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Mystery Misfire P0305

XJsportG

SouthEast #4328 (RVA)
NAXJA Member
Location
Richmond, VA
I've searched and searched. To no avail am I find anything to help at this point.

He's the back story:

Took the Jeep down to the transmission shop last week to have my bent to hell tranny pan replaced on the AW4. While it was there, they did their normal diagnostics checks. They came back and said I had these engine codes(no CEL going in or out of the shop):

-P0320
-P1687
-P622

They recommended I do a tune-up which I happily did my-self.

- Changed oil
- Change Serpentine belt
- Replaced Spark Plugs (NGK V Power Copper)
- Replaced CPS (Echlin Ignition)

After the TLC, I took it for a drive and this is where the mystery begins.

When accelerating the Jeep right between the 2nd -> 3rd shift around 2500rpm the XJ will start to misfire on cylinder 5. I pulled off, turned the jeep off and cleared the code. Get back on the road and same thing.


Things i've noticed:
- Misfire starts when shifting 2nd -> 3rd @ +2500rpm
- Easier to reproduce when the engine is cold or on the first drive
- Drove 10 miles to buddies shop (non-interstate) 45mph Speed-limit, misfired whole way off and on.
- Turned off while waiting, turn it on and take a lap to get the misfire to happen and now it won't misfire after engine is hot.
- Temp is normal 193-210(fan kicks on) cools off.

Things i've checked:
- Replace Coil Pack
- Replaced Spark Plug on #5
- Checked Gap on all plugs
- Replaced TPS
- Swapped Injectors, misfire did not follow
- Replaced CPS

Not sure where else to go from here. This isn't the typical heat soak misfires i've had before (multiple cylinder misfires) on long trips or frequent off & on with hot days.
 
2001?

Bad 0331 casting, crack?

This part makes no sense:

They came back and said I had these engine codes(no CEL going in or out of the shop):

-P0320
-P1687
-P622

They recommended I do a tune-up which I happily did my-self.
I had a recent engine misfire 301 code, not sure when it would trigger, but it turned out to be clogged cat converters from prior cracked heads and head gasket coolant leaks over the years into the combustion chambers that eventually clogged up the Cats (it was a Ford 96 Taurus).

Other than that I got nothing short of bad cylinder issue

One thing you could try is to seafoam the hell of it in case there is carbon built up in the combustion chamber that is getting and staying red hot, enough to cause a miss fire.
 
Last edited:
I had that same code along with about 4 other varied codes, not seemingly connected on the Ford last week. We had disconnected several main harness connections to get to the transmission for valve body work. Disconnected and cleaned the pins and guts of corrosion oil and dirt from their being tossed around for 2 weeks in a dirty area, and all those codes vanished including the PO305, so you might some connector refreshing any where you have been lately in the wiring, like wiring the transmission shop moved around, disconnected etc.
 
Might also check the injector wiring harness where it bends around the fuel rail in the rear. Mine chaffed through and started shorting the wiring for injector #5 giving me the CEL and p0305.
 
2001?

Bad 0331 casting, crack?

This part makes no sense:

I had a recent engine misfire 301 code, not sure when it would trigger, but it turned out to be clogged cat converters from prior cracked heads and head gasket coolant leaks over the years into the combustion chambers that eventually clogged up the Cats (it was a Ford 96 Taurus).

Other than that I got nothing short of bad cylinder issue

One thing you could try is to seafoam the hell of it in case there is carbon built up in the combustion chamber that is getting and staying red hot, enough to cause a miss fire.

No other symptoms of cracked head. I've pretty much narrowed it down to being an electrical issue.

I had that same code along with about 4 other varied codes, not seemingly connected on the Ford last week. We had disconnected several main harness connections to get to the transmission for valve body work. Disconnected and cleaned the pins and guts of corrosion oil and dirt from their being tossed around for 2 weeks in a dirty area, and all those codes vanished including the PO305, so you might some connector refreshing any where you have been lately in the wiring, like wiring the transmission shop moved around, disconnected etc.

Pins look good and all the connectors are tight and secured.

Might also check the injector wiring harness where it bends around the fuel rail in the rear. Mine chaffed through and started shorting the wiring for injector #5 giving me the CEL and p0305.

After some testing, this is what we thought it was. We spliced in a new connector and thought it was over, but sure enough it came back.

We believe the PCM has gone bad in some way, but I'll update when I get a new one in on Weds.
 
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