I purchased an '09 Mercury Mariner Hybrid AWD a few years back for the kids. I've never been impressed with the gas mileage, and lately it's gotten downright awful for a car this size. I have seen upper 20's on occasion, but average since owning the vehicle I'd say has been about 24 MPG. I just borrowed the car & noticed the mileage computer showed 21MPG. It's not throwing any codes & runs well. I did replace the O2 sensors a couple years back to see if that made any difference, but I can't say I noticed any change. I know the battery & electric drive is working to some extent as I can drive for a short distance on just electric if I'm extremely gentle with the throttle.

My son mostly drives it to school which is only about 4 miles from home. Could the fact that he's driving such short distances explain the poor mileage, or might there be something wrong here? Have any suggestions on how to troubleshoot this condition?

Thanks for your assistance!

 

Even in winter I got 28mpg. Short trips too but the car had to warm up to go into electric.My highest calculated was 34, highest displayed for a while was 36. What hurts gas mileage for me is the A/C compressor. When I first had the car, the injectors were really dirty and cleaning those helped a ton. Check short/long fuel trims with OBDII. Make sure the O2 fuel sensors you replaced were decent quality and not the wrong ones.

 

I have bolded the above.

My son drives my old Prius. 1.5 miles to work 2X/week. 5.2 miles to school, 4X/week.

He gets 34mpg... in a PRIUS. He's actually a reasonably conservative driver... unlike his dad. When I drove that bastid all over the valley, my average was 38mpg over about 50K miles. I drive aggressively... well as much as you can in a Prius. If I make an effort, I can squeeze 42-43mpg out of it pretty easily.

Short trips are MURDER on gas mileage. Period. I get dozens of calls from people concerned about how poorly their Prius mileage is because they think they're supposed to be getting 50mpg. Without fail, one or more of these things is happening:

1) Use of cabin heat
2) Use of defrost
3) Short trips.
4) Multiple consecutive short trips (hitting different stores nearby for 20-30 minutes at a time).

The best thing you can do for your mileage is swap cars and drive it as you think you should drive a hybrid for a full tank's worth. Then you'll know if it's the car or the driver.

 

I basically want to put my 2 cents, for someone else that may have low MPG in Escape/Mariner Hybrids my SUV had a poor MPG and suspected Stuck open thermostat but before I do that, got forscan and check under (PCM module) the CHT (Cylinder Head Temp) AND Coolant temp ICT(internal coolant temp) CHT tested OK on the HOT side since it has 2 thermistors in it I had to test the Cold side so waited next day and confirmed values found https://f01.justanswer.com/nmwagscha..._2014_f150.pdf the Cold side was out of range so I changed the CHT and Voila!! my gas mileage went up 7 MILES per gallon!!! SO before changing the thermostat ALSO CHECK CHT
You cant do this too manually when COLD take the CHT out without touching the probe, check your ambient temp let it sit for 10 min so it gest the ambient temp,, use a multi meter Black lead goes to Battery ground and the positive goes to lead (1) from the connector in my case I had type A, it read 5v so i know the PCM is sending the correct value, then Connect the lead (1) from connector to an alligator clip to the corresponding pin in the CHT, the other pin goes to the Red lead from the Multi meter mine read 5v on 60 degress Fahrenheit and as you can see in the tables return voltage should be between 3.73 - 3.26v so definitely out of range. Resistance Values were OK maybe that is why was not triggering check engine light codes.
if you want to do this HOT you will have to leave the CHT in-place (installed ) warm up the car and splice to the corresponding wires and measure voltage
the most important is the voltage Return VOLTAGE.
Since my CHT Resistance value was correct I didn't trust in that simple because it can be reading the resistance from the hot side,, it was not my case on the Hot side so I did not bother and simply replaced the CH.

the reasoning behind this is that since the escapes/mariner does not have a Coolant temp sensor CHT sends signals to the PCM (power module control) and with the incorrect values the PCM makes the wrong calculations and the car was running RICH, in my case the CHT was sending signal value as if it were constantly cold so the PCM
kept injecting more gas than needed -hence poor MPG.
(not sure if 2010+ has a Coolant sensor in housing)

Cheers

https://www.fordforums.com/threads/0...ileage.304322/

 

I guess i spoke too soon, as i still had problems with the MPG, I ended up replacing the thermostat i Installed Motorcraft-RT1194 Thermostat that opens at 190 degrees, i think the model -RT1193 is supposed to be the correct part number the tr1194 fit perfect in my MMH 08 ,,, i think the 1193 opens at 180 or less , but is just a thought anyways RT1194 fits and now it runs better than ever