Diesel and TDI engine runaway engine causes and how to stop and repair it
How to stop a runaway diesel engine
All diesel engines are "throttled" by controlling the amount of fuel they consume. If a diesel engine starts to eat its own engine oil, you could have a diesel engine runaway. This article shows why a TDI engine runaway happens and how to stop it.
A runaway is when engine RPM suddenly surges and races by itself as if the driver had stomped on the throttle or accelerator pedal. If the car is in gear, the car will also accelerate which could create an emergency situation. Burning engine oil will also create a smoke screen out the exhaust or around the engine. This article shows some causes of unintended acceleration from a runaway diesel engine and suggests some corrective actions for North American VW and Audi TDI 1996-present.
If this or any other unintended or unexplained acceleration occurs, the driver's first priorities are to concentrate on safe operation of the vehicle, keep it under control through braking and steering input, and to regain control over engine power or shut the engine off as soon as is safe and practical. Pull the car over to the side of the road only as soon as it's safe to do so. It's not worth getting into an accident to try and save the engine only to lose your life and maybe someone else's. Any information on this page is just a generic suggestion not specific to your exact situation because it's not possible to foresee every cause or response to a potentially dangerous situation. The full responsibility of decision making and the reaction to a runaway engine is on the driver and is specific to each situation. See the TOS for the full legal disclaimer. As with any other preparedness drill, you will have the best results and least panic if the response is first practiced under safe, controlled conditions. While many of the ideas on this page apply to all diesel engines, the possible solutions and mechanical details are only for Volkswagen and Audi TDI. Other diesel cars may have different systems and therefore, different possible causes and solutions. Reacting in an unsafe manner or letting the engine runaway and the car accelerate without control could cause a dangerous situation. Lastly, a lot more TDI engines are ruined by faulty or neglected timing belt changes than runaway engines so I wouldn't sell my TDI or not buy a diesel because of this rare but possible condition.
Basic technical background
Diesel fuel is basically an oil and engine oil will burn through compression in a diesel engine like regular diesel fuel. Most diesel engines can even run off of propane, vegetable oil, coal dust, or even ambient gasses in the air (like if a backhoe breaks open a natural gas line). Older pre-TDI diesel VW engines had a common problem with blowby oils entering the intake path and causing a runaway but TDI engines are set up differently so their runaways are normally due to bad turbos. Gasoline engines or pre-TDI could also experience a runaway from the throttle cable sticking, the gas pedal sticking, or something falling into the throttle mechanism linkage or jamming it internally, any of which could hold the throttle open.
All VW and Audi TDI use Electronic Power Control, or drive by wire "throttle". The major component in drive by wire is the accelerator pedal - it uses an electrical sensor to detect pedal position. An example of a TDI accelerator pedal and sensor is shown below. There are only electrical wires coming out of the sensor to tell the car's computer how to control engine fueling - there is no mechanical cable to move an air throttle. Since the first TDI in 1996 (North America), all TDI computers are programmed so that fueling is cut down to idle after about a half second if the brakes are applied even if you're still stepping on the accelerator pedal. If the accelerator pedal signal is lost, the car's computer will limit engine RPM to a high idle or about 1200 RPM. This will let you drive slowly to the nearest repair.
While unintended acceleration could have many causes, the most likely cause of a runaway engine in modern VW and Audi TDI diesel engines is from leaking turbo oil seals and bearings. Below left is a video about runaway diesels - it's for domestic diesels but it explains the basics of a diesel runaway engine. Below right is a BMW diesel in the middle of a runaway.
The most likely cause of a runaway engine in VW TDI or Audi TDI - the turbo oil seals
A worn turbo can send engine oil into the air intake because air must pass through the turbo on the way to the engine. Oil can leak out of the seals. With proper care and good synthetic oil, the seals can last the life of the turbo. However, excessive thrust movement and pressures (caused by manufacturing issues, bad oil, or worn bearings) can cause excess wear and oil leaks. The compressor and turbine sides of the turbo can respectively leak oil out the intake or exhaust sides. Oil that goes out the exhaust side is burned up and causes black or "blue" smoke. This can shorten the life of the catalytic converter due to melting or clogging. Oil that goes into the intake is ingested by the engine. For more information on turbos, please see 1000q: Turbocharging FAQ.
The line between a leaky turbo and an engine runaway is when the engine suddenly increases in rpm and draws the engine oil out of the turbo seals and feeds off that oil, raising the rpm, drawing even more oil out. Once the runaway gets started, the feedback cycle increases until you shut the engine off by cutting fuel or air or all the engine is burned up and the engine seizes. Again, the reason why you don't hear about this on gasoline engines is because gasoline engines can't run on engine oil. In a gasoline turbo engine, oil in the fuel effectively reduces the octane of the fuel and makes the engine more likely to detonate. In a diesel engine, it can result in a runaway engine.
Note that the crankcase ventilation system does put a light oil mist into the intake. This settles in the low spots like the intercooler and bottom of the hoses (like at the turbo outlet hose). It's normal to have a little oil pooled in the bottom of the intercooler but too much oil could be from a leaky turbo. If you drive the car hard and rev the engine to a high rpm, it can blow out a little bit of this oil and burn it up along with any carbon deposits. This will often create a small puff of smoke when you accelerate hard. It's possible that a short burst of acceleration which stops on its own is from a little pooled engine oil being burned up. The line between a little burst of acceleration and a runaway engine is how much oil was just consumed and if it leads to the feedback cycle of higher rpm leading to more oil consumption.
Some less common causes of runaway are overfilling the engine oil by so much so that oil mist floods back into the intake or worn piston rings creating too much blowby which sends an oil mist back into the engine. Because of the modern cyclonic oil separators on modern TDI, oil from blowby mist is more likely to settle back into the engine. As mentioned before, ambient combustible gasses could also cause a runaway. If the engine inhales natural gas or propane, these will be burned in the engine and increase RPM until the gasses are no longer present or air is shut off (or the engine breaks).
How to stop a runaway diesel engine
An engine needs fuel, air, and compression/spark to run. You can't control compression and a diesel has no spark plugs. Therefore, you must cut off fuel or air to stop a runaway diesel engine. When you take your foot completely off the accelerator pedal it helps cut off the fuel but if the runaway is strong, the engine is already feeding off the engine oil and won't stop on its own.
Again, the driver's first priority is to maintain positive control and safe operation of the vehicle through braking, steering, and power control. Traffic and conditions permitting, the first reaction to a runaway TDI engine is to apply braking and turn the ignition key to "OFF". As mentioned above, drive by wire will reduce engine RPM to a high idle if you step on the brake and accelerator pedal at the same time after about a half second. You are also conditioned to pull the key out of the ignition slot when you shut the engine off but don't do that (explanations below).
Do not hit anything with your car in order to stall the engine or stop unintended acceleration if it will result in injury or damage to you or others. The engine will continue to runaway but it's not worth saving the engine if it means possible injury, death, or damage to you or a 3rd party from intentionally hitting something. No matter the cause of acceleration, shifting the transmission into neutral (applies to all transmissions like auto, manual, or DSG) will prevent power from the engine being transmitted through the transmission and immediately stop any further acceleration from engine power.
Shutting off the fuel to stop the runaway engine
In mk3 and mk4 TDI that used the Bosch VE injection pump (see 1000q: injection pump vs. pumpe duse vs. common rail for details), turning the key off will close the fuel shut off solenoid and cut fuel to the fuel injectors. (The solenoid is basically an electrically controlled valve). Mk3 TDI don't have anti shudder valves so you know the fuel cut off solenoid is working or else your engine would never shut off. If you have a mk4 with injection pump, you can test the function of the solenoid by having a helper turn the ignition ON but without starting the engine. This will give 12V to the solenoid and you will hear a small click, meaning it opened. Turn the ignition off and you will hear it click closed (the default position). The location of the solenoid is shown below. The right picture is a closeup showing the solenoid wire.
In mk4 pumpe duse TDI and newer TDI (model year 2004 and newer in North America), the fuel injectors are controlled by the car's computer. Turning the key to OFF will stop the computer from injecting fuel at the fuel injectors which cuts the diesel and will shut it off if it's not running away on engine oil.
Shutting off the air
Turning the key to "OFF" will also close the anti shudder valve or electronic intake flap "throttle". This shuts off air to the engine which should stop the engine even if it's still getting fueling by burning its engine oil. Mk3 VW TDI did not have these valves, only mk4 and newer do. If you have a 98-03 mk4 TDI, test the operation of the valve by manually closing it with the car idling. The general location and the lever are pictured below (yellow arrows). The default resting position of the lever/valve is open. The lever and valve will be hot if the engine is also hot so use a glove or rag to shield your finger and hand while you close the valve. Move the lever against the spring pressure and it will close the valve inside. Make sure you close the valve all the way. The engine should shut off. If it doesn't, there is an leak downstream of the valve, the valve isn't fully closing, or you aren't closing it all the way. You won't hurt anything by moving the valve manually to choke the engine. (Disclaimer - don't stick your fingers, loose clothing, hair, necklace, or anything else near the alternator belt or any other moving belts while the engine is running because they could get caught and it could result in severe injury or death.)
The solenoid which controls the vacuum operated anti shudder valve in the 1998-2003 mk4 TDI engines gets 12-14V until the engine shuts off or you press the clutch pedal. It's partly based off engine RPM. In other words, when you shut the engine off using the key, the valve closes for only a second because the engine stops immediately. This may be because the car computer knows that the engine is shut down or because the valve is vacuum operated and when the engine stops running there is no longer a source of vacuum. If you shut off the engine while coasting downhill or during a runaway engine, this valve will close until the engine shuts off or until you step on the clutch. This was first tested by someone named "Ski in NC" and verified by others. The forces acting on the valve should be about equal on both sides (because it pivots in the middle) and it doesn't require much force to close but there are cases in which the vacuum operated anti shudder valve failed to stop a runaway engine. This could be due to the runaway being too strong, carbon buildup preventing full closure of the valve (see 1000q: intake cleaning for ALH engine, articles for other engines can be found in the FAQ) , a broken vacuum line, or other unknown factors.
1998-2003 engines use a vacuum operated valve (shown below). 2004 and newer engines use a more robust electrically operated valve that should be better at stopping an engine runaway. Because the later ones use an electric valve you can only test it through a VCDS tool output test. Start the software and select "Engine". There is a button for output tests which will cycle this valve to test it. You cannot close it by hand. It's possible the gears inside the flap assembly that move the valve get stripped due to wear but this should throw an error code for the intake flap valve if it's not working.
Manually choking off air to the engine
It's possible to manually close the anti shudder valve on 98-2003 mk4 TDI to starve the engine of air. Again, the tests mentioned earlier show that a normally operating valve stays closed until the engine is stopped. I would also be cautious of manually closing this valve during a runaway because if the engine fails in a manner which cracks it open (not likely but possible), metal bits could be forcefully ejected from the engine and injure you. If the engine really does blow open, my total guess is that standing to the passenger side (for left hand drive cars) of the engine and near the windshield may shield you better and reduce the chance of injury vs. standing in front of the engine (in front of the car). This is because you are somewhat shielded by the fender and wheel well and you don't have to reach over the engine to manually close the anti shudder valve. In addition, standing to the side means you are only facing the #1 cylinder. All 4 cylinders are facing you if you stand in front of the engine. Some remove the valve to remove the EGR and restriction in the intake but I would leave the valve in place since every little bit helps. See 1000q: EGR FAQ for more info on the EGR system.
While it's possible to starve the engine of oxygen by flooding the air intake with something like a CO2 fire extinguisher. If the intake manifold hose happens to be off or you could overcome the spring clamps and pull it off, you could hold a rag over the intake to choke off the engine but I personally wouldn't. Light diesel engines rev much quicker than larger engines and can runaway so fast that by the time you've recognized the problem, stopped the car, and opened the hood, the engine is already damaged. The other steps on this page are faster and safer and should work for other possible causes of runaway. If you decide to carry a CO2 extinguisher in the car, make sure it's securely mounted in the car to avoid being a crash hazard or getting stuck under the pedals. A cloth crammed into the air intake will get sucked in and jam the turbo/slow the airflow. It will damage the turbo but it's already damaged so no harm done. Don't risk personal injury by grabbing hot hoses, accidentally getting caught by any exposed running belts on the engine, or be exposed to scattering projectiles if the engine were to fail catastrophically.
Stalling the engine with a manual transmission
All manual transmission VW and Audi TDI can also stall out the engine by upshifting into top gear and applying the brakes. This is a very effective method of stalling the engine. When you push in the clutch pedal or put it in neutral during a runaway the engine has no resistance from having to move the car. Therefore, the engine would continue to rev higher until it died. If you upshift or downshift the car may accelerate. An upshift to top gear with firm application of the brakes will stall the engine because the brakes have much, much greater force than the engine can make to move the car. Again, don't risk getting into an accident just to save the engine so only brake when it's safe and practical, conditions permitting. If it damages the clutch or flywheel, this is preferable to damaging the engine or risking personal injury.
Hazards when shutting the engine off, pulling the key out while moving, or responding to unintended acceleration
The first hazard of shutting the engine off is removing the key from the ignition slot and activating the steering wheel lock. With the car parked and key off and out, try turning the steering wheel left or right. You will hear a click past a certain point and the steering wheel will not return to center. This is the steering wheel lock activating for parking the car. The steering wheel lock is why on some cars, you have to turn the steering wheel a little to release a key stuck in the ignition switch - the steering wheel lock is on the edge of its mechanism and is jamming the key. After you turn the key to the OFF position (if it was previously ON), turning the wheel shouldn't activate the steering lock - try it to confirm. However, just inserting the key in the OFF position will not unlock a locked steering wheel - you have to turn it ON.
You are conditioned to pull the key out whenever you turn the car off and physically and mentally practicing this will help unlearn that habit in the event of an emergency situation. If the car was in the middle of a turn and you pulled the key out by habit, this could lock the steering wheel to the side and result in loss of steering control and a crash. If you accidentally pull the key out of the ignition slot, make sure the steering wheel won't lock by putting the key back in the ignition slot and turning it to the ACC position and not ON, RUN, or START.
When the key is off or out, the brake lights will still work as normal. The headlights may shut off and the turn signals may not work. The hazard lights (emergency flashers) will. If the car is having a runaway there will be a big cloud of smoke behind you from the burning oil so if possible, turn on the emergency flashers.
If a car is following too close or can't see you, maximum braking could also cause an accident. Again, it's not worth having an accident to save the engine but the sooner you can brake and stall the engine and halt any further acceleration or runaway, the better.
You may also see or hear a low engine oil buzzer and warning message or light on the instrument cluster. The normal reaction is to shut the engine off but because it's running away your focus should be on keeping the car under control and safely stopping it as soon as possible, conditions permitting.
The power assist on the brakes comes from the vacuum operated brake booster. If engine isn't turning over because you shut it off and the transmission is in neutral, it doesn't make vacuum for the brake booster which results in only having power assist of the brakes until vacuum brake boost is depleted. This is normally about 1-2 pumps of the brakes. If there is no power assist on the brakes, the brakes will still work but the pedal will be very stiff and require much more force to actuate the brake calipers to slow the car. Without power assist, it takes a lot of effort to slow the car at all. All TDI that I can think of get vacuum from a vacuum pump that is on the driver's side of the camshaft. Anytime the engine is turning over, the vacuum pump is running. Assuming that everything is working correctly, vacuum for the brake booster means you have normal power assist on the brakes. An example where the engine is turning over but not running is if the engine is off and the transmission is in gear while the car rolls down a steep hill. The kinetic energy of rolling down the hill is what's turning over the engine. The engine will only stop turning over if you take the car out of gear.
To test how the brakes feel without vacuum boost, with no traffic or people around, find a small hill that you can roll down safely with (disclaimer) plenty of room to the side and adequate runoff at the end in case you can't apply the brakes. Testing under controlled daytime practice conditions and with a driving instructor is the safest way to test no-power-assist braking feel. In neutral gear, shut off the engine but don't pull the key out for the reasons mentioned above. Step on the brakes. You'll find that the first pump and maybe the second pump feels normal. After that the brake pedal will get much harder to press but it's still possible to stop the car. You just have to press the pedal really hard and stopping distances will be much much longer. If you have a manual transmission you can also test the system in gear with the engine off and the car coasting. As long as the engine is turning over, it will make vacuum for the brake booster.
On a side note, very few gasoline car engines have a vacuum pump because they get vacuum from the intake manifold. The pressure difference (vacuum) is present because the throttle is partially shut on most gasoline cars during normal operation. If a gasoline engine experiences a runaway (like from a stuck throttle pedal), the throttle is wide open and there is little-no vacuum present in the intake manifold. This means the brakes will only have power assist until the vacuum reservoir is depleted. This could be a contributing factor of why people who experience runaway engines on the highway say they lost their brakes - if they only halfheartedly pump the brakes they will lose power assist for later brake applications resulting in a rock hard brake pedal. Again, the TDI vacuum system is the opposite of most gas engines because they get vacuum from an engine driven pump - it makes vacuum whenever the engine is rotating.
Once the engine is stopped
Once the engine is off and stopped it won't restart on its own. If the car still has momentum it should be able to roll forward in neutral gear to the side of the road - pull over as soon as is safe and practical. If you've stopped a runaway, don't start the engine until the engine has been examined by a diesel specialist. Seriously. Many people experienced an uncontrollable runaway immediately after stopping a minor runaway because they continued to drive the car. Have it towed to a diesel specialist and explain that the diesel engine had a runaway. If the car is not stopped in a safe place, leave the car when possible and it's safe and wait for help away from any moving cars. The smoke cloud or a stalled car could distract other drivers and it's not safe to be close to moving cars since you or the car could be hit.
The aftermath - don't restart the car!
If you immediately stopped the engine and the engine oil level didn't go down it's possible that the engine will run fine after replacing the turbo. At best, there may have been a little oil pooled in the intercooler that got sucked into the engine and was consumed by the engine but with no drop in the oil level. If that case it's really not a runaway because it didn't go into the feedback cycle amplification of a runaway. You should do further diagnosis to make sure the turbo isn't leaking oil into the intake path and that nothing else was damaged. If you have a mk4 car, see 1000q: turbo replacement for more details. Search the FAQ for the DIY for newer engines.
If the engine revved far above redline, it's possible the valves floated and were smacked around and damaged. If the engine raced for a while and then stopped on its own, it's probable that something was damaged. The engine either sucked enough oil that the engine seized from lack of lubrication or bent something. If enough oil goes into the engine cylinders, the engine will hydrolock and probably bend the rods first. Low engine oil and high revs may have also damaged the turbo, camshaft, or crankshaft bearings. Further diagnosis is needed. Always drain the intercooler of oil as well. If lots of oil comes out it's very likely that the turbo oil seal blew. Remember, you can replace an engine but you can't replace yourself or others so always react to the runaway with safety for people first, then property. Although a runaway diesel engine is rare, it does happen so be prepared.
Why doesn't Audi or VW include an emergency procedure in the owner's manual for stopping a runaway engine (just for TDI)? People would at least be aware of this potential problem but they really don't include detailed procedures on how to react to any emergency situation. I don't know if it's for legal reasons, if it falls within the realm of driver education and training, or because runaway engines is bad press. It won't help sell cars but all gasoline cars could catch fire and blow up from the flammable gas vapors igniting (static electricity) during fueling and I don't think this is common knowledge either. As a final note, this cause of runaway engines is possible on any diesel engine and even gasoline engines can experience uncontrolled acceleration.
Have more questions on why the TDI engine and all diesel engines can experience unintended acceleration or race on its own? Please ask in the Volkswagen and Audi TDI discussion forum linked at the top and help improve this FAQ.