Cummins NT855 F2 on fire pump

[sjs2011]

Hello all,

We have an old diesel engine built in 1975. It is rated for 285 HP @ 1750 RPM. The problem is, as soon as engine loaded with fire pump, it's rpm drops down. For some reason it is not maintaining constant rpm. It drops down to ~ 1300 rpm (per manufacturer spec, there will 8% of rpm drop but this is more than that).

We have checked the fire pump and looks fine. We suspect it is the fuel system of the engine, however, two different companies had looked over the fuel system and didn't find any problem.

I believe that fuel pump also working good since I can bring up rpm by moving throttle shaft setting manually on loading condition. I am not sure if the governor setting inside the fuel system box has to do with this behavior. In other words, it seems that something is not telling fuel pump to crank up to meet load requirement or to maintain certain rpm.

All engine heads temperature are consistent and no sign of black smoke indicating combustion issue. Fuel and air filter have been checked and are ok.

Can any one help to resolve the issue?

Thanks in advance.

 

[rmw]

What does "looked over" the fuel system mean?

If it means that they came and passed their eyes on it and waved their hands a few times that isn't good.

If it means that they took the pump off and the injectors out and took them to a specialty shop with a test stand and ran them and calibrated them on their respective test stand(s), then that is not good.

Has anyone run the overhead on the engine meaning set the lash on the valves and the injector plungers. Sorry, but it has been a long time since I did this and I can't remember the settings. I can remember that when the engine I owned got 'sleepy', running the overhead would often 'wake it up'.

If I am right, the fire pump engine is a naturally asperated engine isn't it? If not, then you might have the turbo checked out by the same people that do the pump. I'm betting it doesn't have a turbo, however.

Has anyone had their fingers in the pump at all? There are some places where guys can 'monkey' with things and that can make things better or worse depending on what was done. It is not a good thing for people who don't know what they are doing to adjust a pump on an engine. The test stand is the best place to do this. When you get it back from the shop, put finger nail polish on the ports where the pump can be tampered with. The shop can tell you where those are. Drivers who couldn't keep their fingers out of the pump cost me more than one engine.

rmw

 

[CRG]

Has the engine/pump combination ever worked correctly since you have owned them?

What does “We have checked the fire pump and looks fine” mean? Have you matched the fire pump performance with the manufacturer’s pump curve? Keep in mind that the fire pump will require more horsepower the lower the discharge pressure. You will be able to see this on the pump curve.

What does the manufacturer list for the discharge pressure that is associated with the 8% drop in rpm?

 

[rmw]

I looked this engine up after posting above, and it is turbo'd. So it needs to be checked as recommended above.

That said, if it is the pump flowing more water against a lower discharge pressure than the engine is rated for, the main symptom will be smoking and smoking bad black.

That series of engine wasn't the high torque rise type and running at 1300 RPM would produce lots and lots of smoke.

Since the OP didn't mention smoke, I never suspected the pump to be overloading the engine.

rmw

 

[sjs2011]

Thank you guys for your reply. I will try to include response to everyone:

1. Fuel pump has been tested by 2 independent diesel engine companies and found to be ok. Believe they didn't tell how they check it. Probably it is bench testing. However, I haven't seen it this process, so I can't totally eliminate this part.

2. Yes, it is turbocharged.

3. No significant change in the smoke almost same throughout testing. So we think that no issue with engine itself.

4. We called fire pump system supplier and told about the issue. They don't deal with engine but said that if you uncouple driver from pump and can rotate pump by hand then pump has almost no issue. Pump is rated for 2500 gpm and 289 ft. We could rotate pump shaft freely, didn't feel any resistance.

5. The last known working year was 1996 and It was also tested in 2002. However, at no load they started with ~ 2100 rpm, so load applied (water flows through pump) it dropped down rpm to 1760 rpm which is sufficient for pump to work for rated capacity and it passed the test. Note that 2100 is 20% over speed of 1750.
We were not owner of this system until couple of years.

6. Inspection company adjust the pump flow rate to rated flow capacity and then measures discharge pressure. Of course it means that pressure is way less than rated press. Since rpm is not at where it is suppose to be.

7. We measured fuel pressure while testing the engine. At no load, it is ~50 psig and between 50%-100% load it drops to 40 psig which is not good. However, if I can bring up rpm manually by adjusting throttle shaft and that gives me 100 psig fuel pressure. I read in the manual that fuel pressure can 50 - 200 psig on this model. So I suspect fuel pump itself is ok but the mechanical feedback system that tells pump to crank up pressure is not doing good.

8. Nothing mentioned about pressure. They just say that moving from no load to load condition can cause 8% speed drop. I guess that the way it has been designed.

 

[mechanic6]

i am assuming that this is a firepump on a fire truck.

usually when a diesel egine will start and run ok,but fail to pick up a load as in your example THAT is a lack of fuel supply.

You said the pump was checked and found to be alright,how about the fuel supply to the pump?.

I have seen this many times,it is not as common as in years past but it still happens

Check that the fuel system is free and unobstructed right from the tank,fuel hoses,filters(put new ones on).
I have found on older trucks that sometimes a small screen was installed on the fuel pipe inside the tank.
this screen WILL plug ,anyway check that out.

the fuel hoses may be deteriorated inside and are blocking fuel flow as well.

try that.

 

[dicer]

Its time to dyno the engine, any large truck shop, especially a major engine one like cummins will have one.
Need to see if the engine is making rated power.

 

[Sabinomecanico]

The fuel pump has a "feed" pump, commonly known as gear pump...

Gear pumps typically fail due to low flow/pressure at low speeds (cranking) if you can take a photo to the fuel pump and the gear pump I can help.
Who tested the fuel pump?

 

[mechanic6]

dicer

with all due respect,putting any engine on a dyno would be the very last resort.
if a cummins will start and run ok but will not 'pull a load',and the pump checks out ok off the truck,its most probably fuel supply.
unless its something really out of the box,such as squirrel nests in the air intake(yes i have seen that)

 

[sjs2011]

Thank you all again for your valuable suggestions and feedback.

This system is a part of plant fire protection, it's not on a fire truck.

We have nailed down the issue. Even though engine manual says governor maintains constant speed, it doesn't especially with change in the load condition. There is no automatic adjustment on this particular model. In other words, it is more like a constant load engine not speed. It means when you set throttle shaft at no load condition, engine will deliver certain power. However, once load is applied (fire pump) rpm drops down to keep up with constant load. The governor is meant for over speed protection.

Solution would be set the throttle shaft for rated rpm at full load and it's good to go!

 

[waross]

I run into this problem when boat mechanics set up engines for generators. There is an adjustment to limit the maximum amount of fuel that the governor will deliver. If this is not set properly the result is similar to driving up a hill without increasing the throttle pressure. Not enough fuel and the engine slows down. It sounds as if you have found this adjustment and are good to go. This is not actually the throttle, it is the maximum fuel limit but from your description it is the right adjustment. A shop would have to test the governor and injectors together for fuel delivery to adjust this. A change in fuel specific gravity may affect this if it was originally set with a heavier grade of fuel and you are now using a lighter fuel.
But, don't disrespect the other suggestions. This is a good time to check anything that may restrict fuel flow.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rmw]

Bill's post reminded me of the term the drivers used to use when they monkeyed with this setting on a Cummins. They called it putting 'buttons' in the fuel pump and it was something that they did to an orifice or changed about the orifice in the fuel delivery system. What ever it was they did, it gave them more fuel and in my case, fried an engine.

Guys that drove Detriots back in that day (there were still a lot of 2 stroke Detriots out there then in both 6 and V-8 versions) would clamp the fuel return line with vice grips to force up the fuel pressure in the fuel rail system in the heads which would force more into the injectors.


rmw

 

[gruntguru]

sjs has the problem nailed. If operating the throttle manually allows the engine to drive the load to the correct speed with minimal smoke, the engine and fuel system is fine and the problem lies in the governor. Sounds like it has a truck or marine type governor whereas a genset governor would be more appropriate.

Engineering is the art of creating things you need, from things you can get.