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Slight misfire at idle

Petemech

NAXJA Forum User
Location
orange county NY
Hey all, just got my new to me XJ on the road. It has a check engine light for P0301 cylinder 1 misfire. I've changed distibutor cap and rotor, plugs and wires, checked for vacuum leaks and seafoamed it. The PO apparently put 4 hole injectors in it but i have not verified the part numbers yet. The misfire is mainly at idle and just barely noticeable. Does not seen to be effected by engine temp or any thing else. Power seems fine, and i havent driven it enough to see what fuel economy is. Hopefully if my MAC took guy pulls through ill be doing s cyl leak down test tomorrow. Any insite would be helpful.
 
Sometimes there is carbon buildup under the valves.

Seafoam doesn't cut it. See what your leakdown test says.

I have had excellent results using water to break up the carbon deposits.
 
Sometimes there is carbon buildup under the valves.

Seafoam doesn't cut it. See what your leakdown test says.

I have had excellent results using water to break up the carbon deposits.

Ive heard that water works great and have personally seen it on diesel trucks with blown head gaskets. Question is how would you go about introducing the water to the engine?
 
Spritz a little into the throttlebody

Nah. Gotta be more aggressive.

Pretty easy.

Get a cup of water, preferably something like a 24 to 32 oz. cup.
With the air cleaner off, hold the engine RPM at 2000 RPM or higher and dribble the water from the cup, which you've pinched to look like a spout on top, into the carb or throttle body. Go slowly and the engine will bog a bit. Stop and let it recover. Repeat until the cup is empty.

Water%20carbon%20cup_zps2fhqgdr2.jpg
 
I would run a can of BG44K though the gas. It sounds like you have a clogged injector. You could swap injectors between cylinders and see if the error code changes, but I run a can of BG44K every 10k in every vehicle as a preventative. It will clean the injectors and remove the carbon, not a bad thing either way.
 
I kept getting a P0305 code on my '98. Drove me nuts trying to figure it out. Finally found it. The wire loom for the injectors was rubbed through and shorting (intermittently) where it bends around the back of the fuel rail. I'd start there...
 
I would run a can of BG44K though the gas. It sounds like you have a clogged injector. You could swap injectors between cylinders and see if the error code changes, but I run a can of BG44K every 10k in every vehicle as a preventative. It will clean the injectors and remove the carbon, not a bad thing either way.

Never a bad idea. But I've had a few where the BG couldn't cut the carbon which has been beaten down between valve and seat.
 
Ok got an update. So i drove it to work this am and CEL was on. Drove home and decided to get gas. Filled it up and started it, CEL went out. Got home and plugged my scanner in to confirm it was the P0301 that cause the CEL. It did have the P0301 but also had P0320 for ignition/distributor engine speed input circuit and the P1694 for no engine buss that i talked about in a different post also came back.

My tool guy didnt come through for me so i wont be doing the cyclinder leak down test this weekend.
 
I recently had that code and it was a clogged Cat converters that had no other codes at all. In prior times it was a head gasket leak on the same engine. Prior to that a hairline fracture cracked head. 96 Ford V-6. Could be a bad plug, plug wire and so on.
In all cases there were no other signs of problems at all except that one code and slightly low compression on one cylinder for the cracked head.
 
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It sounds like you have a clogged injector. You could swap injectors between cylinders and see if the error code changes,

Yes, a dirty injector can cause this problem. It's a good suggestion to swap injectors and see if the potentially bad injector's miss moves to the new position.

Recently found this to be the cause of my brother's 2000 XJ's slight miss and P0303 code. A fresh set of injectors solved the problem .
 
You could also try having the injectors cleaned on the truck. Many shops can do this for less than $150. Worse case scenario, you spent $150 on an all too often ignored maintainance service.
 
Injectors????

Electrical wiring or electronics issue is far more likely with this:

"It did have the P0301 but also had P0320 for ignition/distributor engine speed input circuit and the P1694 for no engine buss that i talked about in a different post also came back."
 
Really leaning away from injectors on this one. It seems more electrical. I know the ground strap from the head to the firewall is in sad shape (not that they are good from the factory). Im planning on redoing all the powers and grounds with 1/0 welding wire and marine/millitary style lugs, but I dont have the available funds at this moment. Have to wait till payday. Maybe ill try cleaning them up tomorrow as a temp fix.

The CEL seems to be going on and off. I may pull that wiring harness out today and just verify nothing is rubbed or chaffed.
 
So few days have gone by. I've swapped #1 and #2 injector and replaced all 12 injector o-rings. Confirmed they are ford 4 hole injectors and fairly new. Checked the wiring from pcm to injector #1 for high resistance, short to ground and pin-to-pin shorts. Checked wiring to crank position sensor for the same. Checked wiring to distributor for the same. Came up with nothing. Can't find any thing chaffed or rubbed any where in that harness.

Yesterday morning i almost didnt make it off my street because it was hesitating so bad but the its been fine ever since. Cel has't come back either. Seriously scratching my head. I think i'm going to throw a crank position sensor at it just cause. I'm pretty confident what ever it is is electrical because its so intemitant.
 
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