Engine stalls when driving

RAYFIO

MEMBER
2008 MERCURY MOUNTAINEER
4.0L
6 CYL
4WD
AUTOMATIC
191,000 MILES

I bought the car second hand. The engine stalls when either when slowing down or sometimes it stalls when driving on the highway. It is only recently that the car mechanic told me that the car does not have an engine display light. Perhaps the previous owner removed the check engine light. I do sometimes get a fuel cap sign when I start the engine but the mechanic has not figured why the fuel cap sign is comes on. We press reset and it goes away and many times the car is fine after resetting and removing the fuel cap sign.

1 Reply

BMDOUBLE

MECHANIC
1,139 POSTS

The fuel cap light was added on Ford vehicles for only a few years because it is redundant and only is the fault of the cap very few times. The reason that the fuel cap light comes on is because of a fault in the evaporative emissions system, specifically it is not holding a vacuum when the purge valve and vent valve are closed. Think of the system like a soft drink cup from a fast food restaurant, your mouth is the purge valve, the straw are the vapor lines, and if you poke a hole in the bottom of the cup your finger will be the vent valve. When the power-train control module wants to do a check up on the evap system it will command the vent valve closed and the purge valve open to pull a vacuum on the fuel tank, after it has pulled sufficient vacuum on the tank according to the fuel tank pressure sensor, it will close the purge valve and look at the fuel tank pressure sensor for a calibrated amount of time to ensure that the tank and lines are holding the vacuum. Now if the vacuum rises too rapidly, there is a leak in the system like a fuel cap loose, fuel pump o-ring leaking, vapor line broken, fuel canister damaged, vent valve not closing fully, or a faulty/damaged tank. To put your 2 symptoms together I would look for vacuum leak or a stuck open purge valve, this will cause an engine stall and an evaporative emissions problem.