2005 Toyota Sienna driving like I'm pulling a small trailer

Asked by Sep 09, 2015 at 12:19 AM about the 2005 Toyota Sienna LE

Question type: Maintenance & Repair

My 2005 Toyota Sienna has 148K miles on it and I've owned since new.  Van is well maintained, and many things have been serviced.  Also have installed new mass air flow sensor recently.  Problem:  Van's mileage is down on highway from about 26-27 when newer to now 21-22 on highway.  Best way to describe it is that it drives like I am pulling a 500-700 pound trailer.  The least bit of grade and it downshifts from OD to 4th gear (about about 70mph).  If on a nice Kentucky hill, it will even have to downshift to 3rd gear (and jump to 4K rpm) in order to maintain  cruise control speed.  Yes, all this is while on cruise control, but problem is still there with cruise off and me trying to hold constant speed with throttle.  Basically, it is constantly shifting back and forth between 4th and OD unless the road is almost perfectly flat.  It DID NOT used to do this and when newer it could actually accelerate in OD gear (5th).  Any suggestions?

2 Answers

116,055

Fuel pressure and flow, dirty injectors, transmission pressure may be dropping due to worn internal seals, weak torque converter, you may want to have the codes checked just to make sure nothing is in the memory.

I have run about 3 cans of Sea Foam and about 6 tanks with Marvel Mystery Oil thru to clean injectors. Last time my mechanic checked, there were no 'problem' codes being thrown. Did he maybe not look deep enough? Have not checked fuel pressure and flow. Have only changed tranny fluid once at about 80K miles. Fluid still now looks healthy.

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