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I recently learned the hard way about a phenomenon known as "diesel run-on", or "diesel run-away"!

I have a 20 year old Mercedes Sprinter, but with only 80,000 miles on the clock.

Recently she did that thing where a diesel engine sucks in and combusts the engine oil intended for lubrication. Hindsight suggested to me that it would have been better to deliberately stall her, rather than turn her off and pull the key out ...!

  1. Exactly how does this happen? ie, What servicing would prevent it? I've been told oil gets into the air intake or turbo. Is that caused by a worn or defective seal perhaps?
  2. What damage would this likely have caused? ie, Is the engine salvageable? I figured if you burn off all the oil, the engine must have seized, but someone suggested to me that the engine stops because she's run out of effective fuel, and there may still be enough oil in her that the engine could still operate, once the original cause (point 1. above) is fixed, and she is re-filled to the proper levels with fresh oil.

Photo for dramatic effect!

van

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A diesel runaway is caused from exactly what you stated, that being there is fuel (since oil can burn, it is a fuel in this case, too) in the intake tract which makes its way to the combustion chamber during operation. When this happens, if there is enough fuel, the engine will continue to run without any way to easily shut it off. It will run as a fast as the engine will go, which is usually over redline (maximum recommended engine speed). Since it is running this fast and the engine isn't made to do that, it will sooner or later kill itself, usually in a spectacular fashion. In most cases, this is fatal. The engine will throw large chunks of metal out through the side of the block ... you know ... that kind of thing.

One of the most frequent causes of a runaway diesel engine is when the seals in the turbocharger seep oil. When this happens on the cold side (on the intake side opposite of the exhaust), oil may build up and sooner or later it will give the engine enough oil so it will run on its own. During normal operation (when it's not running away :o), when there's a small amount of oil, it will get past through into the combustion chamber and get burned along with the diesel fuel. When the amount becomes larger, it will start pooling/puddling, then when there's enough, it can start transferring into the combustion chamber and then the engine kicks into high motion. When this happens, not only does the engine more readily suck any pooled oil into the engine (further feeding it), oil pressure can go up which causes more oil to come through the seal.

At this point it is self feeding with only no way of it stopping. An engine needs three basic things to run (assuming it has compression): Air; Fuel; Ignition. Stop any one of these and it will stop running. If the intake is accessible, you can plug the intake tract with some large, flat piece of board or similar, cutting off the air flow. In most normal driving situations, this isn't going to be possible, because it's all buried inside or under something. The only other way is to eliminate the fuel source, but in this case in the oil situation, that's fairly impossible. There's no way you're going to eliminate the ignition source, because diesel engines are compression fired and there's no way of changing or stopping that on a running engine (well, maybe you could blow it up ... but ... yah).

In your case, it is probably most likely the engine is toast. It appears there is a large puddle of oil under the vehicle. That usually means something has escaped ... probably a piston or a rod out the side and/or bottom of the engine. There is no coming back from this without a new (or new to you) power plant.

What kind of maintenance can you do? Really, the only thing you can do if the turbo was to blame is to check regularly to see if there is oil in the intake tract. If there is, find the source and try to eliminate it. This is something I'd think a mechanic who is doing regular service on the vehicle would be doing. 80k miles isn't a huge amount of miles (not for a diesel or one of these vans). You'd think it'd be okay. Sometimes things happen. Maybe the seal on the turbo dried out and just started leaking. Really, at this point it's sort of water under the bridge, but I guess it's good information for what to look for when it's running again.

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  • Another cause of diesel runaway is parking over a puddle of gasoline. This can happen at a gas station if the previous customer spilled gas and drove away. The unsuspecting diesel customer pulls in and suddenly has a revving engine.
    – MTA
    Commented Nov 30, 2022 at 0:58
  • @Paulster2, if the vehicle has a manual transmission, the safest and fastest way would be to stall the engine, like the OP correctly suggested themselves, isn't it?
    – EᑎOT
    Commented Dec 2, 2022 at 22:23
  • @EᑎOT - It might work, but more than likely you'd burn through the clutch, or if the clutch grabbed, it could very easily throw the vehicle down the road. Lots of torque going on there. However, anything is worth a try, eh? Commented Dec 3, 2022 at 2:35
  • @Pᴀᴜʟsᴛᴇʀ2 - Stalling always works. Yes, there is a risk to a clutch IF you're having to change gears, but standing on brakes will stall the engine just fine. Granted, you may not know this happened unless and until you want to stop or change gears, but if you are quick to recognize the symptoms, you have a second or two to jam high gear and stall. I personally had no runaway diesel issue, only broken ignition, so it was easy to stall, but still, I am prepared for this contingency. Also, now I know it's not easy to push-start diesel, even if it's measly 1.9D from VW... Took a 6-ton truck.
    – AcePL
    Commented Dec 6, 2022 at 9:03
  • @AcePL - Yes, stalling always works ... if you can get it to stall. Commented Dec 6, 2022 at 10:42

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