Ever since I was a kid, I've been fascinated by things that blow up. When I was barely a teenager, I blew up the garage. I was unsupervised and messing around with some powerful chemicals. As a parent, I can see clearly now that I should have never had gotten my hands on them. One rainy day, in my garage laboratory, I was mixing up combinations of powders to make a home-made bomb. The Potassium Chlorate is very unstable stuff. The result was a blown up laboratory and permanent injuries. Now I confine my fascination to store-bought fireworks.
I may like explosions, but there's one kind that I hate, and that's when an engine blows up. And, exploding engines can be very dangerous to life and limb. They can throw large masses of metal with great force. Everyone in car racing knows about a scatter shield. One of the most feared truck engines with the worst reputation is the Detroit Diesel which has massive flywheels that have been known to fly off.
Actually, it happens in three states. First, a linkage problem throws the engine into an uncontrolled free rev -- full throttle. The linkage for the rule rail is very tricky. This can easily happen when the fuel rack control rods jamb -- holding the fuel rack wide open. The first thing a student in diesel truck school learns about this engine is the importance of the fuel control rods.
If you get them installed wrong, they will jamb the throttle resulting in a ran away. And because this engine has this tendency, it comes fitted with a "Slam Door" and a big heavy metal door that is built right into the intake air box. If the engine starts to run away, you can crop the slam door and close off the incoming air, and kill the engine. At least, that's the idea.
But, it doesn't always work. They don't call them oil burners for nothing. By having the habit of feeding on its own oil, the Detroit engine is one of the worst offenders. The reputation partly comes about because that engine is a two-stroke. It has ports on the sides of the cylinder lines for air intake.
This is important here, because it provides a place for the oil to be introduced into the engine. If the air box gaskets are weak, they will be sucked right into the screaming engine. This unwanted source of air will allow it to keep running away. Then it begins feeding on its own oil as superheated oil, which is being pumped past the rings into the combustion chamber, fuels the fire even more.
Finally, it runs out of oil and seizes up. That's what launches the flywheel, the crankshaft suddenly stops turning and something's got to give! The flywheel bolts shear off, and the massive fly wheel comes crashing right out of the bellhousing.
But that's nothing new to race fans. Exploding clutches have brought the requirement for scatter shields on race cars. I guess it took enough severed legs to find out the need for this protective 'device. But there is nothing to protect you from the flywheel off a runaway Detroit Diesel. I've heard that the massive flywheel will go through concrete walls.
And what a noise it makes! It's one of the most terrifying sounds in engine can make. When I say screaming, a runaway Diesel is louder than a speeding train engine. And, at the same time, it belches smoke like a blast furnace. I happen to know firsthand, because I had an opportunity to witness one that ran away. It happened while I was attending night school for heavy-duty mechanics at Sequoia Diesel Institute.
One of the students jammed the rack levers on a Detroit Diesel and the engine went full rev as soon as it was started. The teacher pulled the lever for the slam door, and the engine slowly died. But for one terrifying moment, it. screamed and roared like a metal monster gone amuck. That's when I first heard the story of the runaway Detroit Diesel flywheel busing through walls. The teacher saw it happen. The engine ran away, exploded when it seized up. The flywheel came off and went through three walls before coming to a stop halfway into a fourth.
And that was where I learned about CO2 fire extinguishers. The teacher explained that in case the slam door didn't stop the runaway, we should use a CO2 fire extinguisher to stop it.
Since CO2 doesn't burn, it stops the engine dead in its tracks by starving it of oxygen. It's quick, efficient and works every time.
Years later, while I was wrenching for a Mercedes-Benz dealership in Palo Alto, I was instructed on the MBZ way of shutting down a diesel engine. According to factory protocol, the proper method is to open the fuel injector high pressure supply lines, starving the engine of fuel. While this worked okay, it was really messy and takes up valuable time. And when an engine is at full-rev, every second counts. First, you had to find the correct wrench size for the injector fuel lines. Then you had to break them open one by one, which creates quite a mess. Diesel fuel spurts over the entire engine compartment.
You're probably wondering why MBZ diesels run away. First, they rarely throw off their flywheels and seldom explode. But, it can happen in one of two ways. The first is when the throttle linkage sticks. This isn't much of a problem as long as you keep the ball-sockets properly lubricated. MBZ fits the engine with a shut-down lever that looks like a choke knob. Also, located right above the injection pump, is a manual fuel cut-off device. But, modem MBZ diesel engines use a shutdown system that's operated by the ignition key. A small vacuum motor mounted on the rear of the injection pump performs the shut down.
The second way is when the vacuum motor diaphragm ruptures. The owner find that the only way to shut down the engine is to get out, open the hood, and press the manual shut down lever. Not the sort of thing that MBZ owners could be likely to do, huh? Well, the replacement of the vacuum motor can be tricky. It has a small tang on it that must catch in a notch on the injection pump rack. If it doesn't, it'll jamb the rack in a wide-open-throttle position and bingo, "Runaway Diesel."
And it happened right next to me at the MBZ dealership where I worked in Palo Alto. The tech next to me was installing a vacuum motor and didn't' get the tang right. This launched the engine into a screaming fit as soon as he started it. And while he was making a mad dash for his toolbox to find a wrench to open the injector lines, I grabbed the CO2 fire extinguisher from the wall and shot it into the air intake. The engine's screaming died right away, even before he got the wrench!
He stood there amazed. And when the engine stopped, so did all the work in the shop. You could hear a pin drop. Everyone was looking at the two of us as we stood there. Me with the fire extinguisher in my hands and the tech with a wrench in his. That's when the stop foreman got involved.
He came storming over shouting, "What the hell do you think you're doing?!" I replied in as calm a voice as I could muster, "Just a trick I was taught a Sequoia Diesel." "Oh yeah?!" he shouted back at me. "You just ruined this motor with that stuff." he said as he began removing the air filter, expecting to find it full of something. When he lifted the metal cover, it revealed what looks like snow piled up going into the air filter -- nothing more. He reached over
and touched the frozen CO2 as it rapidly disappeared into wisps of smoke. He said in a caustic tone as he turned and walked away, "Well that may be so, but we don't do that here!"
The only diesel to run away on me happened when I was working at Larry's Autoworks. And it scared me big time! I was doing a major service on a Peugeot 504D. It's a pretty straightforward diesel engine, there isn't much that you can do to hurt it. But I did find one Achilles heel. The air filter uses foam that resides in an oil-bath housing. The foam stays wet by the oil bath, which is an old method used by trap dirt.
And when I cleaned the foam, I make a big mistake. Unknowing, I cleaned it in the parts washer solvent and didn't dry it before installing it. Oh sure, parts solvents works really well at removing grease. But, it became trapped inside the foam. Since it was combustible, it caused the engine to run away for a brief moment. I remember standing there, not knowing it happened. It was over so quickly.
But, for just a brief second, I thought the engine would explode and I would be killed. And it was all over, and after careful checking, there appeared to be no damage. I didn't say anything to the owner, for fear of losing my job. Well, I thought I had gotten way with my mistake until the next day, when the Peugeot showed up on a tow truck. When I saw it, my heart sank. I feared the worse: a blown engine. Actually, it only wouldn't start.
The compression was excellent, and nothing mechanical appeared to be amiss. Further checking revealed that all four glow-plugs had been burned up by the solvent's fiery blast. Because it had warmed up when I worked on it, the problem didn't show up until the next day, when it was dead cold. Here's when I needed the glow-plugs to start. All it took to fix it was a set of glow-plugs.
But by far the worst story I know about an exploding engine happened on a diesel-powered car. Those of you who have been around the diesel engine trade know about the horrible automotive diesel engine that GM made. They modified the 350 gasoline engine to run on diesel. Big mistake. It was never designed to hand that kind of compression or torque. When trying to get one to start, a mechanic was shooting ether into it. The engine exploded, blowing off of the cylinder heads, which dismember one of the mechanic's arms. But that's another story.
