Phil Thompson

When started from cold the engine always starts on 8 cylinders, but, sometimes, when it's warm it will start and continue to run on 7 cylinders. If turned off and restarted it may run on all 8. Once started it will not change when running - if it starts on 8 it will run on 8 until turned off and if it starts on 7 it will run on 7 until turned off. The CAT warning light comes on when running on 7.
When running on 7 cylinders, disconnection of the ignition coils one at a time confirmed that it was cylinder no 1 which was not working. Swapping the ignition modules with those from another car did not fix it . The coils and plugs were swapped between cylinders 1 and 2 but the problem remained with no 1. I next checked the signals at coils 1 and 2 with my oscilloscope (while running on 7) and they appeared the same as far as I could tell.
VAG-COM indicates fault code 17609 - Fuel injector for Cylinder 1 (N30) : Electrical Malfunction P1201 - 35-10 - - - Intermittent.
I assumed that the CAT warning light came on because unburned fuel was entering the exhaust. This would be the case if the misfire was caused by the lack of a spark, but if the misfire was caused by failure of the injector there would be no unburned fuel in the exhaust and no CAT light.
Am I right or is VAG-COM right?

ohbw

I'm not a mechanic by a long shot, but when I read through your post, the last phrase struck me as possibly untrue: "if the misfire was caused by failure of the injector there would be no unburned fuel in the exhaust and no CAT light".

The first part of that is true, but I'm not certain the second one is. If the CAT is designed to be fed heated exhaust by four cylinders, and it's only getting the heat of three, it's very likely operating under its intended temperature, which might trigger the warning light.

I drive my car around a heavily policed town at low speeds about 80% of the time. I often get "intermittent cat failure" VAGs when I get sleepy and don't hammer on it. When I first started exploring this, I kept clearing codes and going out and driving the starch out of the car (and thus heating the cats up nicely). Can't get a code in a thousand miles when I remember to put it in Tip and redline it a few times after it's warm every time I drive it.

So maybe underheating of the cat caused by the misfire in the cylinder is what triggers the warning. Or maybe I'm a knucklehead. Wiser voices will surely speak!

greenbird

Injectors can fail open too, in fact I see more failed open then failed closed. This would both dump unburned fuel in the exhaust, and cause a misfire more so when hot than when cold.

skiracer

Replace the #1 injector. Easy to install.

Phil Thompson

If it were an injector fault I don't understand why stopping and restarting the engine would make the fault either come or go when I can drive the car several hundred miles without turning the engine off without the fault either coming or going. This would seem to point to an electrical fault in the supply to the injector.

ttuling

... and the computer, or even injector driver hardware, might disable the injector if ever it draws too much current. If the injector is short circuiting, it might be at its worst when its hottest, and that might conceivably be a few minutes after shutting off a hot engine.

Resistance specifications for US model 4.2 4V injectors are 12 to 20 ohms, for the 5V it's 13 to 16 ohms.

Tom

skiracer

hook it up to the command cable for the #1 injector when it is running on 7 cylinders. If the command signal to the injector looks good, then the fault is mechanical. If the command signal is bad or non-existent, then it is electrical.

troverman

Did you ever resolve this? I have this problem on our '99 4.2L and I believe the problem is water got into the computer.