Rough Idle With P0300 Code

txfiremn

Member
Supporting Member
Joined
Apr 29, 2022
Messages
49
Location
Waco, TX
A quick backstory to my Jeep, I bought this from a person who stated the transmission was out, it couldn't go in gear, and it was not driveable. I had a buddy haul it back to the house and we started to mess with it and found that the T case was needing to be rebuilt. So, I rebuilt it and it is back to driving. The motor was always able to be started. The battery was weak because she let it sit but I have since fully charged it and it will start right up. It has always had a rough idle but never had codes. I found a thread where @Jerry Bransford said to check your transmission fluid level. I did, I was down 2 quarts and that really helped with the idling. I also replaced the plugs with NGK ZFR5N. I took it out for a test drive and the check engine light started flashing and it was a dog getting back up a long hill going home. That's when it showed P0300.

I installed all 4 O2 sensors with NGK sensors as per the O2 sensor thread on here by @Chris hoping it would help but I still have a rough idle.

My symptoms are:
Rough idle but smoother when in Park or Neutral
The throttle seems to be responsive and makes the rough idle go away
It looses power when I get up to speed on our county road
I can clear the code but it does come back when I'm up to speed on the county road

Specs:
2002, 4.0, auto 3 sp, 175k miles
 
If you still had the issue after the plug change? Check your work. I’ve changed mine before and not had the coil rail seated far enough. It did the exact same thing yours did.
 
If you still had the issue after the plug change? Check your work. I’ve changed mine before and not had the coil rail seated far enough. It did the exact same thing yours did.

I checked it today. All seems good.
 
Well my problem ended up being bad fuel injectors after I changed plugs. It’s easy to check if you have a fuel pressure tester. Or you could pull the injector rail off and cycle the key to see if fuel leaks out of the injectors. I was having a hard start issue/long crank? And a rough idle. Not sure if you have these symptoms?
 
Well my problem ended up being bad fuel injectors after I changed plugs. It’s easy to check if you have a fuel pressure tester. Or you could pull the injector rail off and cycle the key to see if fuel leaks out of the injectors. I was having a hard start issue/long crank? And a rough idle. Not sure if you have these symptoms?

I haven't had any starting issues so far...
 
So another issue I had was a bad ignition rail. I replaced it at about 60k it was also setting an PO300 which is a random misfire.
 
I’m sure there is a way to test the rails I haven’t really looked into it. Anytime I’ve ever replaced a coil rail ore bad plug wire it was visibly damaged somewhere.
 
I'll check back over them. I know for sure the coil pack is oem and the Jeep has 175k miles. It probably wouldn't hurt to change it anyways. I just didn't want to start throwing parts at it.
 
I was able to ride around in the Jeep last night to see exactly when the light would come on. I did clear the code before I left the house. I drove around the neighborhood and down some back roads to the gas station for some fresh gas. The light did not come on and I had plenty of power to get around as long as I didn't really get on it. If I tried to really gas it up hills, the light would start to flash but letting off the gas would turn it off. I drove like that for a good 30 or so minutes, got it back to the house and no light. I didn't think about checking codes with the reader after driving though. I was just happy to not have a light on.

On a side note, I have forgotten how fun it is to drive a Jeep.
 
  • Like
Reactions: Modoc Guy
@txfiremn does your engine run ok when it's cold and only starts running poorly after it starts to warm up? If so your problem is very likely a bad O2 sensor. The PCM doesn't use the O2 sensors to set the air/fuel mixture when the engine is cold, it uses pre-set settings. It only switches to using the O2 sensors to set the air/fuel mixture once the engine starts to warm up. If you decide to change your O2 sensors make sure to ONLY install NTX or NGK O2 sensors, avoid all store brands and avoid Bosch O2 sensors. Bosch is a good brand but its O2 sensors don't work well in Jeeps.
 
I have replaced all 4 O2 sensors using the NGK brand sensors. It seems to idle rougher when it's put in gear but is way better now since I topped off the transmission with 2 quarts of atf. I also replaced the spark plugs with the OEM NGK plugs gapped at .35.
Last night the check engine light would only start to flash when under hard acceleration but would turn off when I resumed normal driving, I never took it to highway speeds though but it did great otherwise.
 
  • Like
Reactions: Jerry Bransford
I have replaced all 4 O2 sensors using the NGK brand sensors. It seems to idle rougher when it's put in gear but is way better now since I topped off the transmission with 2 quarts of atf. I also replaced the spark plugs with the OEM NGK plugs gapped at .35.
Last night the check engine light would only start to flash when under hard acceleration but would turn off when I resumed normal driving, I never took it to highway speeds though but it did great otherwise.
I don't understand how a low ATF level could cause the engine to misfire but glad filling it up helped (somehow). Make sure it's all the way to the full line or even slightly over when the engine is running. 'Tis far better for an automatic transmission to be slightly overfilled than slightly underfilled. Mine is routinely overfilled by about 1/2 quart to prevent it from sucking air during steep climbs.
 
  • Like
Reactions: txfiremn
I don't understand how a low ATF level could cause the engine to misfire but glad filling it up helped (somehow). Make sure it's all the way to the full line or even slightly over when the engine is running. 'Tis far better for an automatic transmission to be slightly overfilled than slightly underfilled. Mine is routinely overfilled by about 1/2 quart to prevent it from sucking air during steep climbs.

I'm sure it has nothing to do with the misfire but I'll definitely add more. It's it's in-between the full and add "safe" area with the hash marks.
 
  • Like
Reactions: Jerry Bransford