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Tuneup decreases gas mileage

I had my 2004 Toyota Rav4 tuned up and the transmission flushed. After that, the gas mileage dropped 2 mpg! I took it back to the shop and they did not see anything out of the ordinary, and he cleaned the throttle body just in case that was the issue. All he could say is that it is because the temperature dropped and that affects gas mileage. But the temperature is not that drastically different and I am getting consistently 2 mpg less than before it was tuned up. The air filter was changed too. Any thoughts on what to look for? Maybe wrong type of plugs or something?

Have you checked your tire pressure?
Have you switched to a different type/brand of gas?

As to the spark plugs, I am a firm believer that only the spark plugs (brand + model number) listed in your Owner’s Manual should be used. There have been cases of decreased performance due to using “equivalent” plugs, rather than the factory-spec ones.

Did your mechanic install plugs different from what is listed in your Owner’s Manual?

Who did the work? “transmission flush” has me thinking ‘quicky lube’ kind of place, lots of room for errors there.

It was a reputable repair shop. I actually pumped up the tire pressure hoping to get better mileage but not much help. The gas is the same, I will ask them what type of plugs they used.

Also check on the ATF. What else did they change or adjust?

Any Toyota should only use NGK plugs. These are the OEM supplier for Toyota. Stay far away from Champion and Autolite spark plugs. I’ve seen reduced mileage and performance with Toyota’s using these.

@BustedKnuckles a former colleague of mine had a sister who put cheap Champion or Autolite plugs in her high end Benz. It was running rough and actually misfiring. When he put in the factory platinum plugs, all those problems went away.

Measuring mpg is fraught with problems, so the OP’er should consider that maybe this is a measurement problem and the mpg hasn’t actually changed. If however the OP’er has measured mpg on pretty much every tank and has done this right up until the tune-up, well, maybe there is something not quite right. I concur w/@BustedKnuckles to stick w/NGK for Toyota plugs, and use the exact one spec’d in the owners manual, and check the gap is to spec. I think if plugs other than NGK were installed, or if there’s a question whether they’ve been correctly gapped, that’s where I’d start. A tune up should either not affect mpg or increase it slightly. I guess I’d suspect next – if several mpg tests over several tanks of gas show the same thing – the ignition timing might have been changed, possibly advanced, but is this even possible to adjust in a 2004? I don’t know. But ignition timing is possible to check, so that’s something that can be readily done by a mechanic and compared to spec. My second suspicion would be that a vacuum hose might have not be reconnected the way it was, or is loose, so checking the vacuum at the various spots could be done.

@GeorgeSanJose how would you check the ignition timing on that car? I believe it’s coil on plugs.
Does it even have any spark plug wires? Well if it does, you could hook up the timing light to #1.
But I’m fairly certain that the timing is not adjustable.

Well you got me there @db4690, I don’t know how timing is checked with that kind of plug. But it seems if there is a wire connected to the plug, there must be a way. Maybe somebody here knows?

I think I’d want to know what they used for ATF when they did that flush. The fact that it was flushed causes me concern.

This engine requires Iridium plugs. Iridium plugs have a finer point on the electrode which causes a more dense corona to form and a hotter spark than a regular tipped spark plug. Make sure the right plugs were used. The NGK Iridium plug is the best to use on this engine. These are good for 120k miles and there is no reason to change them sooner. They have a very good anti-seize plating on the threads so they wont stick in the heads.

Why did the temperature drop? Did they put a lower temp thermostat? If so it should be changed to the right one.

@oldtimer11, the temp he was referring to is amient air temp because of winter, not coolant temp.

Thanks for all the good feedback. As for the transmission, I called it a flush, think the shop called it a service. I probably am using the wrong term. One other piece of the puzzle (and yes the gas tank on this vehicle is so small that I am measuring mpg on every tank), is I would always put the gas cap on tight but not until it clicks. Never had a “check engine” light come on before, but after the service, if the gas cap is not turned to “click”, the check engine light will come on. Have no idea how that is related, but feel it must be.

I have found that checking mileage from one fill up till the next is inconclusive. Seeing the data on fleet vehicles that are driven 300 to 700 miles per day might surprise many. Weather can play a bigger part in fuel usage than most people imagine. Vehicles parked out in winter weather over night must be run at idle until the windows are defrosted and often left running to keep them warm when the driver makes a stop.

And on a few vehicles moving the cold air intake to the radiator shroud to heat the intake actually increased mileage and improved driveability.

@Jimdbridge, your comment that the gas cap/check engine light workings have changed since the tune-up might be consistent with an unintended change to the car’s vacuum system. I don’t have any experience with your car in particular, but some cars use engine vacuuum as part of the gas tank venting system, for example through the use of an EVAP Solenoid controlled by the ECM (or PCM).