What could cause an increase in city fuel consumption while highway fuel consumption is pretty normal (~10 L/100km)?
The vehicle is a 2009 Toyota Venza V6 AWD ~160k km
The total (city + highway, mostly city) average fuel consumption, calculated at each fill-up, has gone from a previous 12-13 L/100km to over 15 L/100km.
The change occurred suddenly a few months ago after an oil change and tire rotation in March 2022, but that might be coincidental.
- Using regular fuel, from different major brand gas stations, no E85 etc
- Checked for brake drag (cleaned and greased hardware and slide pins) and adjusted parking brake to max tolerance to rule that out
- Tires pressures verified at the specified 32psi
- Tires are only about a year old. Tire balancing and wheel alignment were done at purchase
- Air filter is clean
- Throttle body was cleaned a year ago
- Mass air flow sensor was cleaned a year ago and still looks clean
- Engine oil at correct level, changed regularly, no loss of oil between changes
- Transmission fluid drained and filled (and level checked) a year ago
- Transfer unit and rear differential gear oil changed a year ago
- Engine coolant at correct level. Was drained and filled a year ago
- Engine temperature warms up quickly, does not overheat
- Haven't found any vacuum leaks (so far) including while using a smoke machine at intake
- Spark plug gaps are not over spec
- Noticed some darkness/wetness around some fuel injectors, but doesn't smell like anything and didn't seem to present a vacuum leak
Always looking for anything else to check...
Noticed that adding Seafoam to the fuel tank and/or separately to the crankcase (just days prior to an oil change) very noticeably improves the engine/drive feel/performance in terms of how smoothly/effortlessly the vehicle moves, and also improves fuel consumption in short order, going from ~15 L/100km to ~13 L/100km average with Seafoam even though it was not in the system for the full period being averaged. However, the effect is temporary. Not sure if this provides any clues.
Another odd observation is that touching the brake pedal increases the instantaneous fuel consumption (decreases mpg) reading in the car's in-dash display. During braking, (and with the foot off the accelerator..) the fuel consumption spikes to the maximum reading (in L/100km, would be minimum in mpg) until a few moments after coming to a complete stop. I don't know if this reading is real or a quirk of the car's computer.
Here are some plots of logged data from a scan tool, for both city and highway driving. Notice how the instantaneous fuel consumption spikes off the chart on every start (no drag racing here, just gentle driving), and during every stop before reaching idle.