Nightmare Scenario: What to Do if Gas is Put in Your Diesel

May 26, 2023

If you’re a diesel owner, filling the tank full of gasoline is one of your biggest fears. Sometimes it’s an honest mistake. A simple slip of the mind. Other times, it’s a family member or friend that’s borrowed your truck and mistakenly topped it off with 87. No matter the cause of contamination, it can spell disaster for your diesel’s injection system—and it only takes a matter of seconds for this honest mistake to cost you thousands of dollars. Often referred to as mis-fueling, it’s a mishap that roughly one in seven people experience in their lifetime, be it gasoline in diesel or diesel in gas.

This relatively high percentage of mis-fueling is reason enough to talk about what happens when it occurs. And because gasoline in diesel is much more destructive than a diesel-in-gas scenario, we’re covering the former here. Different levels of alarm exist, and are dependent on several factors: 1) how much gasoline you added, 2) the amount of diesel still present in the tank before you filled it, 3) whether or not you started the engine, 4) if you drove the vehicle long enough to notice engine problems, and 5) if you ran the engine until it quit running.

While there is no way to eliminate the occasional mix-up at the pump, you can avoid the nightmarish damage it can lead to if you catch it quick enough. Below, we’ll outline the standard operating procedure for rectifying the situation once the deed has already been done.

Before delving into the effects gasoline has on the components in a diesel injection system, it’s important to remember that gas is basically a solvent, whereas diesel is an oil-based fuel that serves as a lubricant in addition to being a combustible. Gasoline’s lack of lubrication is to blame for much of the internal damage that occurs within injection pumps and injectors when gas is introduced into a diesel’s fuel system. Of course, the chemical makeup of gasoline is vastly different as well, which leads to (among other things) improper atomization in a diesel engine. It needs to be stated that, in most cases, running a diesel engine on gasoline is considerably more destructive than running a gasoline engine on diesel fuel. Most of the carnage takes place in the injection system, which on today’s high-dollar diesel engines can run you upwards of $10,000. (Gulp.)

Adding gasoline to a diesel vehicle is more common than instances of diesel being added to a gas-burner. But why? For starters, a typical gas pump nozzle will fit right into a diesel’s filler neck, whereas most diesel nozzles won’t fit (or fit without some finagling) the fill point of a gasoline car due to their larger diameter. Secondly, most U.S. filling stations are predominantly populated by gas pumps, so the chance of grabbing a gasoline nozzle is higher than they are for one that dispenses diesel. A third catalyst for mis-fueling lies in some stations’ use of different colored nozzles, such as yellow or black. We prefer the arrangement shown above: diesel is green while gas is red, and (as with most commercial filling stations) both are clearly labeled.

The most inevitable part of mis-fueling is dropping the fuel tank. Whether you’ve started the engine or not, it has to be emptied and cleaned. Simply siphoning “most” of the gasoline out and topping off with fresh number 2 isn’t good enough—not with the expensive injection systems today’s diesels come with. For most trucks, this means removing the skid plate, tank straps, disconnecting all chassis-to-tank fuel lines, lowering it and pulling the sending unit. In a best-case scenario, you realize what you’re about to do before you begin filling up, but if you figure it out after you’ve pumped but before you cycle the ignition, no harm has been done. If the lift pump hasn’t been kicked on (if so equipped) and the engine wasn't started, the extent of your problem is isolated in the tank. Get yourself towed home or to your favorite shop for a $400 to $800 repair.

On older diesels, nine times out of 10 the process of draining the tank, flushing the lines and starting over with fresh diesel rectifies the situation. By older diesels, we mean engines that made use of lower injection pressure and looser tolerances—primarily those with pre-common-rail fuel systems. Full disclosure: we have seen a simple drain and a tank cleaning work on an LB7 Duramax (’01-’04 common-rail system) that was topped off with gasoline, but its owner had the good sense to shut the truck off when it first began to stutter. Note that if a dealership or independent shop is draining the gas-in-diesel mixture, there can be a hefty per-gallon disposal fee charged.

Once out of the tank, a diesel’s lift pump is the first component to be faced with the gasoline intrusion (except on ’01-‘16 Duramax engines, which were void of a chassis, engine or in-tank lift pump). So long as it’s not been subjected to long-term exposure to gas, the lift pump can typically be retained. However, its replacement should be considered for ultimate peace of mind, and especially if your mis-fueling has led to catastrophic failure of the injection pump.

Low-pressure fuel supply lines spanning from the tank to the lift pump and the lift pump to the engine usually don’t warrant replacement. Most often, these are hard lines and won’t break down or corrode at an accelerated rate after having been exposed to gasoline. However, the lines should be thoroughly flushed with brake cleaner and compressed air.

It goes without saying that the fuel filter(s) is always replaced after a mis-fuel, but what about the fuel filter reservoir? Due to its internal ports and passages, some independent shops and dealerships prefer to start with a fresh filter housing, and if you’re opting to start 100-percent anew this is a minor addition to your overall costs. If you’re at home doing this in the driveway, you’ll probably be fine if you remove it and treat it to a thorough cleaning.

Aside from the injectors, a diesel’s injection pump will have the hardest time coping with gasoline. As far as newfangled, common-rail engines are concerned, the Bosch CP3 used on ’01-’10 Duramax engines and ’03-’18 Cummins mills is a high-pressure pump with tight tolerances, but is capable of surviving mild exposure to gasoline (nothing long-term). However, on trucks equipped with the Bosch CP4.2 (’11-present Power Stroke, ’11-’16 Duramax and ’19 Cummins), we would recommend its replacement anytime it’s seen gasoline. Already prone to self-destructing when aeration, debris or a lack of lubrication is present (especially in Duramax applications), you never want to take a chance on this pump escaping unscathed after it’s been fed gasoline.

Depending on the severity of your mis-fuel, the fuel rails can either be cleaned or replaced completely. To be sure, most owners opt for cleaning, but it’s important to realize that it’s hard to determine the cleanliness of each internal passage within a fuel rail, especially if you’re dealing with a candidate that’s had metal debris pass through it. It’s common for dealerships and independent repair facilities to recommend complete replacement.

Along with the injection pump, special care should be taken to inspect the injectors after they’ve seen gasoline, especially those operating in high-pressure common-rail engines. In common-rail applications, the injectors already have multiple jobs to do, a host of precision internal moving parts, ultra tight tolerances and a fairly expensive price tag, so making sure they receive a clean bill of health (and replacing them if they don’t) should be high on your priority list. If you’re handling the repairs yourself, it’s worth sending the injectors out to be tested. They might work fine today, but in time they may show signs of wear due to their encounter with gasoline (hazing at idle, abnormal balance rates, etc.) or fail completely. In our opinion, the risk isn’t worth it. The peace of mind is.