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Thread: Really Bizarre Mileage Decrease

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    Default Really Bizarre Mileage Decrease

    Hey Professor and anyone else!
    I am experiencing some really bizarre behavior with my mileage on my 2016 5.6l Tundra ranging from normal at about 13.5 mpg to only 7-8 mpg. It varies after I fill up. It seems like when I fill the truck, either completely or just 10-15 gallons, the mileage will suddenly drop. At first I thought it was just the computer acting up but when I ran a full tank test, I only got about 250 miles running down to the reserve or approx 30 gallons. Normally I can go about 430 miles on a full tank.

    Now here is what is weird. If I stop the truck, remove the gas cap and then reseal it. It'll go back to normal. It may take a few attempts to do so. And ti has to be sealed just right. Almost but not quite to first click. Realistically, the gas cap has absolutely nothing to do with the sudden decrease, but for some reason or coincidence, it's how I can get it to perform normally. I've checked for leaks visually but obviously if I can ran a full tank normally, it's not leaking.

    There is no check engine light. Performance is outstanding and I can't discern anything wrong with the truck. I'm right at 50k miles and due for services. Might be a quart low on oil (very clean) and am running K&N filters and the truck has thus far given me zero problems. I've also spoken with a local mechanic as well as my Toyota dealerships. The specialist at the dealership said to replace the gas cap. Which I did on in Waco, Tx driving from Austin to Dallas right after I filled up and my gas cap trip wasn't working. The new gas cap fixed the issue for about two tanks and then it started all over again.

    Any thoughts on what this is? I plan to go ahead and get it looked like but it sounds like the dealership was going to have to spend some time with it cause they were stumped, too. Hoping there is some simple, causal thing out there that I can point them to first, or even take care of myself.

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    Default Re: Really Bizarre Mileage Decrease

    Never heard anything like this. Please let us know what the dealer finds.

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    Default Re: Really Bizarre Mileage Decrease

    Quote Originally Posted by tonyspin View Post
    Never heard anything like this. Please let us know what the dealer finds.
    I will do that. I think I read somewhere on this forum on another Tundra forum of something similar but there was no final resolution.

    As an aside, I say that I'm still have the problem but it could be that I'm spooked and not trusting the readouts any longer. To wit, after I bought the new gas cap in Waco it was "fixed" for a couple of tanks but I haven't really been on any longer trips in a couple of weeks. So I get in the truck and immediately notice the "distance to empty" ticking off every .4 miles on the odometer and get to jacking with the gas cap but here the past couple of days I've said screw it and just drive. Now it'll do it for 5 or 6 miles and then it stops so it could just be the computer doing it's calculations and I'm putting to much weight on it on short stop and go trips.

    The fact is I was getting crappy mpg per a full tank test but haven't done another one. I've got a nice road trip coming up for Spring Break and will run at least 4 tanks of gas so I'll know for sure if it was an anomaly that was, for now valid reason, corrected by the gas cap or if it is an ongoing issue. If I can't get it to act up driving from Dallas to Santa Fe, it's going to be real hard getting the dealership to figure out what's wrong especially since that would indicate that the problem is gone.

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    Default Re: Really Bizarre Mileage Decrease

    Quote Originally Posted by tonyspin View Post
    Never heard anything like this. Please let us know what the dealer finds.
    So took the family to Santa Fe for Spring Break. Dallas to SF is about 650 miles plus a lot of travel up to ski slopes. Never did experience any fuel issues so I tentatively think the original issue must have been related to the gas cap which has been replaced. I did notice the the dash display is really squirrelly on on initial calculations and then sort of levels out 20 or 30 miles into the tank. On the trip up I average 13.5 mpg but on the trip back with the "land hurricane" winds on my tail, I averaged 14.6 mpg! Wow! Went an extra 120 miles before my last refuel.

    Not sure what to say as I just don't believe a faulty cap would cause a 50% reduction in economy. And since I don't have any data to support an issue, I'll mention it to the dealer on the 50k service but I don't think it'll move any needles for the technicians.

    If anyone reading this thread can think of any reasons, I'll check every now and again but I think the issue is mostly resolved or maybe a one off with bad fuel or something.

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    Default Re: Really Bizarre Mileage Decrease

    Thanks for the update. I never believe dash readouts for mileage because I have found them often wrong. I calculate by hand, miles driven divided by gallons used on fill-ups.

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