m915a1 runaway engine

[ZiggyO]

So here is the background----- This is one of the m915a1s that made multiple cross country treks back in 2010/2011 and ran like a sewing machine during those trips. After its last trek west in 2011, it has not moved from its parking spot but was started occasionally into 2013. It has not been touched or started since then. two days ago, I threw in new batteries, dumped some power service into the fuel tank, checked all the fluids and attempted to start it. It sputtered and hesitated on startup as if it had air in the system but then smoothed out after about 5-10 minutes idling. I did notice at that point that the rpm's had a long lag time to come back down to idle when the pedal was pressed and released. Other than that it idled great and drove around the yard without issue. I shut it down and left it until today. Today the truck started with the same degree of hesitation as it did a few days ago and also still had the same delay in coming back down to idle when the pedal was pressed. I also noticed that it also seemed to be idling at about 1000 rpm as opposed to the normal 5-600 rpm. In addition, when left to idle, it seemed that the idle was slowly creeping up (at one point it settled on about 1500rpm). I shut the truck down again, checked the linkages under the hood to be sure they were free and fully returning (which they are) and proceeded to restart the truck. This time it went WOT immediately on startup--- got it shut down safely but now I am stumped---- anyone have any suggestions?

Z

 

[NDT]

About all it can be is the PT pump. Time for a rebuild.

 

[simp5782]

Take the bolt outof the throttle arm on the pump and remove the whole throttle arm and ensure the splined knob is back to 0 throttle. and start it and see what happens. i would start by eliminating the linkage all together. I have had that issue on a few trucks. As well as an over tightened throttle clevis or a rusted up one.

 

[sandcobra164]

I agree with Simp. My truck is slow to return to idle but if I pull the pedal up by hand, it's nearly instant. It doesn't affect driveability and it always settles to it's normal idle within about 3 seconds so I've not put any effort into fixing it.

 

[simp5782]

I agree with Simp. My truck is slow to return to idle but if I pull the pedal up by hand, it's nearly instant. It doesn't affect driveability and it always settles to it's normal idle within about 3 seconds so I've not put any effort into fixing it.

That is an overtight clevis. Loosen it up a turn or two. Probably did it when you put your pump back on. That bolt should be slack.

 

[sandcobra164]

Dealing with the OP's problem, he's losing prime which causes the extended start times. He's got a fuel line that is sucking air. I've never understood why a Cummins PT Fuel system sucking air will cause them to run at a higher idle and be slow to return to idle. They are also slow to respond to input on the "go" pedal but once they do, they will go way past the governor setting when there is an input / suction fuel line issue. I'm not sure why it happens, I work the theory in my head and a suction line air leak would cause an engine to run slower. The theory in my head is incorrect though because it doesn't work in the real world that way. I do know that the lift pump which is integral to the injection pump moves alot more fuel than is needed to ensure there is fuel for the return side and to keep the injectors cool. I'm still not sure why additional air being sucked into the fuel line helps them idle faster. My only experience is on the NHC-250, maybe you have an aneroid issue on the NTC-400 and it's fueling more when it shouldn't causing the high idle.

 

[wrenchturner6238]

You may want to check the return line for being plugged the way the PT set up works if the return line is not letting fuel through can cause your systems.

 

[WillWagner]

You may want to check the return line for being plugged the way the PT set up works if the return line is not letting fuel through can cause your systems.

I was thinking this too. try running it from a bucket of fuel. This will verify sandcobras thinking. They can and do go to WOT when air is introduced into the system.