosch shop.

 

[DMR]

Hi Dan. When your Cummins had a hard start problem [that was cured by replacing the ECM] did it also stall at low RPM? I have been having a hard start (takes 2-4 tries) and after starting it stalls at low RPM with the low fuel pressure message. The way to start now is to pump the primer. At idle it has a repeating "death rattle" that gradually gets worse and ultimately stalls out. I will be taking out the rail pressure transducer but wondered if this could be the ECM which was never replaced.

 

[NorthernFocus]

Oh. You didn't mention the "death rattle" which I assume means missing/running rough before dying. I thought you indicated it was idling along fine at 700rpm and then just got the alarm and shut down. Sounds like you do indeed have a physical issue if that's the case. The PCV(aka return valve) does most of its work at idle/low rpm. If as you indicated before the engine is running OK at higher revs then it may indeed be a PCV and/or pump issue. Unfortunately without the Merc software I don't think there is any way to see the actual rail pressure.

I did have the starting issues described by others when we first purchased the boat. Or thought I did. In my case they were easily resolved by installing a starting battery that met the engine spec. The original batteries supplied with the boat were under sized. If I remember right I did verify that the part no. on the ECM matched the number of the upgraded ECM. Unfortunately I've slept since then and don't recall the details. But battery power made my issues go away and 1200 hrs later have never recurred. You can search the forum here and find discussion on the part no. for the upgrade. However from what I remember reading from other peoples posts there were no other issues with the ECM. Just difficulty starting.

 

[DMR]

When I first start it (after priming) and run on idle it purrs but rapidly changes to rattle-purr-rattle-purr and the rattle gets longer and longer until it stalls. Makes me think the valve or fuel pump are not regulating properly or air is getting in. But if air got in, it would not work well on higher RPM. And at higher RPM it works fine.

 

[DMR]

Hi Dan,
I pulled the sensor (after pulling the fuel rail and cursing a bit) and the sensor number seems to be 816 >PBT-C-30< (or CF30). I tried to find a Bosch web site that listed this fuel pressure sensor but could not find one. The Mercury part number ordered from my local Mercury dealer it is $500. Any suggestions for how to find a less expensive Bosch version?
David

 

[NorthernFocus]

Sorry for slow reply but I just returned from a week on the PWS w/no web access. Not sure if you've resolved your stalling issue or not by now.

Regarding the sensor I'd try walking into a NAPA store with it in hand. The fuel system parts are common to trucks/autos. However, per my last post what you are describing with the engine sputtering before shutting down it does sound like a physical problem with fuel delivery. While in the NAPA store you might want to inquire if they know of anyone locally who can service Bosch fuel system components. Crowley on line sell the sensor for under $400 but that's still pretty steep. I'd keep looking for a direct Bosch supplier.

When you said in a previous post that the engine runs fine at higher RPM were you talking about loaded or revved up in neutral?

 

[DMR]

Hi Dan,
It worked well under full RPM with load. The update is thatI have checked with NAPA and other auto stores for cross listing of the sensor and they can find no Bosch or other version. So I ordered one from Mercury for $475 but have to wait a month - they are back ordered. I have been looking through the fuel system more carefully and find that there is no shutoff valve as listed in the RT25 manual. Two years ago I removed the Mace diesel generator (too noisy) and just connected the fuel in to fuel out and wonder if that could compromise flow to the engine. I am thinking of just disconnecting the old Mase fuel loop from the primary filter housing. Two diesel mechanics thought it was a fuel restriction problem that manifested only at low demand when the draw pressure was low (draw pressure increases with RPM) and recommended I add a vacuum gauge to the fuel line after the filter to try isolate the restriction (e.g., gel/junk in tank, vent restriction, filter). I don't know how to get access to the tank to polish the fuel.
David