I am having the same issue as rizzman13 is above in FL in 2019 with my 2014 top of the line MKZ Hybrid. I bought the car used certified with only 22,000 miles on it. It was getting 39 miles to a gallon. Once summer hit here in AZ my mileage dropped to 35. I was given it’s ‘the heat’ explanation and I thought well okay it runs harder because it really gets hot here int he summer. Then the following summer I started hearing a noise coming from the air conditioner. I took it in. They said that is normal. Well now my gas mileage is down to 31 MPG. I called again to tell them that. They said they had some other complaints about the same thing in gas vehicles not just hybrids. The air condition is not using the hybrid battery and the car is using more gas. I have an extended warranty and I’m told that they will not fix anything until it’s broken. So I am now digging into what can be done about this and I am not going to stop until I find an answer. I am getting 8 miles less per gallon. I might as well own a regular gas vehicle even though I absolutely love this car. I love it over every Mercedes I’ve ever owned.
Just to be sure, you haven’t changed your driving habits, have you? I get over 40 mpg in my car when I’m driving at speed on the highway, but only about 25 mpg on short trips at low speed. Since you have a hybrid, the first few miles will be on the battery, then will switch to gas. If you drive in heavy traffic after the battery gives way to gasoline one power, it could account for reduced mileage.
The a/c is powered by the hybrid battery on my 2011 MKZ hybrid, and I think yours is too. The drop off to 35 mpg in summer is what I’d expect. The 31 value is low. The first thing I’d check are the tires. Are they at the specified pressure first thing in the morning? I add 2 psi over the door jamb sticker. Are they new tires? I lost 2 mpg with my latest tires. Any unusual wear? Bad alignment will cost mpgs.
That’s not how Ford’s system works. The electric side assists the gas side and takes over in low load situations. You can accelerate from a stop on battery only, but very slowly.
The most common mechanical reason for reduced mpgs is the engine is running at a temperature below the designed spec. Faulty thermostat is where to look first. You’ve got a 5 year old thermostat I expect that’s getting a little cranky. If it hasn’t been done recently, this would also be a good time to replace the engine coolant with fresh stuff.