2006 VW Jetta Mystery Misfire Pt.2

I started this string over a year ago (see below). My car continues to suffer the same problem. It now has 62,000 miles on it. I finally decided to take it to the dealership seeing as my local mechanic and 2 VW specialists could not figure out what is wrong - 2 new sets of coils, 2 news sets of spark plugs, and several expensive diagnostic tests that reveal nothing later. The dealership also has no clear idea what is wrong with the car and VW itself has not been any help either (rather useless really). No service bulletins have been posted for my car. I have tried several additives (Techron, Sea Foam) but that did not yield any results. The one thing I have noticed is that the misfiring ceases to occur when the gas tank is a little less than half full but begins to occur again as the gas gauge reaches the refill zone. It misfires like crazy when the tank is full. I am thinking of replacing the wiring harness for the fuel injectors but this is yet another best guess as to what might be wrong with the car. Any assistance in helping me solve this problem is most greatly appreciated as this car is driving me crazy!

Old Post:
I have a 2006 Jetta with a 5 speed manual transmission and 35,000 miles. The engine randomly misfires causing it to stutter or stammer. The check engine light is on and has been for several weeks. Occasionally the check engine light flashes. I took it to a local mechanic who ran a series of diagnostic tests (no vacuum leaks) and they recommended having all the ignition coils replaced for all 4 cylinders. They also replaced the fuel filter. I did so but the problem soon returned to which they referred me to the dealer. I decided to take it to a VW specialist in town and they replaced the spark plugs for all 4 cylinders. They noticed that some of the plugs were black with a buildup and some were clean. The VW specialist also recommended that I take it to the dealership as they too had no clear idea as to what the problem is. I finally took the car to the dealership and they also had no clear idea what is wrong with the car. I have tried several additives (Techron, Sea Foam) in the hopes that cleaning any carbon build up The car did suffer rodent damage in 2007 but was still under warranty so all the wiring that had been chewed on was replaced. I use premium gas. I would like to fix the problem to avoid any catastrophic failure but am trying to avoid throwing money away on any more guess work.

Codes listed:
P0202 - Injector Circuit Open Cylinder 2
P0300 (2 separate listings) - Multiple Misfire Random
P0302 (2 separate listings) - Multiple Misfire Cylinder 2
P0304 (2 separate listings) - Multiple Misfire Cylinder 4
P0301 (2 separate listings) - Multiple Misfire Cylinder 1
P0201 - Injector Circuit Open Cylinder 1

Has anybody tested the injectors . . . fuel injector balance test, measured resistance, etc. ?

Has anybody checked compression?

What are your fuel trims like?

Any vacuum leaks? . . . I have seen plugs foul due to severe vacuum leaks

Looking at those fault codes makes me wonder if #1 and #2 injector might be open circuit . . .

With 2 injector codes, why has no one checked/replaced the #1 & #2 injectors like db4690 said?

You could also try moving those injectors to cylinders 3 & 4 and see if the misfire and open injector circuit codes follow the injectors.

It was confirmed that there are no severe vacuum leaks. Isn’t the dealer the ultimate authority on this? I took it to them twice and they still had no conclusive idea as to what is wrong with the car. A previous VW specialist mechanic switched the injectors but the codes that registered were exactly the same. How would you explain that the engine doesn’t misfire all the time? At times, it runs exactly as it should. Could intermittent electrical impulses to the injectors be the problem? This is why I am thinking of replacing the wiring harness for the fuel injectors?

If switching the injectors didn’t change the “injector circuit open” codes then I would suspect the wiring as you said. Especially since you know rodents have chewed on wires. It’s possible not all the rodent damaged wiring was found and fixed. Electrical problems due to damaged wiring, etc, are often intermittent, so that could also explain why the misfire is intermittent.

To save money you could try swapping in a used harness from eBay:

If that doesn’t fix it, it didn’t cost much and you can move on to the next part of the wiring for troubleshooting.

The second time I had the coils and spark plugs replaced the car ran perfectly for a week before the misfires returned. Today, I filled the gas tank only half way (as mentioned in my post) and the car is running as it should. Any idea how these two situations caused or cause the engine to not misfire? Do these things substantiate a problem with the wiring harness for the fuel injectors or is this pure coincidence?

Someone smarter than me would have to come up with an explanation for how the amount of gas in your tank produces an “injector circuit open” misfire code, because I can’t make the connection. So if you ask me, it’s coincidental.

Let’s start with what you know for a fact: the computer says two of your injector circuits are intermittently open. And you say that swapping the injectors didn’t change the open circuit codes. Which says to me that somewhere between injectors 1&2 and the PCM is an intermittent break in a wire or a connection. Or the PCM itself could be bad.


The engine continues to misfire no matter how much gas is in the tank. I was hoping I was finally on to some predictable pattern. I continue to suspect that the wiring harness is the cause. What baffles me is how this was not tested for or examined in detail by all of the other VW specialists (including the dealership) I took the car to in the past?

It may have been tested and examined. But there aren’t any foolproof ways of sorting it out. There can be visible and obvious kinds of damage that can be detected. And then others - that aren’t visible or amenable to detection. Part of that does depend on how hard someone tries.

But what of these costly and presumably sophisticated test I am contiuously charged for? They pin-point nothing?

Well, that is unfortunate. But the mechanics are trying to find a difficult intermittent problem, and you’re paying for their time, expertise, and equipment. Just like you pay a doctor for his time while he’s trying to diagnose your rare disease. He may or may not be able to figure it out. But he gets paid for his time and expertise, regardless.

hmmm … well, this is definitely frustrating, I can understand that. I’m assuming the prior efforts have convinced the mechanics at VW and elsewhere that the injectors are connected and being pulsed correctly. That’s relatively easy to determine with the shop equipment they should have. I guess it is possible there a bare wiring insulation problem that only causes a problem when the car is moving, jiggling is necessary to affect the circuit in other words. If that’s the case, then replacing the injector wiring harness would help, fix it probably. You’d think the mechanics would have tested this idea by jiggling the wires or jumping on the bumpers while testing the injector pulses. Still, there’s a chance it isn’t possible to jiggle the way it needs to be jiggled with the car sitting in the shop, it has to be driven for this to happen. So replacing the injector wiring harness seems worth a go. I wouldn’t recommend this tack usually, except that you have known damage to the wiring due to rodents chewing on the wires. It’s sort of like if rodents chewed on the wires inside your computer, there’s no telling what problem would occur or whether it would be possible to diagnose. Everybody pretty much assumes the wiring remains connected to what it should be connected to. With a computer, you’d just buy a new one if that happened. Not so possible with a car though.

What else could you try? At this point, since you’ve already tried the dealer and inde shops, might need to go for the long ball. Just start replacing stuff. Could get lucky. A new set of spark plug wires might be worth a shot. Replace the crank and cam sensors?

I’m also intrigued by the relationship to the fuel tank level. There’s a slight chance there’s a vacuum forming in the tank which shouldn’t be, making it difficult for the fuel pump to pump enough gas to the fuel rail, due to some undiagnosed fault with the evap controls or just a collapsed vent line. I’ve diagnosed something like that before on my 70’s VW Rabbit by connecting a fuel pressure gauge to the fuel rail (or what was equivalent on that Rabbit) and driving the car while watching the gauge. If its not holding steady pressure just tooling straight down the highway at a constant speed that would be an important clue.

The other thing I might try is to simply loosen the fuel cap when this starts occurring, see how that affects things. That will probably turn on the CEL, but if the engine runs noticeably better with the fuel cap loosened, that would be a big clue. Don’t try any fuel system experiments like the above yourself, let someone trained in the proper safety procedures do it for you.

Is changing the wiring harness for the fuel injectors a relatively simple procedure? From the images I’ve seen of the wiring harness, it looks like simply plugging in the right connectors to the corresponding fuel injectors.

The shop should have the proper test equipment and the expertise to be able to determine if the injectors are actually getting the signals to fire and, tracing backwards for the cause, if the crank and/or cam sensors (I don’t know your system) are providing the signals to the ECU to say "fire cylinder #x). That would be the first place I’d look. Or perhaps, as often happens, the dealer just isn’t invested enough in the problem to really want to bother… they often do this with older vehicles. You may want to give another shop a chance.

You may also have an EVAP system problem. There is a float operated valve in your tank that shuts off the tank-venting path to the charcoal canister, where the vapors in the tank go before being vented to the orifice next to the fill-pipe, when the tank is full. If that valve is sticking or not closing properly, it could be allowing fluid gas to run into the charcoal bed before the pump shuts off. That could be partially saturating the charcoal, choking the inlet to the tank and making the pump’s job difficult. That could also lead to premature pump failure, which would account for the rough operation when the gas gets low and the weight of the gas is no longer helping with the fuel pressure (technically that weight is called “head pressure”).

NOTE: there are a number of valves in the EVAP system. Too many people think only of the purge valve, but that is not the one I’m alluding to.


Perhaps people always think of the purge valve, because that’s often the only one they can see, when they pop the hood

To see the purge valve you’d have to crawl under the car, or rack it, in many cases

In cases where the top of the tank isn’t accessible, you may even have to drop the tank to get at some of the valves. Much of the installation of everything gas tank related, including the EVAP stuff, will be through the top of the tank. In many vehicles today the charcoal canister along with EVAP stuff is under the floorboards in front of the tank, and there’s really no way of working on this stuff without a rack.