Tech is stumped. What causes this misfire?

When you bought the used Prius were you aware that it had problems? Or did it take some time for the misfire to develop?

As Chap pointed out in post #10, there are three major requirements for a gasoline engine to run correctly:

1. Ignition (it seems like you've beaten this to death since the spark plugs and igniters have been replaced. Still, you should check for water condensation in the spark plug wells immediately upon experiencing a misfire episode.)
2. Fuel delivery (which includes fuel line pressure and proper injector operation. This could also include the hole in the fuel filler neck - I've not heard of such an issue coming up before. Why is the hole there to begin with?)
3. Engine cylinder compression to verify the condition of the piston rings, cylinders, and valves (you say that this was checked, but it would be good to know the outcome of the test)

Eventually, with the expenditure of sufficient time and money, the root cause of your car's misfiring can be resolved. It sounds like you've put so much money into the car at this point that you want to get it to run so that you can say you are getting some value for the investment. Good luck.

 

AND I WILL REPEAT THAT FOR EMPHASIS.....since I already said it a few posts back.

Another option is:
If it remains serviceable, you could just drive it and learn to live with the misfire......until it finally breaks altogether.
You really don't have much to lose doing that........and you have kind of been doing that anyway.

 

Let me offer a caution there. You do have something to lose. Misfiring leaves unburned fuel that travels to the catalytic converter and is converted to heat. It is likely eventually to destroy the converter, in such a way that the melted guts block your exhaust flow. From Toyota, the part is high quality, stainless, built for a lifetime, and worth every penny they charge for it, but that's a lot of pennies. Or you'll end up at Bubba's Muffler Shop looking at some sort of hack-n-weld.

A Prius that works holds more of your value than one that doesn't ... I can't make the call for you as to where you need to call it quits, but tracking a misfire, just as Patrick pointed out, is just a matter of keeping focus on the only three areas that can be responsible, keeping track of what you've already ruled out to avoid duplicating effort, until you've got the cause.

-Chap

 

You didn't tell us how long it was between the tank replacement and your misfires after the rain. Had you been driving for a while?

Could be, but I still have my doubts...

 

I think you should still plug the hole.
Too many ASSumptions on the part of the mechanic.
And.....the water in the tank may NEVER be removed without some additive in the gas.
Ethanol helps but with just E10 it is very limited in the amount of water it will hold.

 

I agree with him about the injectors and the ignition coils. The injectors just squirt the stuff. Whether it burns after that or not makes no difference to them, and won't hurt them. The ignition coils just make sparks, and whether the spark lights anything or not, the coil is fine.

As for the cat, check page 147 in your owner's manual:

• Use only unleaded gasoline.
• Do not drive with an extremely low fuel level; running out of fuel could cause the engine to misfire, creating an excessive load on the three-way catalytic converter and the Toyota HC adsorber and catalyst system.
• Do not turn off the ignition while the vehicle is moving.
• Keep your engine in good running order. Malfunctions in the engine electrical system, electronic ignition system/distributor ignition system or fuel system could cause an extremely high temperature of the three-way catalytic converter and Toyota HC adsorber and catalyst system.

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I only harp on this 'cause I've been there myself, with a different vehicle where I took about a month to track down a misfire problem and by then it was too late for the cat, which was melted into a gob inside and passed exhaust about as well as a potato in the pipe.

The one thing that made that my lucky day was that a converter for that vehicle cost about a quarter of what yours does.

-Chap

 

I disagree with the concept that any substantial amount of contaminated fuel will be left in the system after the fuel tank is replaced. Further you've indicated that the car only has a problem after heavy rain. That is not consistent with the idea of pre-existing contaminated fuel. In that case the car should misfire at all times.

Regardless, rather than argue about that, if your mechanic believes the fuel is contaminated with water then the obvious solution is to disconnect the fuel line where it connects to the fuel rail at the engine compartment, and attach it to an extension hose routed to a drain container.

Then apply 12VDC to the fuel pump within the fuel tank so that the fuel can be pumped into the drain container. This requires removing the rear seat and an access panel in the cabin floor, for access to the fuel pump wiring.

An alternate approach to powering the fuel pump would be to remove the circuit opening relay (aka fuel pump relay), use an ohmmeter to identify the coil terminals and switched terminals, then insert a paper clip into the relay socket to short the switched terminals together. Turn the ignition switch to the IG-ON position to power the fuel pump. If this approach is used, put a battery charger on the 12V battery to keep it fully-charged.

The reason it is necessary to bypass the circuit opening relay is that if the engine ECU detects the engine is not running it will remove power to the coil terminals of the circuit opening relay after a brief period, so that the fuel pump does not continue to run.

Draining the fuel tank is a lot better than you having to drive the car another 150 miles to see what happens.

If you want to see if all other problems are resolved first prior to replacing the fuel filler pipe, is the hole in a place where you can cover it with duct tape? That would be better than doing nothing.

I also am wondering why the fuel filler pipe was not replaced at the time the fuel tank was replaced? It should not have cost much extra to do that. For that matter I am wondering why the fuel tank was replaced to begin with. If the only issue was water in the fuel, that could have been pumped out as I described in this message.

 

In my experience, the wiring at the fuel pump is a PITA to reach even after removing that access panel.

I put together a video for CClay that shows which terminals and how to do it at the circuit opening relay, which is very easy to access. (The only reason I haven't posted that vid yet is I'm embarrassed at the audio quality and haven't had time to redo that. But I can send it if it would help.)

If I were put to the trouble of pumping the fuel out to a container, I would use a clear container for at least part of it, so I could let it stand and see if it stratifies into gasoline on top of water. Then it would be known whether that was any part of the problem or not.

-Chap

 

Pressure testing can be done on a fuel system. An inert gas like nitrogen has to be used (introducing pressurized oxygen-rich air into a space with fuel vapor crosses the line into bomb-making) and the pressure has to be carefully regulated to a very low value (the tank is not designed as a pressure vessel, after all) ... see the warnings on the green Evap-test-port label under the hood.

What is puzzling me here is that the ECM does its own routine leak tests using vacuum during normal operation, and ought to be throwing codes if it is detecting a leak. Odd.

-Chap

 

Here's a good reference on the fuel and evap system.

I had forgotten that the exhaust O2 sensor plays a part in how the automatic leak test works. I guess if the O2 levels are already wonkers because of the misfiring, that might make the automatic leak testing inconclusive.

-Chap

OK, I also forgot there are two automatic leak tests. The one for a leak in the internal bladder is the one that depends on O2 sensing. The other, complete-system leak check does not, it just relies on drawing a vacuum and confirming it with the vapor pressure sensor.

 

I'm going to see if I can get a look at my filler pipe. Our experiences are remarkably similar, except that I'm managing mine by throwing in the occasional bottle of conditioner.

I had a rotten fuel filler pipe on an old BMW and wrapped it with fiberglass... That's a good cheap DIY fix.

 

From the thermonuclear flyswatter department:

Mod the fuel line to add a fuel/water separator filter.

Then (a) water, if present, doesn't get to the engine; (b) it's easy to drain some out of the petcock and see whether there is any water or not, and recheck after trying various repairs.

-Chap

Cons: could cause its own problems if it restricts fuel flow enough to lean out the engine...

 

So here's another harp:
That hole can be fixed temporarily with the application of some METAL duct tape or maybe even some self-stick muffler tape.
Given the overall condition of your car, I STRONGLY suggest that you do a temp fix on that hole. NOW.
Are you listening ??
If the hole is in an area where the tape can be applied several layers thick by wrapping it around the pipe, it might be considered a permanent fix on a car that old.

I doubt that the cat is going to be damaged here IF the problem really is fuel contamination because what you are getting likely isn't a complete mis-fire but just a partial one. As someone else said, if one cylinder quits completely, it will shake your teeth out and your description doesn't sound that bad.

And finally, I don't think it is a good idea to put a whole can of HEET into less than a half tank of gas (about 5 gallons) because it was designed to be used in a whole tank of a full sized car, which is upwards of 15 gallons. Methanol in high concentrations can damage fuel system parts.
That stuff can diffuse a remarkable amount of water so you should see a difference by the end of your first run through......if that really is the problem.

 

I found the source of water on my car. It appears that the filler pipe has a corrosion hole just inside the nozzle on the top of the pipe. Tried to load pictures here but that function doesn't seem to be working right now. I can see corrosion on the inside of the pipe where water has come in from the top.

The pipe is exposed in the wheel well, where it would be subject to a steady stream of water from the wheel in wet weather. That completely explains my experiences, including the emissions DTC.

Update: I wrapped the pipe with self-adhesive rubber tape. Not that hard to get at with the wheel off and the canister removed.

 

It's good that you determined the apparent root cause of the misfire.

Rubber tape is not a very good fix for the hole because gasoline will tend to dissolve the tape. It would be much better to use metal duct tape which would be more resistant to deterioration.

Even better would be for you to have that pipe replaced now that you have developed a reasonable theory regarding how that defective pipe is causing the misfire problem.