Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
Newbie here. Just joined and I am desperately in need of help.
Quick background. I'm a shade tree mechanic. I have done virtually all my own mechanical work for almost 40 years. I have decent mechanical and electrical skills.
The car in question:
2004 Jeep Liberty Sport
3.7L AT
279k
Car has been running fine. Friend was driving it to my house for me to fix other things (Ball joint and ignition lock actuator pin). About 45 minutes into her drive, the check engine light started flashing. (Check engine light has been on for over a year for O2 P0038.) She pulled over on freeway and called me.
When I got to her car, this is what I found:
Slightly rough idle
At idle. Check Engine Light steady on (not flashing).
If attempting to drive, flashing check engine light
Scan with BlueDriver scan tool
Confirmed DTCs
P0300 - Random misfire
P0038 - 02 sensor 1/2 Heater circuit high
Pending DTCs
P0300 - Random misfire
P0700 - Trans Control system
P0038 - 02 sensor 1/2 Heater circuit high
Given no other misfire codes. I guessed it was a bad fuel pump, plugged fuel filter, or plugged catalyst.
Very low power. top speed 45mph.
We decided to limp the car home on the back roads.
Did not feel like a plugged fuel filter (based on previous experience.)
Once home I checked all the injector connections and coil plug connections. All ok. (No surprise since there are no cylinder specific DTCs)
Pulled plugs.
Left bank all evenly black. Not fuel soaked, but definitely too rich
Right bank all very white. Not tan. But not quite yellow. Please pardon the poor pic attached. (I wasn't specific enough when I asked friend for a pic of the plugs, lol)
No mechanical engine noises.
Noticeable smell of something hot. But no rotten egg smell.
I hit ends of the catalysts with a plastic deadblow. No rattles.
Fuel pressure at idle 52PSI 60 PSI if revved (I think)
Cleared codes.
Disconnected battery.
Touched battery terminals to potentially clear adapts.
Fixed broken bank 1 downstream 02 sensor wires. (Clearly been that way for a while. But pcode remains)
We tried cleaning the plugs and reinstalled for a quick check.
No improvement.(Should have swapped left and right, just in case, I suppose)
DTCs now.
Confirmed DTCs
P0300 - Random misfire
P0038 - 02 sensor 1/2 Heater circuit high
Pending DTCs
P0300 - Random misfire
P0038 - 02 sensor 1/2 Heater circuit high
At this point I mostly suspect a plugged catalytic converter. But not sure which one or how to verify non destructively, nor how to diagnose if it is something else.
I wouldn't let the P0300 mislead you. Just because the code description says 'Random/Multiple' does not necessarily mean there are multiple cylinders acting up. Yours would not be the first Jeep to set a P0300 over a single cylinder misfire.
Does the engine still have a rough idle? If so, you can perform your own power balance test by disconnecting ignition coils one at a time with the engine running. Listen for a change in engine speed when you disconnect the coil, and compare that change to the other cylinders. Any cylinder with no change or significantly less change is a suspect.
Will your scan tool allow you to look at live data? Fuel trims would be a great place to start, especially given the condition of the plugs.