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http://www.myturbodiesel.com/1000q/runawaydieselengine.htm

In light of Toyota's unintended acceleration problems, I wrote an article explaining how and why any diesel engine, specifically a TDI engine, can runaway. While it doesn't happen often, and never happens to most, it could develop into an emergency situation if the driver does not maintain control of the car because the car can also accelerate. Some of this was in the turbocharging FAQ but I added info and pics and made it a separate article. The videos also fit into this article better than in the turbocharging article.

I also included some cautions if you turn the ignition switch off - the steering wheel may lock if you have to make a sharp turn and the brakes only have a couple pumps with power assist.

I'm not 100% sure of the differences in the throttle position sensors and I couldn't find the reference but I believe mk4 and newer TDI have 2 separate sensors. Most modern drive by wire cars have 2 separate sensors. The Bentley service manual says that the engine will go down to a high idle if there's a lost signal or malfunction of the pedal but I seem to recall seeing 1500 rpm as the high idle value. I can't find the reference but if I can verify I will edit the FAQ as needed. And of course, this assumes that everything is working as intended.
 

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It would be hard to open the air intake in time to stick a bag over it. If you remove a clamp and put the bag over the hose it better be a thick bag and not a supermarket bag because it can suck the bag in half.
 

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runaway diesel

approx 1965. 2000 hp diesel (submarine) wouldn't shut down. dumped 3 co2
fire extinguishers into intake. nfg. shut intake duct valves. vacuum collapsed intake ducting. nfg. shut exhaust duct valves. cylinder relief valves started lifting. nfg. now, boat filling with exhaust smoke from exhaust relief valves lifting. this is not good. man in oxygen breathing apparatus shut individual injector supply valves. engine stopped. we were in port. japan. 30+ passed out men lined up like dead soldiers on deck. fairbanks-morse 38 d 8 1/8, 2 cycle, 10 cylinder 20 piston, vertically opposed. no one died, some passed out guys injured pulling them out of the boat. i love diesels, but this was a bad day. jp
 

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:eek yikes. What safeguards were in place back in 1965 to shut it down other than air cut offs. No fire suppression system for the engine?
 

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runaway

on a really big diesel, if your rings are shot, you can runaway on lube oil. when i say "runaway" i mean "won't stop". like running at thirty rpm. but just won't stop. some of the older white superior or cooper bessimer engines turn really slow. you can actually focus on the flywheel. the fairbanks morse was unique in that could stop it, shift cams, and start it in the reverse rotation (tugboat). the most beautiful diesels i ever worked were 20 cylinder 4 turbo
caterpillar. this was a powerplant in the south seas. jp
 

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If you shut the engine off while driving, is there any possible harm to the engine during the shutdown or restart? Don't you lose power steering if the engine isn't running?
 

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Manual transmission no. Auto, i think some of the ATF fluid pumps may not run but it's not a big deal (I'm not sure). DSG, I'm not sure which pumps run and which don't if the car is coasting in neutral vs. in gear.

You don't lose power steering as long as the engine is turning over because that means the power steering pump is turning over. If you shift to neutral and the engine is no longer turning over then yes you lose it. I will add this note later. mk5 and newer the steering is electric power but even if you lose elecricity the steering column just becomes manual steering.
 
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