Cummins NHC 250 High Idle Problem

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travistodd1

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Camp Verde, Arizona
Hello, I would like to start out by saying thank you to everyone who keeps helping me with my vehicle problems. This forum has been a great resource for getting my M817 closer to running.

I have been working on my M817 for a little while and figure out what was keeping it from running. I can now go out and fire it right off. The problem is it wants to idle very high. I am estimating around 1500 RPM's Here is where I am at with the project.

1. Obtained from the previous owner because it had been parked and would not start after being parked. Sat for several years but unknown just how long.

2. Brought home and flushed out the coolant and changed the oil. Installed new fuel filter.

3. As a note, the fuel tank selector has been bypassed and the lines appear to be in very good condition.

4. It would no build fuel pressure. I replaced the primer pump on the dash and ultimately the fuel pump.

5. The truck fired off but was spraying fuel all over the place. Idle was right where it should be.

6. Discovered a couple fitting had come loose on the pressure side of the fuel line. Tightened the fuel fittings up and it fixed the leak. Now the truck wants to idle high. I am estimating 1,500 RPM (tachometer not working yet).

7. I have been tracing the fuel lines and checking the fitting but can't seem to find where it may be pulling air into the fuel system causing a high idle issue.

The throttle responds as it should, it just will not idle down to where it needs to be. The pump I put on was new and not a reman. It was supposed to be for the M809 series of trucks. Is there an idle adjustment on the pump I am unaware of or something else I need to be looking at? The only thing I noticed different between the pumps was the brass fitting at the fuel line inlet (from the tank) had a different thread size. It was a simple fix with a trip to the hardware store.

Thank you guys for your help. It really is appreciated.
 

WillWagner

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Do easy things first. When the idle flow is set on the stand is is 99.9% right where it should be, I would not adjust it,it does take a special tool to do it. Take the throttle linkage off of the pump....not the lever on the pump, the rod and heim joint off of the lever....and see if that fixes it. Also look for FOD under the pedal that would prevent it from coming all the way back to rest.

Just an insight to how the pump is set, the pump is not driving the stand like it would the engine, the stand drives the pump. All the pressures and flows are published in the fuel calibration books, at a given pump stand speed, there should be XXX amount of flow and/or pressure depending on throttle shaft position. These are made possible with the color coded springs, shims and position of the throttle shaft and screw. This is one of the reason that when I put up the parts manual, I left out injector and pump data. Technically, one can assemble a pump with the correct springs and shims listed in the calibration books and pump will work, the idle and governor cut off will not be correct, but it will make the right power, yes, I did that out in the dirt a couple of times, it is a PITA though.
 

travistodd1

Member
37
10
8
Location
Camp Verde, Arizona
Thank you guys. I will mess with it a bit more. I guess what is throwing me more than anything is how it was at a great idle speed. I turned it off and tightened a couple nuts then poof, high idle. It was leaking really good, I wonder if it is simply getting more fuel now causing the high idle issue....
 

travistodd1

Member
37
10
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Location
Camp Verde, Arizona
I unhooked the linkage and it didn't make any difference in the idle speed. I am doubtful I have the "specialty" tool needed to adjust the idle speed. I am happy to order whatever I may need but was curious how likely you think this is the problem? It is a last attempt type of thing or pretty common?

Thanks again,
Travis
 

simp5782

My safe word is Pork and Beans.
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Pull the 7/16 plug and its a small flathead screw driver inside the hole. Doubt that's your issue though.

Sucking air can cause a high idle. Or something in the pump is sticking
 

travistodd1

Member
37
10
8
Location
Camp Verde, Arizona
What did you tighten before the issue started? Loosen them and see what happens
It was the fuel line from the pump to the front of the head. I was loose at both ends and spraying pretty good. I am hoping to work on it for a few minutes before I go to work tomorrow. If I don't, it will be the weekend before I can get back to it.
 

WillWagner

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Just for S&G, take the return hose off of the steel line on the engine and see what happens. It WILL make a mess, if you can put a hose over/on the fitting and drop it into a container, it will make your life a little less messy. If you don't know where it is, post a full size pic and I will edit it. While you are at it, snap one of the throttle arm.

The issue happened as soon as you tightened the fitting to get rid of a leak? Was it a supply or return leak?
 

travistodd1

Member
37
10
8
Location
Camp Verde, Arizona
Just for S&G, take the return hose off of the steel line on the engine and see what happens. It WILL make a mess, if you can put a hose over/on the fitting and drop it into a container, it will make your life a little less messy. If you don't know where it is, post a full size pic and I will edit it. While you are at it, snap one of the throttle arm.

The issue happened as soon as you tightened the fitting to get rid of a leak? Was it a supply or return leak?
Sorry for the delay, back at work for the week and I don't have a lot of free time. It was a supply leak (pressure side) and was spraying all over. Right now I have the throttle linkage unhooked at the arm and it made no difference. I plan to tackle it again this weekend.
 

travistodd1

Member
37
10
8
Location
Camp Verde, Arizona
Ok, today is likely my only opportunity to work on this thing for the weekend. I've been going over the fueld lines and trying to make sure everything is as it should be. I am curious if there may be a crack in the pickup tube for the tank which may be introducing air into the system. Just a thought...

I pulled the 7/16 plug to I could adjust the idle screw and a small amount of diesel came out. I assume this is normal but wanted to ask just to make sure there wasn't a problem with a bad seal somehwere.
 

simp5782

My safe word is Pork and Beans.
Supporting Vendor
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Ok, today is likely my only opportunity to work on this thing for the weekend. I've been going over the fueld lines and trying to make sure everything is as it should be. I am curious if there may be a crack in the pickup tube for the tank which may be introducing air into the system. Just a thought...

I pulled the 7/16 plug to I could adjust the idle screw and a small amount of diesel came out. I assume this is normal but wanted to ask just to make sure there wasn't a problem with a bad seal somehwere.
Yes that bell is filled with fuel
 

travistodd1

Member
37
10
8
Location
Camp Verde, Arizona
Ok, I backed the screw off a few clicks. I fired the truck up and it died off pretty quickly so I suspected I had turned it a little to far. Gave it a pump and it fired right off and idled almost exactly where I wanted it. I bumped the throttle and it picked up idle back to the higher idle and actually seemed to be slowly creeping up a little so I just turned it off again before things got worse.

The throttle is free and returning back to where it should be.

The only thing I can think is it is pulling air somewhere and I just can't figure out where. I did notice for the first time, it appears I have diesel slowly filling up the fuel pressure gauge. Obviously I have a leak but could this be the source of the air leak also?

Also, I wanted to thank everyone who has been helping me out. I appreciate everyone taking time from their day to give me advice. I am not overly versed with diesel engines, especially large diesel engines.
 

WillWagner

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Eliminate the truck fuel system. Put a line into a bucket of clean fuel directly to the pump and see what happens. Do you remember how many clicks you turned the screw? Is put it back. Also, it takes a bit to get all of the air out of the pump after the housing is drained, so, I guess you could fire it like it is, throttle it up a dozen or so times, shut it off and let it sit and re try in a half hour or so.
 

travistodd1

Member
37
10
8
Location
Camp Verde, Arizona
I believe I only turned it down 4 clicks. I will go back out and put it back to the original setting in a few minutes. I have been looking at the diagnostic from Oregon Fuel Injection https://oregonfuelinjection.com/content/uploads/2016/08/cummins-pt-pump-diagnostic.pdf and I just keep coming back to the air in the system, I can't think of anything else. I will unhook the fuel lines and see if I can bypass them. In all reality, there is very little fuel line to the pump from the truck though.

Am I correct in thinking it would need to be on the low pressure side of the truck since that is where it would be pulling fuel? I would think if there was a leak on the pressure side, it would be spraying fuel all over.
 
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