P0303, bad mileage but otherwise running fine?

[Sleepydog]

I just picked up another XJ, a 2001 with 192k on it. It runs great, but I am getting VERY poor gas mileage (i.e. 6-8mpg). No CEL showing (i think the previous owner pulled the bulb), but I ran the codes and it showed P0303 and P0194. I think the P1494 was due to a disconnected vaccuum hose I noticed shortly after I bought it. I know the P0303 is cyl 3 misfire - But this thing is running great - smooth idle, runs great - Could the code be related to the mileage issue? Going to change plugs first, then go to the injectors - just want some thoughts if you guys have any. I have searched, but anything I find for P0303 always ties back to a rough running Jeep. Mine appears to be running fine.

Thanks.

 

[jeeptec]

there are some know isusses of the 4.0. chrysler has a tsb about the 300 to 306 on the 4.0. they say that the valves carbon up and cause miss fires. the chrysler salution is to remove the valve cover. add air preasure the each cylinder one at a time. at tdc. compress the valve spring and rotate the valve 90 degrees. the run a combustion chamber cleaner throught it when done.if it were me id replace the plugs do a compresion check and do a decarb. if you have acess to a scan tool that reads sensor info then you maybe abe to what the fuel adtives are doing and see if one of the o2 sensors isnt stuck. sorry i said a mouth full

 

[heyhar]

Run a can or two-one at a time!-of BG44k through it. It will de-carbon (un-carbon?) the combustion chamber and valves. See if in-fact there is a bulb in the CEL socket. It should come on momentarily when you start it. Make sure it doesn't suffer from the dreaded 0331 head crack. You don't mention that you own a scan tool, but they're worth their weight in gold (gasoline) as our beasts age.

 

[Sleepydog]

Where does one find BG44k? Is it like Seafoam? I was going to try that...

 

[Tim_MN]

If your mpg's are 6-8, I would suspect that one or more of the O2 sensors are faulty. O2 sensors have a normal service life of about 100-150,000 miles. Typical symptoms of worn out O2 sensors is an extreme drop in gas mpg's. If your O2's are the originals, they are due for replacement. A new cat converter would be good idea also.

 

[heyhar]

I've never used Seafoam-it has its fans and skeptics. But, I was introduced to BG44k about twenty years ago at an oil change store. It made my Taurus run much better. I tried it again a few years ago, when my '96 Country began to not start on the first crack. I diagnosed it as a bad check valve, so a twenty-dollar shot seemed worth the risk. The engine started on the first crack again (still does), but more importantly, it runs great on 87 octane again! It removes the built-up carbon and varnish, and it therefore cools the combustion chamber, negating the need for higher octane. I tried it in my '04 Subie Outback 2.5 when it started tapping on 89 octane, now it doesn't tap on 87. All i can say is that it works for me, and I'm still impressed...

As far as where to get it, their website lists the stores and dealers that handle it. It's not cheap-about $20 a can, and I think that's why the discount chains won't touch it. They like to sell 'miracle in a can' sort of stuff for five bucks. If your engine is as gunked-up as it sounds, I'd try at least two treatment cycles before I started ripping stuff apart. At least it will be clean if you do!

Dittos on Tim's suggestion...And, it's the O2 sensor (you may have two) BEFORE the cat that controls the fuel mixture, not the one at the rear of the cat. And while I'm here, the Coolant Temp Sensor, on the T-stat housing, is the other culprit in the whole 'closed loop' deal. It's a few bucks, so while your hands are dirty...

 

[Sleepydog]

update: reset the codes with my neighbor's obdII tool. Ran the piss out if it and rechecked the codes - all are gone! Suspect the 0194 vaccuum leak triggered the 0303 cylinder code,as it was at the same locatoin at the #3 cylinder. Tore the the dash apart to check the CEL bulb - it was there, but functioning. Replaced the bulb, now it works just fine. All seems right, but still need to run more miles to check the mpg...will update as i progress. I did do the BG44k treatment - will advise as to results. Thanks to all for your input...

 

[birchlakeXJ]

I don't see any mention of "heat soak" as a possible root cause for that P303. The 00-01 are quite susceptible to it and it usually manifests itself as a cylinder 3 misfire. Here is the TSB on it. The factory fix can work for "mild" cases. For stubborn cases, guys have rigged the e-fan on a timer so it stays on for a predetermined amount of time after you turn the engine off to bring down engine bay temperatures (the cause).

Some others have cut louvers into the hood. In fact, just saw this thread which shows what one guy is doing. Good luck.

http://www.naxja.org/forum/showthread.php?t=1085397

And count me in on the BG44K fan club. I use it for symptomatic driveability issues where I suspect a compromised fuel system.
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TSB 1803103 - 4.0L ROUGH ENGINE IDLE AFTER RESTART FOLLOWING A HOT SOAK
Date: 09/05/03 (supercedes 1802702 dated 09/06/02)
Model Year(s): 1999-2004

Description: NOTE: This bulletin applies to vehicles equipped with a 4.0L engine. This bulletin involves the installation of a fuel injector insulator sleeve.

Details: Customers may describe a 20 to 30 second rough idle following the restart of a heat soaked engine. This condition may be most noticeable when the engine is restarted following a prior 10 to 20 minute heat soak in hot ambient conditions of approximately 32 C (90ºF) or higher. This condition may be consistent with short city stop-and-go driving trips and can be aggravated by the use of fuel with a high ethanol content. Depending upon various conditions a MIL may occur due to DTC P0303 - Cylinder #3 Misfire.

This condition may be caused by heat from the exhaust manifold that following engine shut down migrates to the area around injector #3 and causes fuel vapor to form within the injector. This in turn may cause a momentary misfire of cylinder #3 until the fuel vapor is cleared of injector #3. The insulator sleeve lowers the injector #3 temperature to a point below which the fuel will not normally vaporize.

If vehicle exhibits a rough idle, and if a misfire of cylinder #3 is observed following a 10 to 20 minute hot soak, perform the Repair Procedure.

Parts required:
56028371AA Ignition Wire Shield

Repair procedure:
1. Cut insulator sleeve, p/n 56028371AA, to make two (2) insulator sleeves about 25-30 mm (1 in.) in length. See diagram.
2. Install one sleeve around injector #3, with the slit on the upward facing side of the injector. Install the other sleeve with the slit on the downward facing side of the injector.
3. Confirm sleeve is flush to intake manifold surface around injector.
4. Check injector #3 wire and ensure that the injector is rotated to a 2 o'clock position.