05 MPG down to 35...what's wrong?

My SWAG:

Could a hole in the exhaust pipe/muffler be causing the MPG drop?

Two scenarios:
* between ICE and Calytic Converter
* between CC and tail pipe.

In first case, wouldn't there be a code thrown?

 

Please do get back to us on the exact make, model and size of the current tires. If possible, let us know that of the previous ones.

It's odd that they're "mandatory". 2nd gen Priuses came w/15-inch wheels and touring versions had 16-inch. 17-inch wheels will hurt fuel economy vs. smaller ones. If those Toyos are high rolling resistance, that'll make things even worse.

Did you ever tell us details about your trips as in distance, time, how much stopping or stop and go? Also, did you ever actually get your alignment checked? It "feeling good" isn't a check.

There's a reason why I put up http://priuschat.com/forums/gen-ii-...-answer-these-questions-esp-if-youre-new.html. I think you should answer at least these:
- How are you determining fuel economy? (trip computer or manual calculations)

- How much of it is city vs. highway? Roughly what's the average speed in overall and and of each segment? Is there a lot of stop and go driving?

- If reporting a mileage drop, did anything significant change on your car (e.g. accident, hit a curb or big pothole throwing off alignment, oil change/other maintenance/repairs, changed tires or wheels, etc.) or your commute?

- How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking?

- Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on?

 

17 inch wheels on a Prius? There goes your mpg, IMO. For optimum mpg, you should have std size wheels, and 40-45lbs psi.

 

I think it's more than just an opinion about lost mileage due to larger wheels. When CR tested base (has 15-inch wheels) vs. touring Prius (has 16-inch wheels), the latter got worse mileage. See http://web.archive.org/web/20080618...-advice/most-fuelefficient-cars-206/index.htm.

On the 3rd gen, we were told by the chief engineer that 17-inch wheels would knock off ~1 mpg vs. the 15s (http://priuschat.com/forums/gen-iii...m/57380-what-did-we-learn-chief-engineer.html). http://priuschat.com/forums/gen-iii...ill-wider-tires-decrease-mpg.html#post1096974 also mentions a mileage hit for 3rd gen 17s.

When the OP responds w/his make, model and size of tires, he should also tell us the max sidewall pressure listed.

 

Whats the recommended tire pressure for 17' wheels? Usually low profile tires require a higher pressure i think.

 

There have been repeated suggestions that the OP should have the wheel alignment checked. I would not minimize the adverse impact that excessive toe-in can have. I recommend that the OP should have front toe set precisely at 0 and the rear toe set to be slightly positive (need to use shims to adjust the rear wheel alignment, a specialty wheel alignment shop can do this) for the sake of vehicle high-speed stability, consistent with the alignment specs.

Further, one item that has not been previously suggested is that engine coolant temp needs to be checked, after the engine has warmed up - drive 10 miles or more at freeway speeds. Scan Gauge can report engine temp.

Another way to check this is using a handheld IR temperature gauge, pointed at the thermostat housing or the radiator hose attached to that housing. Also check the temp at the engine coolant sensor screwed into the engine head. Operating temp should be around 185 degrees F.

If the temp is much lower then the thermostat is probably stuck open. This will not log an engine ECU DTC. However the engine will run inefficiently if below the correct operating temp and this will have a huge mpg impact.

 

Great suggestions Patrick. Several others related to the gas engine (that we often forget or take for granted) are also wise.

 

I agree for optimum mileage, I run 45 series 17" rubber, this summer my daughter used the car during august, my mileage was down in the 51.5ish range (about 10 tankfuls since a reset) but am now at 54.6 just going to .7,

I'll give up 2 or 3 mpg's for a much more secure ride, ie:great tyres
err, some people just drive in a manner that is not going to get very good mileage, even in a Prius.

 

It sounds like a frustrating problem Ken, you seem to have checked everything but still no joy. I agree with you about the tires, they're not optimal but if they've been on the car the whole time then it's not likely to be the cause of the decrease.

Since you've tried just about everything you can it mightn't hurt to try the following. Could be "clutching at straws" but one user who posted a similar story to yours of declining MPGs said that they restored their MPG with the AMSOIL "PI" fuel additive listed here : Featured Products

Please note that this story was very anecdotal (at best) and the user never followed up on whether or not the increased FE persisted after using the product. Given all that you've been through it couldn't hurt to give it a try. See reply #35 here : http://priuschat.com/forums/gen-ii-prius-fuel-economy/60794-reduced-gas-mileage-4.html

 

Maybe the two kids have being growing fast and adding tons of weight to the car? Sorry, couldn't resist.

Now seriously, I would think twice before using any additives because

1. There were other users having bad experience with additives, and
2. If there is indeed some mechanical issue as the OP is suspecting, then masking the problem with additives won't help solve the root cause.

Just my two cents. Good luck!

 

I should probably keep my mouth shut ...but... Occam's razor keeps slapping me in the back of the head as I read your problem.

Since the problem started around the time the inverter coolant pump failed I'd be very suspicious of everything about the replacement pump and the inverter cooling circuit:

Is it possible a peice of the impeller from the old pump broke off and is stuck somewhere in the cooling circuit?

Is it possible the hoses were put on backwards when the pump was replaced?

Is it possible there's a kink or twist in one of the hoses?

All of the above would partially block coolant flow and cause the inverter to operate hotter but maybe not hot enough to register a code.

Maybe one of the people who know what they're talking about could tell you how to monitor the temperature of the inverter. Good luck -- you've really done your homework and deserve an answer or solution to your problem.

 

I think we need to go back to examine what changed before and after the MPG drop. It looks like the pump failure is the only big thing. (It may have failed sometime before the warning messages got triggered.) Was it the inverter coolant pump? If so then here is one possible hypothesis. What if after the pump failure the big power transistors in the inverter were thermally stressed to such a degree that their internal resistances have permanently increased? If so then more fuel will be wasted as ohm heating. Unfortunately it will be very expensive (an inverter replacement) to prove or disprove the hypothesis.

 

Getting the inverter temperature from the computer messages is doable but not trivial. I like your hypothesis better in the sense that it is more easily fixable, unlike mine. Good points!

 

What you describe sounds like the hybrid system just isn't functioning correctly. I don't think it is related to the inverter, though, since power to the A/C is usually the first thing that gets cut off.

Possibly the HV battery air duct is blocked? This is located on the passenger side between the back seat and the door. If you can hook up a 1 inch hose to a shop vac and snake it down in there, it might pull out a bunch of crud, and that would be a good indication of the problem. I remember being a kid and stuffing things into all kinds of weird places --- dropping rocks down plumbing vent pipes on the roof was a special favorite because it made a really schwoomp cool noise, and it made the toilet overflow too. Through karma, my own kids regularly turn the back seat into a rolling landfill nowadays.

Anyway, if that duct is blocked, then there is no way for the fan to cool the batteries. Basically, they are already hot when you start the car due to the weather, then get even hotter due to the preferential use of the HV battery to propel the car as the ICE warms up, then if they don't get cooled off by the fan you will only see limited boost/regen, and thus lower MPG. If the fan itself isn't working, then it should throw a code. But several scan tools can test it, including AutoEnginuity.

Monitoring the engine temp was a very good idea. If the car was not reaching the full hybrid mode stage post-warm-up (coolant temp = 165F+), then fuel economy would be impaired. This might still be the case, that there is some other temperature sensor (cat. converter?) that is not up to normal operating temperature and is causing the engine to stay in a rich-burn mode. This also means that full hybrid functionality is not available (warp steath, no-arrow glide).

Finally, there is a possibility that I didn't see anywhere was having your injectors cleaned and tested. This is not entirely consistent with what you have described, which seems like a problem with the hybrid system rather than ICE by itself, but it is always a possibility. DON'T let someone put something in the gas tank or fuel injector rail. Your dealer would love to run a can of BG44K through and charge $150+ for it. Best to take them out and have them cleaned ultrasonically and flow tested. There is probably someone in your area that performs this service, about $80 for 4 injectors plus the labor to remove and install. Try this last since it means money out of pocket.

 

I suggest that you reset the MFD mpg indicator, then drive at 40-50 mph for 30 miles or so, on flat roads with minimal traffic and stoplights - then return to the starting point. That should produce close to the best mpg that the car is capable of delivering. What indicated mpg do you see after that test drive?

 

I am having the same problem. 2005, 117k mi

3-4 months ago I was getting a constant 46-48 mpg averaged over 200-500 miles. Now I can't get it to average more than 38 mpg.

Apparently, there is no standard fix for this.

G