NOTE: The car is 2006 Mercury Milan 2.3L 4cyl.

About ten months ago I have hit a pothole and my exhaust fell and the check engine light immediately came up.

I've put back the exhaust and drove home.

Later, I plugged the code reader and I got a note saying:

"Very small EVAP leak"

I erased the code and started driving the car. A week later the code came up again (same code). I started noticing my fuel consumption getting way higher. I had 27 MPG which dropped to 16 MPG over time. I had no time to fix the car then and the leak was very small to affect the power of the car. So I let it be.

Anyway, recently, I performed Smoke Test on the EVAP system. It took them a while pumping smoke into the EVAP system and eventually they sprayed the Vent Solenoid Valve with some liquid and noticed very small leak there. They changed the clip and sprayed again and there was no leak anymore.

We are still not sure if that was the exact problem or not. I have been driving it for a week and the light still has not come up (hopefully it doesn't).

What is the real reason behind the drop in fuel consumption? Could the Vent Valve cause the problem? I have asked a friend of mine and he told me that in some cars, the check engine light might fool the car to run in Rich Mode all the time. It makes some sense because when I erased the engine codes, the fuel consumption got better (until the code came back and I was screwed again).

Could someone help me understand the situation?

  • Tell us more about the car type, manufacturer, year and variant. Commented Jul 1, 2017 at 20:39
  • @NumairAidroos just added the information.
    – Everyone
    Commented Jul 1, 2017 at 23:47
  • How are you calculating MPG?
    – Moab
    Commented Jul 2, 2017 at 1:17
  • A car going into rich mode because a check engine light came on doesn't make much sense and I've personally never seen this happen. I'd be interested in seeing before and after lamda values and fuel trims if available.
    – Ben
    Commented Jul 2, 2017 at 14:14
  • @Moab The car displays the MPG on the dash
    – Everyone
    Commented Jul 3, 2017 at 3:17

1 Answer 1


Well there was a vacuum leak. Plain and simple logic dictates a drop in fuel efficient performance as the gases are not being recycled as the Evap system would when it's running without leaks at optimal efficiency.

The moment you ran the test and fixed it, your problem vanished.

Now the theory with the check engine light is accurate in your case as it was indicating a leak. And even in most circumstances anything close to the intake and exhaust manifold failing may show up by a sensor via the check engine light. Understand the ECU is not programmed to run rich, its just a consequence of the recycling process failing that leads to this. If your theory were true, no offense, we'd just reprogram the ECU to run lean. Its not as simple as they made it sound. Just because we switch to sport plus mode on the transmission doesn't mean fuel economy will drop, we have to drive the car in a spirited nature at high RPM to see that drop. Even in normal mode if i drive rough my FE will be less.

I hope I'm not confusing you with bad grammar here. I believe you got the point.

  • 1
    I had my doubt on the theory (which isn't really my theory) and I didn't believe it to be true since my RPM value drops after a couple of minutes running on a normal day. If it was running RICH the RPM wouldn't drop. Thanks for the response though.\
    – Everyone
    Commented Aug 2, 2017 at 3:14
  • 1
    How is the fuel economy in figures now? Commented Aug 2, 2017 at 13:53
  • 1
    I still haven't replaced the vent solenoid. I just got the leak even smaller and the fuel economy got better. I am replacing it soon and checking up again.
    – Everyone
    Commented Aug 3, 2017 at 8:00
  • Please get it replaced soon. Commented Aug 3, 2017 at 10:50
  • 1
    How is your FE now? Commented Jun 21, 2020 at 9:10

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .