2001 Sport here, <89000 miles with P0306 cylinder 6 misfire code that will not go away - goal is to clear the P0306 Cylinder 6 misfire code

We thought we'd list our steps of diagnosis:


Perform a tune-up:
- 6 new plugs - no effect; later recheck found new #6 plug to be cracked, R/R'd again - no effect
- 1 new coil pack - no effect
- 1 new cam position sensor - no effect
- 1 new crankshaft position sensor - no effect
- 1 new MAP sensor; engine ran like crap at idle would only appear to run well above 2200 rpm. Removed new MAP sensor and compared it to the old MAP sensor and we noticed the hole on the new sensor was just slightly larger than a pin hole where the old MAP sensor hole was a bit larger. Put the old MAP sensor in and the engine ran better at idle. - no effect
- 1 new air filter - no effect
- swapped #5 & #6 fuel injectors - no effect
- cleaned throttle body with throttle body/map cleaner - no effect
- made sure injectors were angled to the 2 o'clock position - no effect
- disconnecting the injector connectors one at a time resulted in engine changes, except for when disconnecting injector #6 connector (hence the misfire code?) - no effect

+/- noid light in all injector connectors show good firing (but not necessarily good voltage/timing!) - no effect
+/- Thermal wrapped all injectors as well as the fuel rail. Why? During the course of all this troubleshooting we noted that the aft section of the fuel rail closest to the firewall was extremely hot to the touch where as the mid to forward sections were tolerable to the touch - no effect
+/- Disconnected and flushed the fuel rail with Seafom - Why? The injector fuel rail sits at an angle where the #1 is higher than the #6; possible collection of rust/crud at the lower #6 end in the fuel rail causing insufficient fuel flow to #6 injector? Cleaned it good... - no effect
+/- Seafoamed the gas tank - no effect
+/- Seafoamed the intake - (didn't smoke bad at all) - no effect
+/- Found a vacuum leak on the brake booster - the plastic plug-in on the booster had a crack in it; replaced it and engine became smoother, but P0306 still happens - no effect



* Fuel pressure holds steady at 45-47 psi (different references showed 49 +/-5 and then 49 +/-2)

* Rechecked Compression check @ 5 revolutions (versus 3) -
6 = 155 (!)**
5 = 180
4 = 170
3 = 170
2 = 170
1 = 170

Things left to do:

1. Replace IAC valve - (but don't think this will change anything)

2. Rotate valves on cylinder #6 - (but don't think this will change anything)

3. R/R intake/exhaust manifold gasket (but don't think this will change anything)

4. Check intake manifold for cracks

** 5. Based on compression test, could be burnt valve or crack between valves on #6 cylinder


Conclusion to date: Do the compression and fuel pressure tests upfront before you decide to start buying those expensive little parts that may not fix the problem.

We most likely need a new head... or just a valve job on #6... if you're gonna do one, might as well do all of them.

So, we're looking at a *NEW* head for $495.00

Head bolt replacement set for $36.00

Head Gasket set from for $137.00

and I want a set of those new Bosch injectors for $150.00


...so maybe another $900 until we get it right. This doesn't count any bad lifters we may run into.

 

did you spray carb cleaner all around aera to see if the intake may be leaking in affected area do you have one coil or coil packs if you have coil packs that could be the prob

 

yup, sprayed carb cleaner all over looking for any other vacuum leaks - the service manual says to spray a water mist around possible areas

replaced 1 large coil pack with 3 coil bumps for every 2 spark plugs -

 

i use carb cleaner my self the code may have to be cleared by the scan tool?

 

spark plug wires and cap replaced?

 

You need to clear the code, it won't just clear its self right away, the PCM needs to see so many good trips and ignition cycles in order to clear the code.

 

How do u know 5/6 coilpack is getting its signal to fire? There's 3 pcm driver signals for that rail pack.Check ur signals first. 155psi compression is not horrible ,not that great either.Drop a little oil in that cylinder and recheck to rule out fuel washout. If numbers don't change then u have a valve/head problem. At that point just get a new head especially if u have a 331.

 

head gasket...$20

head bolts...$37

intake//exhaust gasket set...$13
t-stat gasket, $3


the new head, I would assume, comes complete with valves and seals. at least it should, at that price. order a head for a '95 xj. that is the best year head you can get.



the worst you could do is get a head from a motor at the junkyard, rebuild it yourself and install it for $300. it's an easy job.

 

Check your cam before you rebuild the head.-----------(question if he gets a 95 head whats gonna hold his coil pack????HMMMMM) ---------- Did you replace the whole coil rail and new boots also? A bad boot will cause misfire terriable

 

it's not like the coil pack is gonna fall off....there are 6 spark plugs there, ya know...

and it's a better head...

 

I didnt know if you new how to make a bracket for this or had some other idea to hold it,cause it should be held steady I beleive.

 

make a bracket that ties into a couple or a few of the valve cover bolts. easy.

 

complete new coil pack, checked new coil pack for spark, it works fine... OBD tool does clear the P0306 code but it keeps coming back.

.. new head comes with valves n springs installed and has mounting holes for coil pack bolts

 

why bother replacing the head with a 2000/2001 head?

they are known to be poor quality heads, they are known to warp and crack.

the '95 head is ported better, probably the same price, and is cast properly. sure, you have to make a bracket to hold the coil rail/packs, but it's gonna net more power, better mileage, and less chance for failure.


that said, don't drop $150 on gaskets. you need an intake/exhaust gasket ($13), head gasket ($20), head bolts ($36), and t-stat gasket ($3), and a GOOD rubber valve cover gasket (which is NOT included in the $150 gasket set....they include a cork one...)...that is $44.

so, you get the new head, and spend $90 or so on gaskets, and are less out of pocket.

you don't need to pull the fuel rail, sensors, or throttle body off the intake to do this job...

 

... going to try the exhaust valve rotation procedure before buying a new head...