Dramatic Drop in Fuel Mileage - Please help

<div class='quotetop'>QUOTE(richard schumacher @ Jun 27 2006, 10:33 PM) [snapback]277805[/snapback]</div>

Hello everyone - thanks for the continuing help and illumination about the reflash. That would make sense with the ECUs I have had experience with in the past - the computer goes back to the base memory table and then relearns from there. The impact on the Prius, because of it's high mpg bracket, is more pronounced and at first surprising. I will keep track of the tanks and see if it is coming back up or not.

I have had the oil changed back to Mobil 1, and this time made sure it was dead center between the divits on the dipstick. Previously the dealer had overfilled it a bit above the top divit. They hate to throw away that last 1/2 Qt I guess. I checked the throttle body, it was very clean, dry, no sign of oil infiltration. The next tank has crept up to 43-44mpg. That's an improvement over the 38-42 I was getting on the previous several tanks. It's anyone's guess what it is actually attributed to - the relearning or the oil.

I also had the car aligned again - slightly out of whack with a minor toe out (yes out - thanks to the dealer again). That makes it less twitchy, not sure it had any measurable effect on the mileage.

Stay tuned - I won't do much else until I see what the next several tanks exhibit.

Thanks again - !!
I love this forum!!



<div class='quotetop'>QUOTE(Alnilam @ Jun 27 2006, 11:33 PM) [snapback]277858[/snapback]</div>

Hi Bob-

Some of the 2004s, not sure of 2005s, had to have a reflash of the ECU program to correct a potential set of issues resulting in a FORD (Found on Road Dead) symptom in the Prius. Some owners had a complete shut down of the system while driving. That resulted in a recall where Toyota downloads a new program to overwright the previous one (software patches). In so doing, the learned memory table that determines operation of the ICE parameters, like mixture, timing, etc, goes back to virgin and has to have some experience (as all virgins do) to get back to it's previous state of operation. That's the theory - I'm testing it for myself, others here seem to have had similar experiences.

That help?

Gene


Ya know - I think there may be something to this "better than before" mpg you guys are getting after the initial dip (or crash) post the reflash. If you were as shocked as I was - you started driving as economically as you could to get those numbers back up there! (Give us numbers and we'll try to beat them.....us engineers are a scary bunch).

My bet is that most of the "better" comes because we adopted better driving habits. I know I did!!

Ya think?

(Grin)
Gene

 

Here's an update on the status of my search for my lost mpg.

I got the car aligned by Miller Toyota - they found that both wheels were out of toe and the centerline was off. That means the previous dealer (Bill Page again - groan), simply whacked the steering to one side to zero one toe out and put all the correction in the left front wheel - that makes for an eternal tracking problem. Fixed that and now it handles as it should, much better. Perhaps a slight increase in mpg, too early to tell yet. They did this realign check before installing the recall for the steering to give me the info I needed, then realigned it after the recall was installed.

I also had the TSB installed to change the filler pipe (my VIN does not need the computer swap). The dealer thought that may also be part of the mpg loss - messing with the ecu's assumption about the amount of gas in the tank, but I don't think so because my loss in mpg is calculated by me, not the MSD display. So - it's easier to fill, but no idea what effect this will have on recovering the lost mpg.

Wash DC and Norhtern VA is eternally on "10% Ethanol" - we never go off and haven't been off it for several years so that should not have dropped the mpg.

Dealer checked the A/C function and cooling - it's blowing 42-45 degree air which is right where it needs to be. Scratch the A/C running too much as a source of the lost mpg.

The tires should not affect the mpg by this much, according to the dealer, and our forum chatter.

The dealer checked the fuel trim and said it was okay - nothing out of whack so they cannot figure out why the mpg has dropped.

So -we really are down to the reflash of the ecu, and hopefully a relearn of the settings. I've run only one tank since all this work, and it is still around 42 and not good around town at all. Grrr.....

Gene

 

<div class='quotetop'>QUOTE(dagerlach @ Jul 10 2006, 01:12 PM) [snapback]283901[/snapback]</div>

Hi Dave - thanks for your reply. Yes it appears there were a number of factors involved, however as I systematically remove each one of them, the mpg is not recovering much. The chatter on this board about Comfortreds seems to vary between no impact to mpg, to maybe 1-2 mpg. So, if it was just tires, then I would still be getting around 50mpg, rather than 38-42 I was really working hard at getting. You're right, they are little changes for the Comfortreds, and shouldn't make dramatic changes of this kind. Actually, they would come into play only if some overriding aerodynamic was not overshadowing their effect. My guess is that is why people are seeing no impact in some cases - they have something else going on that is out in front of the tire effects. When it comes to aerodynamics, not all conditions are additive - it is an onion peel more or less, of first, second and third, yada yada effects coming into play as you continue to refine and improve the whole architecture of the beast. The Prius is not that sensitive to small changes - but it is nicely tuned from undercarriage to roof.

Clearly the alignment was a problem. I have had one tire be low on pressure vs the other side and seen a fall off of a couple mpg, so I can easily attribute the first bit of recovery of mpg to a good alignment (or might I say corrected one).

These reflashes erase the learned memory in the vehicle, which is the fuel map for actual operation. It replaces, slowly, better matches for conditions initially set by a universal base map that is loaded in your car when it is first assembled, and again when it is reflashed as in the last ecu update. From everything I have seen so far, it is likely due to that and other folks have seen a gradual return to previous levels after this reflash.

If that ain't it - it's going back to the dealer for surgical dissection of it's brain.........

Thanks again -
Gene