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Old GMC Runaway


Notice that it is the old guy that keeps his wits about him and saves the day. What causes these diesels to runaway like this?

I am not a Diesel expert but I understand that the air intake is open all the time and it is the injectors that control the power. I would think that the injector system has a problem.

What a nice piece of history, that truck. I wonder what the flames were in the video.

Yes, that is correct about the intake. It's wide open, no throttle plate or choke. The engine speed is controlled by either the pump output by means of a swash plate (Bosch/Roosa-Master/Stanadyne), or in this case, by a control rack that is part of the injector unit itself (also common on older Cummins engines). Usually, the governor will control the max rpm, but if they fail, well....
There should be an emergency shutoff valve in the inlet for the blower that chokes off the the air, but it might not have been working on this truck. Tossing the rag in basically did the same thing.

Hy Allen Lane, what usually causes a runaway is the injector fuel rack getting stuck in the full load governed speed position. The old racks were a two screw design, so that if even one injector was to stick, it would hold all the injectors in the same position. Newer racks had only one screw and a spring to replace the second screw, so even if a single injector stuck, the other injectors would still operate independent of the one that was stuck. The positioning of the fuel rack for "high idle" and full load governed speed is quite different, and unfortunately injectors tend to stick in full load position. Before attempting to start any engine which has been out of service for an extended time, make sure your fuel control system is functioning correctly, and is in the idle position, hope that helps.

Less than 6 months ago I lost an N14 Cummins. The shaft in the turbo broke which dumped oil into the intake. The driver had been driving a truck for over 20 years, but he had never heard of a runaway. All he had to do was put her into hi gear and dump the clutch to kill it. Instead, he panicked and called the fire department because he thought the truck was on fire with all the smoke. It ran until the No. 1 rod starved and spun the bearing and then the piston hit he head. We haven't even bothered to disassemble the engine. I will sell it to the first core buyer that comes around.

Diesels have a nasty habit of being able to run on any "fuel" that resembles lightweight oil- - - -including lubricating oil. If one gets much above its designed governed speed, it can suck up oil from the crankcase and run on that, even if the fuel supply is completely shut off. One of my neighbors got a runaway on an International Diesel farm tractor, and he put the front bumper against a big tree and let the clutch out in high gear. The old engine had enough torque to bury the tires, and the engine didn't stall down until the rear end housing was on the ground. The tires dug holes almost 2 feet deep before they got enough traction to overcome the engine torque.
Jerry

I had a GM 8.2 non-turbo take off on me. I was replacing the heads and was in the process of adjusting the injector height and rack settings. I had just bumped the started with a push button switch to go to the next cylinder and away it went at full speed. I was between the front tire and frame rail with no place to go. Everyone new it could happen se we kept a wood clip board close by to shove over the intake opening. It only lasted a minute or two and finally slowed down enough I could get a hold of the rack to idle it down, but that was enough to scare the pants off me. Everyone else took off thinking I was away from it.

Joe

I saw a Detroit 6-71 runaway.
One of the new injectors jammed at WOT which held the rake at max rpm.
The valve cover was off so it was running off of it's own lube oil as the intake was directly above the open head. Finally got it shutdown by stuffing rags down it's intake.
It ran perhaps a full minute but it seemed like hrs
Dave

Aside from fuel racks, governors, running off oil, diesel fuel in the oil, in a shop or industrial applications can be real serious if there are fumes or gas leaks present (acetyline, or lpg leaks especially) are things that are overlooked versus a wide open throttle setting, theres nothing to govern what the engine will suck in

Once again, learn something new every day. Been around diesels for 40 years and never heard of it or experienced it - mostly farm tractors. During the arab embargo in the early 80's I was unfortunate enough to buy one of those converted gasoline core's to diesel engines in a cutlass supreme that completely ruined me on diesels. I bought 2 big batteries every winter, went through cases of starting fluid but was a poor college kid, and had to put up with it. If anything was going to runaway would have thought that piece of **** would have been it. Had I known about runaways, would have definately carried a big rag in the trunk at all times. Thanks for the education & stories.

Back in the early 70's in votech school industrial engines, we learned all about the runaways. The instructor was a cummins mechanic by day,we disassembled a cummins engine, new cylinders, rebuilt the injectors and injection pump.Had to set the rack and shim the covers on the cam? I believe. One of the no it alls started the engine while the instuctor was in the tool room, and it took off.The prof ran over with a clipboard and a couple of racks and got it stopped. Was an interesting learning experience, we even worked on Budha engines out of a tow boat, That thing was HUGE!!

We got 80 brand new CNG powered school buses this year. Every morning they start up the diesels I keep waiting to hear one run away since they park next to CNG fill stations and buses. The CNG,s are still having teething problems and leaks keep showing up, that poor diesel that gets a good whiff of gas is really going to take off !

Joe

I love watching that video, Especially when the kid tells the older guy it sucked in the rag and almost his arm. I watched him hold the rag walking away scared as all hell.

When I was a tech for vw a Diesel engine ran until it blew up. I even tried the rag thing but too little too late.

Saw a smaller excavator on a jobsite self destruct once. He was digging and hit a natural gas line. Didn't get it shut down in time and the natural gas took it over. That got entertaining trying to clamp the gas line while the excavator engine was failing.
*Please Lord, no sparks!*

had a diesel runaway, used a phone book to block off the intake....
darn near ate thru the whole book before it stopped....

saw a old Detroit 3-53 that was in a log skidder run away once. we didn't have anything to plug the intake so we used our shirts.

I don't have much diesel experience but this is definitely something I will keep locked in the corner of my brain so I can dig it out if I'm ever in a "situation"!!!!

Every Detroit 2 stroke I've been around has an emergency shut down valve to the entry of the blower.