The wife had the Trooper out and it started to run rough and a flashing CEL popped up. She took it to Autozone and the guy said cylinder misfire #1.

At idle it sounds like one cylinder is not firing. The idle speed is not surging.

Thinking it was maybe a bad coil pack I swapped the one from cylinder #3 to #1 and vice versa (for those with other vintage rigs this vehicle has one coil pack per cylinder). Started it back up and it still sounded like one cylinder was not firing. Not sure if the problem had moved to #3 I unclipped the electrical connector to the #3 coil pack and the idle got much worse. Clipped #3 back in to return to the bad idle rather than the very bad idle. Then unclipped #1, no change. Conclusion - I don't have a bad coil pack or the problem would have moved to #3.

Pulled the spark plug in #1. It looked fine and was not loose like in a similar recent post. It has about 20k mis or less on it. Swapped it for another known good plug. The problem remains. Conclusion - the spark plug is not the problem.

Did a reasonably good check of the wiring, including the injector wiring, and for a vacuum leak using carb cleaner. Came up with nothing.

About the only thing left I can think of is a bad/plugged injector. This would seem to be a rare problem for the 3.5 engine though.

The vehicle has about 60k total miles. No recent work. Have about 1/2 tank of gas left (i.e. no recent purchase and the filter was replaced at the start of the year).

Did I miss something obvious? If you agree with the injector theory any pointers would be appreciated as that is a service I have not yet had to attempt.

Unclip the electrical connector from the injector and repeat your idle test. The fact that the sparkplug is clean points to an injector problem. Is it cleaner than the neighboring spark plugs? Might want to take the #1 coil out, put a sparkplug in it, and have someone crank the engine while you check for spark. If there's spark, then move on to the injectors.

You could rig up a "noid" light using a 12V LED. You can buy a universal one from Snap-On or Craftsman, but a 12V LED and a resistor are only a few bucks from Radio Shack. Up to you. Just make sure that the resistance across the LED is same or higher than the resistance across the injector itself or you will fry the drivers on the ECU. Add a resistor in series to get the correct resistance. Unclip the injector connector and connect the LED across the connector terminals. Then start the engine and see if the LED flashes. If it does, then you know for sure the problem isn't in the wiring. I would say it's a bad injector at that point. Good luck

Alin

Thanks for the help.

I had already done your first suggestion. The second suggestion of checking for a spark was what I was coincidentally out doing in the garage. I had spark once I remembered to ground the plug. I had also done a voltmeter check and everything was the same as the adjacent cylinder.

I proceeded on to the upteemth repeat of jiggling all the wires associated with plug #1 hoping to find a short. No luck. I next took a screwdriver and gently tapped on the fuel rail about a half dozen times from start to end. Just as I made my final tap the problem dissappeared. I have a couple of miles since then with no issue. I am hoping I dislodged something through the system. I also just added a can of Techron to maybe help clean everything out.

may your fix banish the misfire permanently. what's scary is how young your engine is. hopefully it was something as simple as getting some bad gas.

i used techron a little while ago because i was starting to get a diesel like rattling sound and have been using chevron gas most of the time ever since. that rattling has since not returned. note: i was advised that adding techron to your gas can affect the oil and to change the oil out sooner rather than later. good luck, taro

Taro, thanks for the well wishes and I hope your problem is similarly gone forever.

So far with 1/2 day of commute the problem remains gone. Like I said my theory is that some piece of junk, in spite of a relatively new fuel filter and having bought gas quite some time ago, managed to lodge in the #1 injector. The slight tapping I did was just enough to dislodge it. While I am not real confident in this explanation, it is at least a possible cause and if true should be a one time fluke.

Also, thanks for the heads up on injector cleaners and possible oil effect. I am not sure that is much of an issue given I only used on bottle. In any event my next scheduled oil change is about 2 weeks away.

Kind of funny how what goes around comes around. This time in a good way. You have a cylinder misfire problem and I try to help. Next thing you know I have such a problem and you help.