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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.

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  • You didn't state the "when" in this equation. Since when has the fuel mileage gone down? Has it been in the last 6 months or year or ?? Aug 19 at 11:12
  • If you have a scan tool, please edit your question to include short term and long term fuel trim numbers. These numbers may reveal a problem that's not immediately obvious, such as a small undetected vacuum leak.
    – MTA
    Aug 19 at 13:39
  • @Pᴀᴜʟsᴛᴇʀ2 This issue started suddenly 4-5 months ago.
    – adatum
    Aug 19 at 16:49
  • @MTA Added plots of logged scan tool data including fuel trims. Total fuel trims (short + long term) is typically around +10% at idle since we got this vehicle almost 2 years ago, and has been consistent since before this issue began.
    – adatum
    Aug 19 at 16:51
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    I once detected a fuel leak by observing dripping directly from the fuel line when the stuff under the hood was cold. When it was warmer under the hood, there was no dripping, since the fuel evaporated at that warm temperature immediately instead of dripping. There was occasional fuel smell, though. Fuel leaks can however happen in other places too, like in the lines from the tank to the engine injectors.
    – juhist
    Aug 19 at 17:00

1 Answer 1

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The scan tool plots are very helpful. Your fuel trim is out of whack. It should trend toward zero after warm-up at idle and while running at constant speed and constant load. Yours is running at around 10%, which means that the engine is running lean and is compensating by adding more fuel. This can be due to a vacuum leak.

In city driving, the engine spends a significant amount of time at idle. If fuel trim is almost always positive, especially at 10% or more, this will have the greatest effect on fuel economy at idle when fuel consumption should be at an absolute minimum. (It never reaches that minimum.) In highway driving, the effect is minimized due to lack of time spent at idle.

One way to find a vacuum leak is to take a propane torch, remove the burner tip and attach a length of rubber hose where the tip was, then attach the rubber hose to a length of metal brake line (or similar metal tubing). Pinch the end of the metal tubing to leave just a small opening for propane to blow out of. You want a gentle hiss, not a blast.

While the engine is warmed up and idling, taking care to stay clear of belts, fans and exposed electrical parts, turn on your propane wand and slowly sweep your plume of propane over various engine parts. Concentrate on the intake manifold, the throttle body area, all vacuum lines, etc. Don't forget vacuum lines that go to your braking and HVAC systems.

Listen to the engine as you sweep the plume of propane slowly over engine parts. As you approach a vacuum leak with the propane plume, there will be a momentary change in rpm -- possibly a quick increase, then a return to normal idle speed. As you pull the plume away, rpm will drop, then return to normal.

If you or someone else can monitor fuel trim during the exploration, the vacuum leak detection will look quite obvious: a drop from +10 down toward zero or into negative territory, depending on the size of the leak.

NOTE: You said that fuel trim has always been around 10% and it has not changed recently. I'm acknowledging that, but ignoring it for now in the interest of either confirming or ruling out a vacuum leak. If you find no vacuum leak, then I suggest you explore other reasons why fuel trim is / always has been so high. Perhaps a new question on this stack would give you more ideas.

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  • Appreciate the detailed response. I've been looking for any vacuum leak for a long time, including with a smoke machine on the intake recently, but haven't found any. I don't feel very comfortable with the combustible method of leak detection. A master diagnostic technician mentions +10% even +15% can be normal as long as there aren't other issues. youtube.com/watch?v=fDwq4OR6FT8 O2 readings look ok too. Plus, as we both noted, fuel trim hasn't changed since the sudden appearance of the fuel consumption issue.
    – adatum
    Aug 19 at 20:29
  • A vacuum leak implies higher than normal idle rpm. Propane in only one of several ways to diagnose a vacuum leak. Using plain water from a spray bottle will do eht same except water ingestion in a vacuum leak/high idle would simply lower rpm since water isn't compressible and non combustible.
    – F Dryer
    Aug 19 at 23:14
  • Do you have any baseline data from a normal engine run? This would be helpful to compare original baseline info against this problem. Baseline data was used to determine a faulty maf sensor.
    – F Dryer
    Aug 19 at 23:18
  • @FDryer Idle rpm is 700-800 rpm which seems to be normal. I haven't noticed an increase in idle rpm since the fuel consumption issue started. I did previously use soapy water to try to detect potential vacuum leaks with bubbling, but did not find anything. P.S. Tagging my user name is helpful, otherwise I do not get notified of your comments or replies to someone else's answer.
    – adatum
    Aug 20 at 23:52

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