Efficiency dropped by 25% - 30% out of the blue

The Tundra in question is a 2008 with a 5.7 engine. The car has 98,000 miles.

The truck has dropped in efficiency by 25% - 30% in June of 2021. The truck had a carrier barring that was going at the time and it was on a 2nd treatment of fuel injection cleaner. The issue continued after changing the carrier barring and is progressively getting worse.

The manual calls for the spark plugs to be changed at 120,000 miles, but I decided to do it early to see if doing so would resolve the issue. The computer was reset by disconnecting the battery for over 30 minutes at this time. We also replaced the PVC valve and I cleaned mass air flow sensor with mass air flow sensor cleaner about 200 miles ago.


I just ran a full tank of fuel with Royal Purple fuel injector cleaner through the truck. A mechanic I know said that is one of the best fuel injector clears out there and he has seen this resolve this issue in other cars he has worked on. Nothing changed.

I just brought the truck to Toyota to have it checked out, but the guy only looked for codes and didn't go into live data to see if he could find any issues.

Does anyone have any ideas on what I can do from here?

Thank you for your help, Ray

 

Welcome!

Did you buy this truck used from a dealer?
How are you measuring efficiency?
Are you using the computer or hand calculating your mileage?

 

I hate to sound like a broken record, but the number of carrier bearings I've seen installed upside down on this site is astounding. If the carrier bearing were installed upside down, it could cause extra friction, in theory, but not that much.
Care to post a picture of it?

 

Read the short and long term fuel trims (STFT, LTFT).

 

No change of tires around that time? Going to a LRE tire from a SL tire makes more than a bit of difference to mileage in my experience, even with no size increase.

Colder weather, and winter fuel blends have a negative effect as well, but clearly this isn’t your first winter with it.

A a couple ideas other ideas immediately come to my mind, all else being equal.

First would be one or both of the upstream O2 sensors (Actually air - fuel ratios in this application, same but different), and looking at fuel trim data may help narrow this down. Other tests that look at the sensor response times are useful too. Just because there are no codes does not mean everything is good.

The sensors aren’t cheap (use Denso parts) but if you’d be paying someone to diagnose, it might make more sense to just put two in as it sounds like you’re a DIY kinda person.

Second, any dragging brakes? A seized caliper piston or two, parking brake not fully releasing? Might be hard to detect, but an infrared temp gun should turn up extra heat on the brake(s) after a highway drive.

 

I bought the truck new and the miles were calculated by the computer and manually.

 

I've had it happened with three, not just one of my old cars that they suddenly dropped both in power and fuel economy, only because H2OS was basically dead, but ECU did not trigger an error yet. Looking up in Bosch specs (the cars used Bosch ECU and all engine-related sensors), it said the lifetime is 150 kkm or 250 kkm for more expensive version, based on the average mileage per year. If your truck was idling a lot, or Toyota puts a less durable oxygen sensor, or whatever else we don't know, it could be about a time.

I'm personally replacing upstream oxygen sensors every 10 years.

 

Thank you for the info. Everything was the same at the point that it dropped. The only change was the fuel injector cleaner in gas and I did put a K&N air filter in that has been replaced with an OEM one. There has been an oil change since then, breaks have been checked, and the mechanic checked the truck from head to toe. He is a good mechanic, so I don't think he is missing anything.

 

I'm sure you already aware of idling the truck will kill the MPG.
If i drive point to point no idling i get 14.5 mpg on average (best case)
if i drive and idle (coffe stops, kids from daycare waiting on seat belt installation, etc) then i end up around 12.5 mpg.

That's a 10% variance from fill-up to fill up (currently driving a lot so that's every 3 to 4 days).

Otherwise, check your tire pressure. and check for brake dragging.

Also, i drive with shifter in 6 instead of drive. seems to allow the truck about .5 mpg. (i think it lets the truck roll on downhills without trying to shift down as much?)

Good luck. Hopefully you find something simple that fixes it.

 

Thank you for the info. Everything was the same at the point that it dropped. The only change was the fuel injector cleaner in gas and I did put a K&N air filter in that has been replaced with an OEM one. There has been an oil change since then, breaks have been checked, and the mechanic checked the truck from head to toe. He is a good mechanic, so I don't think he is missing anything.

You recommend checking
Check fuel ratios
Fuel trim data
Sensor response time
Check for dragging breaks and emergency break

 

The MPG were the same with the bad one and once the new one went in.

 

Cool, I was asking cuz if you were going off the computer because if the battery was recently unplugged or changed, that would throw things off since the computer will reset. Hope you figure it out and it's an easy fix.

 

Adding to above ^^ monitor the Knock Correction Value too! If it's under 20, chances are the ignition advance is not up where it could be to improve performance and economy.

At a steady speed of say 55 mph it should be in the 20's to 25.

Also -
Tire pressure
IR gun to calipers/rotors to verify brake drag isn't killing you.

Maybe hit the trans, diff's, x-fer case after 25min of hwy use. Should be 100 - 150F on all but the transmission.