Bad gas or something else?

My truck recently ran incredibly rough the day after a downpour and being down to 1/4 tank on gas. I had to shift it into neutral to avoid stalling at lights. Truck ran better but a little rough at higher speeds (> 20mph).

Once it got down to 1/8 tank I put in $20 of regular unleaded and presto! Problem gone driving out of gas station. Hasn’t been a problem since, but am not down to 1/4 tank.

Bad gas/water in gas/something else? Things to check out? Thanks.

year of Toyota Tundra?
what Engine?
Last time fuel filter was changed?
has fuel pump ever been replaced?

03 Tundra, 190K, 4.7L V8, never replaced fuel filter, fuel pump is original.

Did you check the spark plug wires because water could get past the insulation and start to arc.You get a misfire in this case like if the engine wants to stall.

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That doesn’t really explain why it would misfire until immediately after I put $20 more in gas.

A weak fuel pump can cause this problem.

The level of gas in a gas tank creates a pressure head. This pressure head can assist a weak fuel pump deliver the proper fuel pressure and volume of fuel to the injectors.

As the level of gas in the gas tank drops, the pressure head is reduced. Until it no longer assists the weak fuel pump.The engine will then run rough. Then when the tank is filled, the pressure head is restored and the engine runs fine.

Drive until the gas level is back down to a 1/4 tank. If the engine starts running rough again, I see a new fuel pump in your future.



Other than the fuel pump, Is it unlikely to be just water mixing with the gas when it gets low enough? I tossed in some dry gas today with another $20 in gas to see if it helps.

how do you expect water is getting into your tank?

I concur with Tester’s assessment. I’d probably try installing a new fuel filter first. If it is truly the original filter, it could be straining an already weakening pump.

Maybe a bad partial fill at a prior station? Water is heavier than gasoline and sinks to the bottom of the tank. Maybe the ratio of gas/water the pump is sucking up is too low when the tank is almost empty? Maybe a hole in the line from the tank to the pump that gets exposed when the gas is low in the tank mixing gas w/air?

Just spit balling here… I dunno. I never opened a gas tank to understand the operation in any detail.

I’m going to go with COROLLAGUY1 on this one, the damp spark plug wires had plenty of time to dry out sitting on top of a hot engine in the few minutes it took to fuel up.

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To replicate, use a spray bottle filled with water and aim it at the wires.If you hear the engine miss or see spark in the area ,you found the problem.Its cheaper to change s/p wires than a fuel pump.What I think happened is by the time you filled up, the heat from the engine evaporated the moisture caused by rain water. I had the same thing happened on my vehicule recently.

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Yeah, but I was driving around all day the day after it rained with it running rough misfiring. At the end of the day I put $20 in the tank and the problem immediately disappeared. Doesn’t seem to be a coincidence.

Could be a case of bad gasoline as mentioned in your first post.When things got better after you fill up,was the gasoline from a different gas station?

One possibility is a fault in the EVAP system so that air is not getting into the fuel tank. The fuel pump has to draw against ever increasing vacuum. If it runs better after the gas tank is opened, that’s a hint.

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I experienced a very similar symptom in my own truck a couple years ago. In my case it was a clogged carb caused by failure to notice the air filter needed replacement. That and just time, it’s a 45 year old truck.

I think in OP’s case it was just bad gas in the tank. How it got there, who knows. As long as the problem doesn’t re-occur, good to go. I wouldn’t worry about it otherwise.

Yeah, I was going to one of 2 stations for the last 6 weeks. This time I went to a different station near my work.

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In my case, I avoid filling my car in older gas stations because the underground tank could be contaminated.When the tanker arrives he will fill the underground tanks and mix all the crud in the bottom of the tank.Those sediments will end up in your tank. What you could do is drive the truck until the low gas light comes on, fill with fresh gasoline and see how it goes from there.Take a note on the stations that gives you problem and try to avoid them.

Turned out to be bad gas.


I wouldn’t get all bent out of shape replacing parts just yet because it is very common to get bad gas. I would run high octane gas through it or buy a fuel additive and drive the truck before replacing parts. Don’t buy from the same gas station. I say this because we want to rule out bad fuel before potentially wasting money on parts that may not fix the problem, or in fact make the problems worse.

It sounds like when you added more fuel you probably diluted what bad gas was in there and that’s why it ran better.

I haven’t gotten a tank of bad gas locally in years…Maybe an additive would be helpful, I doubt premium gasoline would make a difference…