5 ton rpm

[TadB]

I have a question. A week or two ago, I was having problems starting my 818. When i picked it up from GL, we put new batteries in it, bumped the start switch and it fired up and ran great. Over the next couple weeks after driving it several hundred miles, if i stopped somewhere and let the truck idle, it would start to surge(idle up, down, up, down. It also got harder and harder to start.

I finally got time to fool with it. From everything my mechanical ability told me the mechanical fuel pump was having problems. I could pressurize the fuel tank, crank the truck and it would run until i took the pressure off of the tank. I took the pump off and took it to a diesel repair shop. They bench tested the pump. They said the pump it putting out about 165 psi and it should be pumping about 170, but its fine.

My first question is. What other things should i look at to be causing this( truck hard starting and surging)? Next question, the guy at the shop told me he could shim the pump and change the gov spring and get more fuel pressure and rpm's. Can he really do this and if so whats a safe rpm to run on an 8000 mile 250 Cummings?

At this point im pretty sure im pulling air somewhere in the system. I wish i wouldn't have jumped the gun and pulled the pump. Oh well, cant do anything about it at this point but put it back on and fix it. Any help would be much appreciated.

 

[jwaller]

air leak in the fuel system. hundereds of posts covering the entire system. search can and will aid you in your quest.

start with the spider valve and replace anything not solid. ie all rubber lines.



you can use a little more fuel but NO MORE RPM'S.

 

[Ferroequinologist]

Yep, sounds like air in the fuel lines. We have a locomotive with Cummins model B engines, and they will run away if they get an air leak! had it happen several times. That's why they took them out of OTR trucks long ago.

 

[Stalwart]

How in the world would an air leak cause a run away? Where is the fuel source to cause this? I know badly leaking turbo seals can do it, or any source of fuel being introduced to the intake tract but an air leak?

 

[mudguppy]

how about dirty/plugged fuel filters? weak or non-funtioning lift pump?

 

[Earth]

how about dirty/plugged fuel filters? weak or non-funtioning lift pump?

Yeah, change the fuel filter--NAPA 1954 I think--just to be sure. And clean of the top of the IP real, real good and clean the strainer under the big flat screw--just to be sure. But it probably isn't the filter. As stated, it's an air leak--these guys are very prone to this problem.

No lift pump in a 250 NHC.

 

[Stalwart]

Yep, sounds like air in the fuel lines. We have a locomotive with Cummins model B engines, and they will run away if they get an air leak! had it happen several times. That's why they took them out of OTR trucks long ago.

B engines are still being used today, the current 6.7 is a "B" engine and so were the 3.9 and the 5.9 and I've never heard about them running away from anything but a bad turbo seal.

Now Detroits, the 2 strokes, had a bad tendency to run away and most of the early ones I've seen had emergency stops at the blower inlet.

 

[Earth]

Whoops, I gave the oil filter # (1954) The fuel filter is NAPA #3401

Also, while a current conversation on air sucking 5 tons is in play here, Does anyone have a source, p/n for the selector valve?

 

[Westech]

I am going to change the filter to a spin on type in my 813 also looking in to a air remover pump that the Diesel guys use.. just trying to find a cheaper one rather then spend 300 bucks

 

[Capt Pat]

The fuel system air leak sounds like a prime culprit, the lift pump second. I've a pair of 6B-5.9's in a govt. boat I run and a friend has one on a firepump on his boat. The cummins mechanic stated these are a great utility engine in forklifts and such, but that is about it, otherwise they guarantee him job security.

 

[Hammer]

NHC-250 does NOT have a lift pump. The IP does that function. And if that guy checked out the IP, that would have been one of the tests he SHOULD have performed.

Which is also the reason that any air leak is a PITA for these setups.
Pull the tool box under the driver's door. Replace ALL those rubber hoses with some GOOD hose. Fittings are all reusable, so you only need more hose for those fittings.

To check for air leaks, apply a LOW amount of pressure to the fuel tank, leaving that small vent line OFF. If there is an air leak somewhere, it will start leaking fuel when there is pressure to the system. Repeat for other tank. Don't forget to move the tank selector around when testing tanks.

Also note, make sure the tank selector is ALL the way to either the left or right tank, it shuts the fuel off if left in the middle.

 

[Monty]

NHC-250 does NOT have a lift pump. The IP does that function. And if that guy checked out the IP, that would have been one of the tests he SHOULD have performed.

Which is also the reason that any air leak is a PITA for these setups.
Pull the tool box under the driver's door. Replace ALL those rubber hoses with some GOOD hose. Fittings are all reusable, so you only need more hose for those fittings.

To check for air leaks, apply a LOW amount of pressure to the fuel tank, leaving that small vent line OFF. If there is an air leak somewhere, it will start leaking fuel when there is pressure to the system. Repeat for other tank. Don't forget to move the tank selector around when testing tanks.

Also note, make sure the tank selector is ALL the way to either the left or right tank, it shuts the fuel off if left in the middle.[/quote

Ditto

Check the fuel tank selector.

 

[bevanet]

Just bought an M817. It starts very hard, won't idle very long and surges a lot. Also when I let off the throttle to shift it stays reved up for a few seconds. Does this seem like an air leak as well? What is the best way to "apply slight air pressure to the tanks" Do I connect and air hose to the vent line? About how many psi should I use?

 

[Hammer]

I pull the SMALL vent line out of the short piece of rubber hose that comes off the top of the tank. I take a simple air nozzle and push it against the rubber hose and apply pressure.
Setting your pressure to about 5 psi is plenty for this kind of testing btw. I don't do this, but it is a safer way to do it.
You will feel the excess pressure coming back out the other end of the tube (the top part you just pulled out). Plus hear the pressure relief some where up front.
I personally like to do it with the fuel tank somewhat low on fuel. That lets the tank fill up with air pressure and gives you a little more time to find the leak.
It also helps to not put TOO much pressure in the fuel tank!

Still, it takes very little pressure to find the leaks. Mine all appeared immediately after applying pressure.

 

[Hammer]

Oh, and your situation sounds exactly like the symptoms seen with an air leak of some sorts.

 

[Pappa-G]

NHC-250 does NOT have a lift pump. The IP does that function. And if that guy checked out the IP, that would have been one of the tests he SHOULD have performed.

Which is also the reason that any air leak is a PITA for these setups.
Pull the tool box under the driver's door. Replace ALL those rubber hoses with some GOOD hose. Fittings are all reusable, so you only need more hose for those fittings.

To check for air leaks, apply a LOW amount of pressure to the fuel tank, leaving that small vent line OFF. If there is an air leak somewhere, it will start leaking fuel when there is pressure to the system. Repeat for other tank. Don't forget to move the tank selector around when testing tanks.

Also note, make sure the tank selector is ALL the way to either the left or right tank, it shuts the fuel off if left in the middle.

X2 following these instructions fixed mine!

 

[Hammer]

Oh yeah, when replacing hoses, do ONE at a time, that way you don't reroute your return fuel line to the wrong tank

 

[bevanet]

Hammer, Thanks for following up. I won't be able to get to it for a few weeks because of the snow storms, but I'll post back once I get it fixed.

 

[bevanet]

I am about to head up to the truck. Does anyone know what size of fuel line it uses. It is a ways to a part store, so I would like to bring some line to save a trip. Thanks

 

[groundog]

should be half inch for pressure and 3/8 for return---I would also re-place return line on top of fuel tanks---I removed the step as well as the too box