2006 VW Jetta Mystery Misfire

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 problem soon returned. 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. 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 experimented using premium gas and regular gas. Premium gas seems to make the problem worse. Also, the misfires seem to lessen as the weather gets warmer. 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.

You only have 4 spark plugs, so are 2 sooty and 2 clean? Sooty usually means running too rich. The mis-fires could be lean misses from clogged or dirty injectors on the two cylinders with ‘clean’ plugs, and the other two are running rich due to the ECU trying to balance the system.

That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.

Every time I had taken it to a mechanic they would run a diagnostic test, read the codes, and presumably attempt to repair the car based on the codes displayed. Both shops attempted to correct the problem but the problem returned. Both shops couldn’t figure it out and referred me to the dealer. The codes (10 in all) listed by the reader at my local Autozone are:
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

Any guidance would be greatly appreciated as I have already spent a good deal of money on trying to have the problem repaired.

It looks to me like you have wiring problems to the injectors, likely from the prior rodent damage. A mechanic should confirm one way or the other whether injectors 1 and 2 are getting the electronic pulses from the ECM and injecting or not. Mechanics have tests to easily determine this. I expect injectors 1 and 2 aren’t. Or at least not reliably. And the misfires are a symptom of the injectors not injecting gas when they should or in the amount they should.

So why is there a misfire in cyl 4? Because there is a time delay between when the injector fires and when the crankshaft speeds up as a result of the firing. The ECM diagnostic is saying it appears like cyl #4 isn’t responding like it should to a firing in #4 cylc, but it is actually lagging because #1 or #2 didn’t fire correctly. That’s what it looks like to me anyway.

A mechanic with expertise in auto-electrical systems and the fuel injection wiring diagram could confirm/deny this theory without much problem.

A wiring problem is what makes the most sense to me. However, the first mechanic did look at the injectors and ran the engine for many hours attempting to duplicate the problem without any luck. Both mechanics ran a diagnostic test presumably more sophisticated than what AutoZone is capable of providing free of charge. Nonetheless, both mechanics (one being a VW expert) recommended I take it to the dealer because they couldn’t figure it out. Is there a specific part of the wiring system that I might direct a mechanic to pay attention to?

If this were my car, I’d measure the electrical pulses by back probing the connectors at all 4 injectors and look at the pulses with an o-scope. I expect that no’s 1 and 2 will look different than no’s 3 and 4.

Carbon buildup on the intake valves is a common issue on the “direct injected” engines, no fuel on the back of the intake valve to clean it, you have to go in an clean the carbon buildup, also a few injector issues on these cars.