Please help! P0155, P0300, P0301, P0306

Hi there! Trying to diagnose my cylinder misfire issues that have been ongoing. I feel like I've tried almost everything and I now have a shop telling me I need a new motor which may be the easiest but also most expensive solution.

My 2000 Jeep Cherokee 4.0L inline (168k+ miles, automatic) has been running quite rough lately and the check engine light is on, again (codes listed in subject line). It takes a few turns for her to start and it takes a min or two for the idle to smooth out. More recently idle is sometimes dipping down to 250 rpm when I'm at a stop but so far, no stall.

So I bought her a few months ago and she ran fine for a bit. About a month later the check engine light came up with cylinders misfiring. I changed the spark plugs which were pretty corroded and not properly gapped. This fixed the issue for about a month when the check engine light came back and she began running very roughly. I took her to a shop where they said a couple fuel injectors were non functional. I had ALL of them replaced along with another set of new spark plugs. She ran beautifully for about another couple months, until now. I'm getting the same codes. I've swapped coils 1&2 and 5&6 to make sure they weren't the culprit. The misfire did not follow the coil. I purchased the O2 sensor that set off the code and replaced it. Code came back. The other day I pulled the throttle body and IAC and cleaned them thoroughly. I've also run seafoam through the engine. When I pulled the spark plugs they were again corroded and covered in oil. Is there something else I'm missing or is a new motor really the way to go?

This is what the throttle body and the plugs looked like when I pulled them...

Ask the guy that said motor was bad, what the results of the compression test was ? Not likely they even did one.

Not looking up those codes, it sure looks like a lot of un burnt fuel. Misfire could be as simple as bad spark plug wires too, which I didnt see as a part of your tune up. Weak coil, weak spark ? New plugs and runs fine til plugs foul up from un burnt fuel ? Something to think about anyways.

I'm sure someone will chime in with more related to the codes.

the '00 is coil on plug if I remember right, so no plug wires. Most likely either rings or valve guide seals, something dumping lots of oil into the cylinder.

Codes are for O2 sensor malfunction, random cylinder misfire and cylinder 1&6 misfire. It does run well until the plugs get gummed up. Gonna try out compression test to see if that tells me a bit more.

First, find a new shop to work on your Jeep.

Fix the P0155 and the P0300's should clear up.

Is it California or Federal Emissions ? What brand and type of spark plugs were installed ? What brand of O2 sensor was installed ? The Jeep 4.0L is well proven to have have a preference for certain brands, and will run poorly and even misfire with the wrong plugs or the wrong O2 sensors.

Since you have a 2000 with Coil on Plug ignition and with the 03331 head, you should check the cam sensor synchronization, and your should inspect and test for a cracked 0331 cylinder head.

Read this -> Scan Tools, O-Scopes, and toothpicks

P0155 Jeep - O2 Sensor 2/1 Heater Performance

Possible causes

- Faulty Heated O2 Sensor Bank 2 Sensor 1
- Heated O2 Sensor Bank 2 Sensor 1 circuit fuse
- Heated O2 Sensor Bank 2 Sensor 1 harness is open shorted to ground
- Heated O2 Sensor Bank 2 Sensor 1 circuit poor electrical connection

California emissions. The O2 sensor I replaced is a Bosch and it was bank 2 sensor 1. The plugs are Champion although I'd have to pull them for the specific type since they're the new ones put in by the shop. I have Sunday off and plan to toy around a bit more with further testing. Thanks!

Nice link on the Toothpick. I took for granted the CPS has no weak links other than working or not!

BOSCH O2 sensors are well proven to create problems in the Jeep 4.0L, use the Google for proof. You should be using NTK sensors