My 05 Tundra is running rich..

So i noticed recently my truck was burning fuel, like bad.. 300 k per tank rather than 500.
Took it into shop.. they did some stuff chekced the spark plugs, MAF seson 02 sensor.. the tech ended up cleaning what he said was a very dirty MAF sensor.. ran like a dream for 1 tank of gas and now its right back doing the same thing.. weird cause it didnt happen gradually, just boom, right back to same crappy mpg's.

i did find this thread on another tundra forum.. bringing it back into the shop and was wondering if any of you have dealt with this and have any ideas so i can share with the tech.

Thank you!

 

I mean, when I see gas-chugging, that's not a one-cylinder deal. And when someone tells me things went back to old ways after a tank of gas, that's roughly one re-ECU calibration cycle. I think both are important to point out.

You see virtually zero cases of bad injectors on here, and maybe one may fail, not all at once.
Same with coil packs, one bad coil pack is likely not going to cause all cylinders to chug gas.

So considering combustion only relies on measured air, fuel, and spark, and the ECU dynamically adjusts fuel-air ratio based on (a) how much air it detects coming in and (b) what combustion mixture it senses from the O2 sensors before and after the cats, what does that leave you with?

My guess, without seeing a live monitor, you've either got an issue with the device measuring how much air is flowing in (i.e. your MAF), or the data your ECU is reading after-combustion (i.e. your O2s) is invalid. There's a distinct possibility there's an issue with your throttle body being clogged, dirty, or bad position sensor so it's letting in more air than expected, but ...

If your issue was mine, the first thing I'd do is go to the local auto parts store and buy a replacement Hitachi brand (specifically Hitachi, and NOT from fleaBay or scAmazon) MAF. You can easily install this yourself in 5 minutes, seriously. Pull your negative battery cable afterward for 10-20 minutes to clear the ECU. Drive, expecting it may run weird at first while it recalibrates.

If it continues, next place I'd be eyeballing is my O2 sensors. And if any of these items were replaced by your shop, I'd be suspect of what brand of part they used.

 

This right here ^^^ is the perfect plan of attack.

 

Oh, and it's worth mentioning. If you have a digital multimeter, it's really easy to test your MAF to see if it's in spec. I don't have the factory service manual page handy to tell you what the resistance(s) should measure out to be, but ... it's very do-able if you have access to a DMM and more time than money.

 

Right on, thx so much for the detailed response. The mechanic didnt replace any parts, he just cleaned the MAF sensor which he said was pretty dirty.

will use all of this info moving forward. cheers,

 

cool. Hopefully its something simpler like this and not the secondary air injection smog set up like buddy was dealing with in that link i posted. again, i appreciate the advice.

 

SAIS is only running for the first little bit after starting, I can't remember if it's 30/60/90/120 seconds but something like that. When SAIS is going out, you'll usually hear a high-pitched whistle at startup that goes away after that initial period. But you shouldn't see stuff like this.

The reason I'm going to MAF and O2 sensors with a 'possible' on the throttle body or its position sensor is this, at least as I fundamentally understand how EFI engines work:

If the MAF is telling the engine more than the actual amount of air is entering the engine, ECU is going to tell the injectors to dump more fuel into the cylinder to get the fuel/air ratio correct. MAF is a relatively cheap part, even for the Hitachi model that's equivalent to OEM, so no-brainer to throw in. Just need to clear ECU after to force it to recalibrate with the new part.

Likewise, say the O2 sensors pre-cat sensed a weird combustion mixture, as if things were too lean on the back side, maybe the ECU would adjust fuel to compensate. I have no idea if that's possible, but I know O2 sensors generally check combustion before the cat and after the cat for emission purposes. I'd expect this to throw a code, though.

It's entirely possible if this was going on for a while and you've been burning rich, your cats are clogged or shot, I suppose. I'd also expect this to throw a code.

Likewise, if a coil pack was bad, or an injector was bad, or a single plug was bad, I'd expect all of the above to throw a misfire code for the impacted cylinder.

Really, the MAF makes the most logical sense. But throttle assemblies and throttle position sensors do go bad on 2UZ enginers. And that's responsible for air intake. And people have seen piss-poor response if the TB housing is dirty, cleaning is a no-brainer, something easy enough to check. So is checking your air filter.

 

On the other thread it was one of the vaccum line valves, had a pin hole in it.. the SAIS was still good otherwise..

i think youre right that it was going on for a while.. took me a minute to get concerned cause it was winter time, running heat and 300 lbs of sand bags in the back.. eventually though, i knew it couldnt be effecting the mpgs that bad..

everything youre sayin makes sense. Feels good to be finally learning more about vehicle systems. between that other thread and eveything youve shared, i feel well equipped. thx for sharing your expertise. Cheers man.