Poor MPG engine not shutting off during coast or glides

Alright to start with I've owned the car for over 3 years. It's higher mileage now at 195,000 not using oil. Tires are Bridgestone ecopias and at 42psi. Alignment is good checked last year no big potholes or curbs hit. 12v battery replaced 4 months ago. New air filter and spark plugs replaced 4 months ago. The EGR cooler and valve were disassembled and cleaned and reinstalled by me and no error codes or DTCs have ever come up. When I first got the car I was getting 54mpg (using the cars calculations) while living in West Michigan relatively flat with highway speeds to work and back around 70mph. Shortly before our move out to Southern California the mileage started to drop down to around 48mpg, then to 42mpg still using the cars display calculations. I've tried it with the HVAC controls off and it makes no difference

What I've noticed is when I start driving and the car warms up and the ICE will shut down once its warmed up at a stop. Then I start driving and I'll go to coast down a hill but the instant mpg indicator will only go up to about 30-40mpg and stay there. The engine will still be running with my foot let off the gas for several minutes. I seem to remember it would shut off almost instantly in a coast or glide. Now I can coast all the way to a stop and the engine will finally shut off once the car comes to a stop after a few seconds.

I've replaced all the brake pads and lubed all sliding pins, I've also replaced in the past both front wheel bearings as they were getting loud. I need to do the rear wheel bearings soon I think just because of the mileage, also the axle-shafts as I get some clicking when shifting from forward to reverse. These old axles and bearings could be bringing down the mpg but would not make the engine stay on. I'm thinking either a thermostat or some electrical sensor is giving bad info and causing the engine to still be on during glides and such. Anyone have a clue where to start? I've done a fair amount of googling and haven't come up with any good threads or suggestions. I'm also open to suggestions on obd scanning software. I have Andriod phones and a laptop to use any suggestions on dongles or cords software ect?

 

Hey guys thanks for the replies. Sorry on the super long delay, since moving I've used my car about once a week lol so its been a back burner kind of deal. I will def look into the battery harness and charger as an option. I've thought of going the hobby charger route too as I didn't pay a ton for the car I don't want to drop close to $1k on it to just condition the battery. I've got all the time in the world here as I'm currently a stay at home dad with a 3 year old.

So on a side not I did pull apart the rear brakes again as the rust build up on the lips was making a bunch of noise and rubbing so I removed all that. Lubed the caliper pins ect and made sure the caliper pistons were free and retracted when spun. I kind of exercise them about 2 times each, I bring them all the way back remount them, then go in the car and gently push the brakes until they are seated and do it again. I found one slightly hanging rear pin so thats remedied and I went from 42mpg average over 800mi to 45mpg over 700mi. I'm going to get a set of replacement rear wheel bearings as I do get more noise than usual from the rear. I did tow a garden trailer with motorcycles around town and on a trip or two. I know the bearings are original and I know I've put more weight on them than they were designed for (yes the trailer was properly loaded ie not tongue heavy). FYI priuses are wicked little tow cars, I'd get 33mpg with a 12ft trailer and two motorcycles over a 800mi trip. Probably could've gotten better mileage had I removed the giant ramp on the trailer (garden tractor ramp at least 4ft long and steel mesh design very heavy and stuck up high for extra wind resistance).

As for DIY type yeah. I was a ASE certified auto mechanic for 8 years in another life. I owned a marine service company (diesel, electrical, electronics install and repair, fiberglass repair, gelcoat repair, sewing sails and canvas work) up until last year. I used the car for a work vehicle as well and its been fantastic. Plenty of storage for all my gear.

Any who I'll report back after new rear wheel bearings and see where I am at.

I'm worried as I get the occasional chugging at high load low rpm when the engine kicks in which is telling me the rings are fouled via the whole crappy PCV set up and I've not done a catch can system so I know its gonna be a problem. (Bought the car at 160K mi so I figured doing a catch can at that high of mileage wouldn't prevent a heck of a lot.

 

Yea I keep a close eye on the batter charge and my power level indicator bar thingy. It doesn't seem to cycle very fast. If I'm in traffic with the AC on 73F it'll kill the battery from the standard 2 bars less than full charge in 20 min or so with some moderate stop and go driving. It will get a full charge occasionally going down the large grades and mountains on the highway here in Socal. Sometimes when the charge is low and the engine kicks on it doesn't seem to charge as fast as it used to but that could be my imagination as I never timed it before.