This often sets off 4xe owners and really none of us want to see this but it’s likely to show up from time to time. If you live somewhere where it gets cold it will be way more common. Much has been written and discussed about this condition, this article will attempt to sum it up.
FORM stands for Fuel/Oil Refresh Mode. This is a process that some PHEV’s use to maintain the internal combustion engine (ICE), the 2.0L ICE in the 4xe is a direct injection engine. A direct injection engine injects fuel directly into the combustion chamber and not into the intake or ports like some other types of fuel injected engines. Direct injection has some performance advantages but on an engine that is on/off frequently like a PHEV this can cause fuel dilution in the engine oil.
Two Versions of FORM, the Simple and the Complex…Simple First
A simple version of FORM is caused by old fuel – if the Jeep hasn’t had more than four gallons of fuel added in over 90 days the Jeep will run the engine to burn off the old fuel. Really, if you are going over 90 days on a tank with regular driving we are impressed – sounds like lots of short trip electric only driving. On the surface exceeding 90 days sounds great but that engine needs to run, it’s just a fact.
Now the Complex
Imagine this, the engine is running and shuts off for a period of time while the Jeep is running on battery. The fuel injector shot some fuel into the combustion chamber just as the ICE shut off. This fuel is unburned and lays in the chamber. Since there isn’t a complete seal between the pistons and cylinder walls that unburned fuel drains down the cylinder walls and ends up in the oil. On a traditional ICE vehicle this usually isn’t a problem but the constant on/off of the hybrid operation magnifies the problem. A traditional ICE vehicle that is constantly driven short distances especially when it’s cold will experience this too. Add a turbo and it gets magnified.
So Jeep engineers programmed the 4xe to enter FORM to eliminate the dilution and prevent engine damage. The big contributing factors in the Jeep’s FORM algorithm are ambient temperature and oil temperature. Getting the oil hot enough will evaporate the fuel that is mixed into the oil and the PCV system will vent the fuel back into the engine to be burned.
The trick is getting the oil hot enough – Jeep says over 169°F (76°C) is the minimum, but normal operating temperature is above 194°F (90°C). Elevated speed and load will warm the engine oil most quickly. The Jeep’s coolant will get up to temperature way quicker than the oil. That takes the engine getting fully warmed up. In the winter with cold air flowing through the grille it will be even more difficult. More on warming things up – Getting a Hot Engine in the Cold.
Dilution FORM in the Wrangler 4xe behaves differently, depending on modeled dilution level.
-Step 0: Normal Operation: Below a bottom threshold, vehicle operation is normal.
-Step 1: Moderate fuel dilution: Between the bottom and middle thresholds, silent start allows electric operation during a drive until the first ICE start. After that, you’ll see the FORM message and the ICE will continue running until you shut the car off.
Note: this mode is actually a bit pleasant – the power of the Jeep ICE plus battery is really readily available in this mode. The jeepfan.com 4xe has been in this mode twice within its first 5000 miles. The 1st time it lasted 2 days thanks to a stretch of 50 degree weather and the 2nd it was about 10 days.
-Step 2: More fuel dilution: Between the middle and top thresholds, EV operation is not allowed. The ICE will start when you power up the vehicle and remain running until you shut down, or the bottom threshold is reached, whichever comes first.
-Step 3: More fuel dilution: Above the top threshold, EV operation is not allowed, and the vehicle will instruct you to perform an oil change.
There are some 4xe owners that get stuck in FORM and never seem to get out. Even an oil change will only clear it for a short time. In most cases the end result was determined to be something else faulty. Work with your dealer and Jeep Cares if you are experiencing this.