Recently, I did a tune up, including the plugs, wires, injectors, distributor cap, and seafoam on my 1999 XJ. for some reason, that i can’t figure out, it starts misfiring on cylinder 6, throwing p0306. However, when I clear the code, it doesn’t come back for a few drive cycles. I’ve tried putting in new spark plugs (again), putting in the old injector, and swapping plug wires around to see if the misfire moves. I’m out of ideas. Does anyone have any idea what could be causing it?

 

Is it a miss you can feel or just throwing a code? How high do you rev your engine?
Some 4.0s of that vintage had an issue with carbon building up under the exhaust valves due to weak valve springs.
A buddy of mine fought that code for months. He even replaced valve springs to no avail.
fuel system cleaners of all types were used.
What finally fixed it was the old mechanic's trick of running it at a high idle and slowly pouring water down the throttle body to break the carbon loose.

https://www.physicsforums.com/thread...r-fact.793344/

 

Again looking through this saga.
Multiple cylinder misfire, parts cannon has been fired, have spark, plug(s) look like they do.
It might be reasonable, considering it's not been done, to:

1)check fuel pressure
2) look for vacuum leaks.

I know you're focused on the ecm because you had that happen on another vehicle but you ought to look at ALL
the typical causes.

EDIT: Actually meant to put this on different thread but I guess it could work here too.

 

Chysler TSB 0900303
4.0L MULTIPLE CYLINDER MISFIRE
Date: 08/01/03
Model Year(s): 1999-2004

Description: NOTE: THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH A 4.0L ENGINE. This bulletin involves inspection of all engine exhaust valves and a decarbonizing procedure if necessary.

Details: The customer may experience an incident of engine misfire during certain vehicle operating conditions. The misfire may occur when the vehicle is operated between 50 - 70 MPH and under light loading conditions, e.g. slight uphill road grades. This condition may occur at all ambient conditions, but is more noticeable when ambient conditions are less than 0 C (32 F).

If the vehicle is equipped with On-Board Diagnostic (OBD), a MIL illumination may also have occurred due to Diagnostic Trouble Code (DTC) P0300 - Multiple Cylinder Misfire. Various single cylinder misfire DTC's may also be present. If the frequency of misfire is high the Powertrain Control Module (PCM) may place the engine in Limp-In mode.

The misfire condition may be caused by one or more engine exhaust valves that are slow to close. Late closure of an exhaust valve may be the result of no valve rotation and associated build up of carbon on the exhaust valve stem.

This condition may occur when the engine is not allowed to run at engine RPM?s that are greater than 3,200 RPM. At 3,200 RPM or higher the engine exhaust valves will rotate if not impeded by high carbon deposits. Low engine RPM's and high carbon deposits are associated with short trip driving where the vehicle engine is not allowed to fully warm to normal engine operating temperatures. Cold ambient temperatures will increase engine warm-up time and add to the opportunity of carbon deposit build-up on the stem of the engine exhaust valve.


Jerry Bransford a senior member of JeepForum posted the following:

Decarbonizing an engine can often be accomplished in 5-10 minutes for free using nothing more than 12-16 ounces of water. With the engine running and the air intake removed from the throttle body, SLOWLY trickle the water into the throttle body, keeping the rpms up a tad with the throttle linkage. Do it slowly enough that it will take several minutes to drain the container. No this will not (!) cause hydrolocking unless you dump the water in too quickly.

The water turns to steam and it creates millions of little shock waves that break up carbon deposits which are then sent out via the exhaust system. This is done all the time on engines that tend to develop carbon deposits when they're driven too conservatively. For example, Corvette mechanics do this all the time on engines that are babied too much by their owners.

Done properly this will not harm the engine in the least and it's an age-old technique that can leave an engine sparkling clean internally. Engines have actually used water injection since before WWII to increase power and the mechanics quickly learned that it keeps the engines carbon free, or gets rid of existing carbon deposits once modified for water injection.

All you have to do is trickle it in slowly enough that it will take probably 2-3 minutes to drain the container... a very slow constant tiny trickle of plain tap water.