Gasoline in a Diesel Tank: Stop Before You Start It
The wrong fuel in a diesel is fixable if the engine never runs on it. What gasoline does to a diesel fuel system, what to do right now, and the fix.
It happens at a fuel island in the dark, with a rental box truck that looks like the one you drove last week, or with a motorhome at a station where the green handle is diesel and the black one is not. The pump clicks off, you look at the receipt, and it says regular unleaded. What happens in the next two minutes decides whether this is an inconvenience or a fuel system.
Here is what gasoline does to a diesel, what to do the moment you realize, and how the fix goes.
Do not start the engine
That is the whole rule. If the engine has not run since the wrong fuel went in, the gasoline is sitting in the tank and nowhere else. The tank gets drained, the correct fuel goes in, and the truck goes on its way with no lasting harm. If the engine has run, the gasoline is in the lines, the filters, the lift pump, the high-pressure pump, and the injectors, and every one of those is a potential casualty.
If you already started it and it is running, shut it down now. Do not drive it to a shop. Do not drive it off the fuel island to a parking spot unless the truck is blocking a lane and there is no other option. Every second of running moves more gasoline through the system.
If you realize while still at the pump, do not turn the key. Not to move the truck, not to check the gauge. Push it or leave it. This is the one situation where sitting at the island is the right call.
Why gasoline hurts a diesel
Diesel fuel lubricates the pumps and injectors that move it. Gasoline does not. A modern common-rail diesel runs its injection pump at very high pressure on very tight tolerances, and that pump depends on the fuel for lubrication. Gasoline through it scores the internals, and the metal it makes goes straight to the injectors. Gasoline also does not compression-ignite the way diesel does, so the engine runs rough, knocks, and can misfire badly enough to damage things on its own. The result of driving on a gasoline mix can be a pump and a set of injectors, which on a heavy truck or a diesel coach is a very expensive way to learn the colors of the pump handles.
How much is too much?
There is no safe amount on a modern high-pressure common-rail engine. Older mechanically injected diesels were more tolerant of small contamination, and there is folklore about diluting a little gasoline with a lot of diesel. On a current engine, the pump and injectors are the reason not to try it. A small amount in a large tank, with the engine never started, is still a drain. The fuel cost of draining is small. The pump is not.
The fix
- Truck stays off. Note how much of the wrong fuel went in and how full the tank was before.
- The tank or tanks get drained. On a truck with dual tanks that crossfeed, both may need it.
- Fuel filters and the water separator get replaced, because they have been sitting in the mix.
- If the engine ran, the lines get flushed and the system gets primed with clean diesel.
- Clean fuel goes in, the system is primed, and the engine runs until it is clearly running on diesel and only diesel.
- If the engine ran for any length of time, the crew will tell you straight what the risk to the pump and injectors is and what to watch for.
The drain and refill is a mobile job. A service truck with the right equipment does it at the fuel island, in the lot, or wherever the truck stopped, and the contaminated fuel gets handled properly rather than dumped. That is a normal roadside assistance call, and it is one we would rather get five minutes after the mistake than an hour of driving later.
Diesel in a gasoline engine
The reverse mistake, diesel in a gas RV or a pickup, is less common because the diesel nozzle usually does not fit a gasoline filler. When it does happen, the engine will run badly and smoke and eventually stall, and the fix is the same drain and flush. The damage risk is lower than gasoline in a diesel, but the fuel system still needs cleaning, and the catalytic converter does not enjoy it.
DEF in the fuel tank
The other fuel island mistake is DEF in the diesel tank, or diesel in the DEF tank. DEF is water and urea, and in the fuel it corrodes the system fast. The same rule applies: do not start it. We covered the aftertreatment side in DEF and Emissions Faults, and the general fuel problems that look similar in Diesel Fuel Problems.
Frequently asked questions
I put a few gallons of gas in and topped it with diesel. Am I okay?
On a modern common-rail diesel, no. Drain it. The pump and injectors are not worth the gamble.
I drove ten miles before I noticed. How bad is it?
Shut it down now and get it drained and flushed. Whether the pump was harmed depends on the mix and the engine. The crew will look at how it runs on clean fuel and tell you honestly.
Can you drain a fuel tank at the fuel station?
Yes. The crew comes to the truck, drains, replaces filters, refills with clean diesel, and primes the system on site.
Will the station help?
Usually they will let the truck sit while help comes. They will not drain it. Call dispatch first, then tell the attendant.
Wrong fuel in a truck or RV anywhere on the panhandle? Do not start it. Call Duckett Roadside Repair at (850) 495-0366. Dispatch is answered 24 hours a day.