Tech is stumped. What causes this misfire?

News from the Toyota service station:

The mechanic said he was able to rule out all of the items on my list from the previous page, but took an additional fuel sample and found that there was in fact water in the (new) tank. Not as much as before, but enough to cause the misfires.

This couldn't have come from a gas station, since my wife and I filled our tanks from the same gas station at the same time right after we replaced the tank, and she hasn't had any problems. My mechanic agrees this isn't the problem, since it's also the same gas station where he fills up.

He thinks that there was more water left in the fuel system than they expected when they replaced the tank and fuel lines, and that the water the are seeing now is just what was left behind. It took a few hundred miles of driving for the misfire to occur, but he didn't think that was odd.

He says that the amount of water in the tank couldn't have come from the hole in the filler neck. Some could have, but he's confident that it wouldn't be enough to cause a problem.

He also says he's confident the water isn't getting into the tank from the rain, so it's just a coincidence I've had misfires during rainstorms. They've already dissembled the fuel system and run two smoke tests, and he says that any way this could happen would have shown up then. (I'm also confident I'm closing the fuel cap tightly when I finish filling up, so water isn't getting in that way.)

His proposed solution is that I drive around to burn off the water and gas. He estimates it will take a few weeks to a month, but fuel additives may speed the process. I'll have some misfires during that time. I asked specifically, "If the car starts misfiring and the check engine light starts flashing, I should just keep driving?" and he said yes, provided I was comfortable driving it.

I asked him if there was any risk of damaging the car by driving it while misfiring, and he said no. Specifically that it wouldn't damage the catalytic converter, the fuel injectors, or the ignition coils.

How does this sound to folks?

This is good news in that the solution isn't buying another part, but puzzling because 1) I replaced the tank the first time because he told me I wouldn't be able to burn off the water by driving it, even if I used fuel additives and 2) I thought the water mixed with the gas was what caused the coils and injector to fail in the first place.

 

I wasn't. The check engine light came on the day after I bought it, and the first misfires occurred about 200 miles after that during a heavy rain.

Yes. I drove more than 200 but less than 500 miles between replacing the tank and the next misfire. I filled up twice during that time, once at the same time as my wife and once on my own. The misfire happened during a rainstorm several days after I last filled the tank.

I share your uncertainty about ruling out water getting into the tank during wet weather, but I don't have much to go on other than two misfires happening during rainstorms, and don't know what I'd do to test the theory. But I should add that it has also misfired when it wasn't raining before we replaced the tank. We've towed the car a lot.

 

Another update. I picked up the car from the mechanic this evening and had a longer chat about what is wrong and what is to be done. The misfires will continue as long as there is water in the tank, so the goal is to get it out as quickly and cheaply as possible. To do that, he recommended driving the car until most of the gas (now about 1/6 tank) is gone. At that point, I should fill the tank about 1/4th full and add a water removing fuel additive. Drive until the tank is nearly empty, and repeat until the water and misfires are gone.

As for the source of the water, he assured me that some water and fuel could remain in the system even after replacing the tank and lines. Looking around PriusChat, I've seen other people report being told similar things when dealing with water in the gas tank, so I have more confidence that he's not just guessing. Still, I'm going to check the spark wells myself after the next rain and keep a close eye on whether the rate or severity of misfires change with the rain.

That leaves the fuel neck. The hole is from rust, and while the mechanic assures me it won't affect anything other than emissions, I'm not inclined to leave it be. However, I'm not inclined to pay the quoted 1k for parts and labor if there's another expensive repair on the horizon. Here are the two scenarios that worry me:

1. Water is getting into the tank from outside and will keep getting into the tank until the source of ingress is found and fixed.
2. We won't be able to remove enough water quickly enough to prevent misfires from damaging the catalytic converter.

Now, my mechanic says I need not worry about either of these scenarios, but they seem like reasonable fears to me. My plan is to try the fuel additive plan for three or four cycles and see if it makes a difference. That's maybe two weeks of driving, give or take. After that, I'll take the car to an independent mechanic and ask them to take a fuel sample, check the cat, and quote a price to replace the fuel neck.

If there's still a lot of water in the tank, I'll know to worry about 1. If the cat is looking bad, I'll know to worry about 2. In those cases, I won't be likely to find the source of one serious problem before it causes another. That's my limit, so I'll sell the car for whatever I can get for it. (Anyone want some very old and very new parts?) Otherwise, I'll have the neck fixed by whoever gave me the lowest quote.

Here are my questions now:

1. Would 3-4 cycles of a partial tank of gas + HEET clear out enough water that I shouldn't expect a watery fuel sample unless more was getting in?
2. Is there there anything I can do to minimize damage to the catalytic converter, or at least monitor it while I am trying to burn off the water?

No, the harping is welcome. I bought the Prius after the cat on my old car failed, then the replacement cat failed. I only had to fight a little to get the mechanic to honor the warranty on the replacement cat, but then the replacement replacement cat failed and my new mechanic found some other problems. Rather than go another round I said "Screw it, I'm getting a Toyota." I've been very protective of this cat for that reason, and have made a point to ask whether it was damaged by the latest round of misfires. The answer so far is that it is in great shape, but the harping lets me know what sorts of risks are on the horizon.

 

The first time the CEL started flashing and the car shaking violently was while driving in a heavy rain, less than a week after I bought the car. It flashed and shook less violently again in good weather after we replaced the fuel injector but before we replaced the tank. Then it flashed and shook during a heavy rain after we replaced the tank. It has been throwing misfire codes in good weather and bad without the CEL flashing, but some of those we've attributed to the injector and the ignition coils. None of that rules out misfires from the rain, but it makes it harder to tell which of the many problems were the cause. I'm not confident at this point that I can tell what this car feels like when it isn't misfiring from when it is without the flashing CEL since it has apparently been in terrible shape all along.

As for draining the tank, I asked the Toyota mechanic why we couldn't just do that, and he said that it wouldn't help. I don't know why it wouldn't. Clearly it would remove *some* water; the fuel sample itself had water in it that is now not in the tank. I asked why we couldn't have used fuel additives the first time we found water in the tank instead of replacing it, and he told me that draining wouldn't have helped, and there was a rusted spot in the old tank. For what it is worth, when he first suggested we replace the tank, I asked the mechanic who replaced the ignition coils for a second opinion, and he agreed that was the only option.

As for the filler neck, I don't know why they didn't see it when they did all of the other work. It seems unlikely to me that the hole didn't appear until after the tank was replaced, but I don't think there's much I can do about that other than be pissed that three different mechanics working on the car multiple times didn't manage to find it.

It sounds like I should be putting less faith in my mechanic's diagnosis and plan here. I'm reluctant to take the car to a *fourth* mechanic to get another opinion, but might it be worth it? Suppose I drain the tank myself. How easy (and expensive) would it be to have a mechanic determine whether there is any way for water to be leaking into the fuel? A pressure test has been mentioned a few times---would that be able to rule out a leak, or identify where the leak was if it there were one?

I'd be happy to give it a shot.

 

Another mechanic, another update.

I had a look at the fuel neck to see if I could patch it temporarily myself, but wasn't able to locate the hole. We have a rain storm coming, so rather than cross my fingers, I called around to see if I could find a better price on the fuel neck replacement. A mechanic recommended by a friend thinks he can do it for significantly less than Toyota, so I dropped it off with him. He'll have a quote tomorrow. If the price is reasonable, I'll ask him about draining the tank while he is at it.

So far, I've driven about 20 miles from the Toyota mechanic to my home last night, then another 20 to from my home to my friend's mechanic today. The first 20 were without any fuel additive, and I had the expected misfires and occasional flashing CEL. That felt a bit harrowing, the Toyota mechanic's assurances notwithstanding. I added a bottle of HEET before driving to my friend's mechanic today and didn't feel any misfires. The CEL was on (the EVAP codes from the fuel neck are sufficient to keep it on), but didn't flash.

I'm a little worried that I added too much HEET. The bottle got stuck in the fuel aperture when I added it, so I ended up pouring almost a whole bottle into about 1/6th of a tank of fuel. If we don't drain the tank, I'll add a few gallons before driving it away from the latest mechanic. On reflection, I don't know why I didn't think to use a funnel.

That may have been my poor writing. We are getting EVAP codes and a solid CEL, which the Toyota mechanic attributes to the fuel neck. If he is right and water *isn't* getting in through the neck, I worry that the water in the tank is getting in through somewhere else. If so, we'll keep getting EVAP codes after replacing the neck, and keep getting water in the tank. I'm not up for more exploration by part replacement, so if I can rule that out before replacing the neck, so much the better.

One thing I'll add: one of the three mechanics I talked to today asked for more details about the water in the tank. She didn't want to rule out the fuel neck without seeing it first, but thought that the residual water explanation was plausible and said she would have recommended the same fuel additive strategy. That and the misfire free trip today gives me some hope that the end is in sight.