Drop in MPG - is our car just getting too old at 220K miles?

This, and the Auto AC (below), make a recipe for rotten MPG. Cast it aside as bad conditions where poor MPG is unavoidable.

My alarm bells are ringing loudly at this point, especially the fact that you haven't checked tire pressure. The new tires will cause a temporary drop in MPG until they are broken in. But you need to monitor your own oil level and tire pressure, not trust someone else to do it for you. Please check cold levels and get back to us. I particularly suspect low tire pressure.

The battery is unlikely to be an issue at this age, but is still worth checking. If the link for the GenII self-check method is broken, I'll ping a moderator to fix it.

Auto mode AC can be a significant drag on MPG. If tolerable, consider raising the temperature setting a few degrees and picking a lower non-auto fan speed. But in some conditions, AC is simply worth more than MPG.

The vehicle's total miles should not be much of an issue. My old Honda achieved it best lifetime MPG just before I replaced it, at 235k miles, with a Prius. The car itself wasn't improving, but the tuning of the nut behind the wheel was improving faster than the car itself degraded.

 

Hi sleelio. As you've probably guessed, that questionnaire is geared more toward the more recent owner who possibly have unrealistic expectations. In your case as a long term owner it seems most things are either unchanged or ok there. I would like to pick out a few points though.

1. "- Is your oil overfilled? (i.e. above the full mark on the dipstick) Don't know."

With the car at 220k miles you really should be checking that dipstick fairly often. Seriously, not knowing that question is a bad omen for your engine.

2. "- Are you using the factory tires and wheels? No. We just had the tires replaced with Pirelli P4 Four Season M&S 88T 185/65R15. These are the same tires we already had on the car for the last 50K miles with no drop in MPG."

While I know that there was no change there, you should check that the pressures are unchanged and also understand that with new tires (even of the exact same type) you may experience a temporary loss of MPG for several thousand miles as they get "broken in".

Also be aware that those tires are not particularly good for LRR (low rolling resistance). In a 2009 "consumer reports" comparison they rated in the second lowest of five categories for fuel economy. I know you used them before so (break in not withstanding) that doesn't necessarily account for the loss. Just be aware that if you wanted to you could pick up an instant 3 to 5 MPG just by better choice of tire.

3. "- Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on? No - never have"

This one is an interesting one. The inclusion of this question in the survey (which I has no part in drafting btw) was no doubt to portray it as a negative, to be avoided if you want optimal fuel economy. Surprisingly however, warming the engine for a short time (typ 40 to 50 seconds) can actually improve your fuel economy under certain conditions. Where this applies in particular, is if your traction battery is aging and where you often notice a significant drop in SOC (the 8 bar graph state of charge) in the first few minutes of the trip. The increased fuel economy here is actually only a side benefit (along with decreased engine stress), with the primary advantage being reduced battery stress on an aging Prius.

Edit: I see fuzzy1 beat me to the punch with many of the same points as I was typing. What can I say but "great minds think alike".

 

you can't judge anything by stop and go traffic. i'm surprised you haven't noticed this over the last 220k. drive a full tank and see what it averages. it's possible the hybrid 'big' battery is going south, but you should have some warning lites. have the dealer check for codes. the battery gauge usually goes from all the way down to all the way up over and over when the battery is bad.

 

If the gas engine is running all the time, then there is a problem. After warm up, you should see the engine shut off when braking to a stop, when coasting, when going down hill, and when sitting stopped. If the HV battery is not getting up to the blue bars range, and settling there most of the time on the highway, then there might be a problem with the gas engine power output. Check the plugs, air filter, throttle body, and anything else that affects conventional engine performance.

Otherwise, the engine can stay running due to a bad (shorted cell) 12V battery, but that does not look to be the case here.

If you haven't changed the transaxle fluid, it is way over due.

What does your battery indicator actually show?

 

(1.) Was that voltage measured shortly after the ignition was shut off? If so, measure again in the morning before the car is started.
(2.) Oil sounds OK.
(3.) Tire pressure sounds OK, as long as the gauge is good (have its readings been compared to other gauges?) and the tires are cold, not driven for the past few hours and not baking in direct sunlight.

I haven't been a Prius owner long enough to experience an aging traction battery, so must generally defer to others. But at your car age and mileage, battery failure is not a surprise.

'Full' charges on the battery normally don't happen on flat roads, but need regenerative braking on considerable hill descents.

In the long highway backup described in the base post, with high temperatures demanding considerable AC, even a good traction battery will deplete very quickly and you'd have the gas engine running for much of the rest of the traffic jam. But if this happens in more normal conditions, with the traction battery charge level (SOC) rising and falling abnormally fast, then it would suggest impeding battery problems. Other threads have better descriptions.

If the battery is latter found to be failing, don't automatically get a new replacement. There are several options for used or rebuilt batteries at lower cost.

 

This is normal, but it gets worse as the battery ages and it's capacity drops. Please read my above reply I addressed exactly that point (3), the cure is to warm the engine (idle) for 50 seconds before driving.

Now you've mentioned two separate but opposite problems. First you've said it's not using the battery enough and now you've said it's using it too much (rapid charge depletion) in the morning. I know that sounds weird but it is possible. If the cooling fan for the traction battery is clogged (quite likely if it's never been cleaned) then as the battery warms the car becomes less willing to use it, so this could be your problem.

In the morning the battery will be cool and the Prius prefers to use electric power while the engine is warming (first minute or two), this is the reason for the sudden drain people often notice. That's why the cure is warm up.

BTW. See this thread to learn about cleaning the HV battery cooling fan. If it's clogged it can definitely inhibit EV usage. Fur and dirt causing major battery problems | Page 2 | PriusChat

 

How does it correlate with the monitor gauge that shows bars of SOC ?
Any reason to think you may have a plugged up battery vent ? (dogs, kids in the back ) ?

Addendum: I see Uart beat me to it

 

That rate of change doesn't seem unreasonable, depending on conditions.

If you have or can borrow an OBDII engine monitor (ScanGauge, Torque, etc), there are programmable codes that can read out various battery block figures. This may be worthwhile.

Without climate control, very high MPG can be achieved this way. But add outside temperatures of 100F and heavy AC use, MPG will plummet. The car is using considerable fuel to power the AC, but rolling up few miles.

With extra heavy AC at slow speeds, and new tires, this doesn't seem bad. But when this happens at regular outside temperatures, then it is too much to be caused by new tires alone. It could be a sign of impending battery failure, but try to rule out the other less expensive causes first.

Do you hear any unusual loud fan noise from the rear seat? This would point towards a battery cooling issue, and otherwise mimic a failing battery. See Uart's earlier post for fan cleaning. Extended parking in very hot sun can also cause heat a battery enough for protective features to restrict its use. I don't believe an aging battery alone will cause this fan to speed up

 

MPG drops as the hybrid battery 'degrades' (drifts out of balance) is common, as the gas engine has to charge the bybrid battery more and more to keep it functioning properly. Users of our Hybrid Battery Rebalancers (Grid Chargers) are reporting 5-7 average MPG jumps after rebalancing their Prius hybrid battery. A common factor as batteries age is more background charging by the ICE which reduces fuel economy. We saw this as well with out own shop Prius, our 130k mile OEM battery pack significantly improved (+6MPG, much more EV mode driving) after only a single grid charge session. Perhaps something to consider for you car as well .

 

Don't bother with that. If there are no warning lights, the dealer won't have any trouble codes to scan, and will tell you everything is fine.