'04 3.9L, 256k miles, transmission less than 2 yrs old. Just did a tune up (new coil pack, plugs, wires, air and fuel filters, fixed all vacuum leaks I could find, replacing a few lines). Ran amazing for about 800 miles. A few days ago I started getting some stumbling while driving, sometimes while accelerating. If I shift into 1st, it'll run up, pull strong, no misfires. Shift into "3," and it'll pull strong through 2nd and 3rd. If I go into D, it'll shift into 4th, and if the RPM is below 2000 it misfires, loses power, then misfires in 2nd, 3rd, 4th. If I just barely push the accelerator, it doesn't misfire as much (nearly not at all). When working under the hood, I can't identify any additional vacuum leaks with carb cleaner, I don't see any more rotting hoses. It doesn't appear to misfire idling in park working on it, only under load. When I crack the throttle open quickly, I get a loud "PSHT" before the engine revs up, and my PCV vacuum hose inflates. I have noticed that on cold starts in the morning, it idles around 1200 RPM until I shift into gear. All fluids appear to be at appropriate levels and not worn out. Weather/moisture doesn't seem to affect it. It started while driving at highway speed, now does it regardless of speed, and I do get (infrequent) misfires at idle at stoplights

Codes I get are P0316 (misfire in first 1000 revolutions), P303 (misfire in cylinder 3), P2195 (Lack of Ho2S-11 switching, sensor indicates lean), P0171 (Fuel system too lean cylinder bank 1).

I'm going to pull the plug for cyl 3 and see what it tells me. My running theories are bad (new) plug, fouled plug, bad (new) coil, dying #3 injector. Possibly #3 wire has rubbed and is intermittently shorting (was too long). Or intake manifold gaskets. I would think if the coil or plug were to blame it'd happen throughout the rev range, regardless of load. If I slowly rev (in P while working on it) I don't hear the PSHT sound. I'm worried most of it is vacuum related, but I can't find a source.

I'd found in one place a mechanic indicating the "PSHT" sound could be related to clogged catalytic converters, and replaced them, successfully fixing the issue after fixing the misfire. I assume the torn up cats would be from excess unburned fuel from the misfires (previously had pretty bad misfires due to other issues I've since fixed.

I'm open to suggestions, knowledge, or previous experiences from you guys. I really need to make this van last at least a few more months.

 

So I found and fixed the source of my misfire. During the process of trying to diagnose it I replaced both bank 1 O2 sensors which took care of the lean codes.

Turns out the coil I'd just installed had gone bad after about 800 miles or less. It's now back to running with no codes, and even after heavy rain and driving through puddles today it's working flawlessly.

https://www.youtube.com/watch?v=Bu-l...ature=youtu.be