Hi ~ I'm a semi-retired ASE-cert'd "Master Mechanic" and I'm trying to help a friend with his 2005 Liberty 3.7 He brought it to me running rough, with the CEL lit. He scanned it and found codes P0300 (random/multiple misfires) and P0302 (cyl 2 misfire). OTOH, I scanned it and found P0302 AND P2305 ("Ignition Coil ‘B’ [cyl 2] Secondary Circuit").
I initially suspected either the sp/plug or the coil. I swapped the plug with another cyl (#1), and the misfire remained with cyl 2. I then installed a new coil in cyl 2..... to no avail (still misfired). Next, I checked for "key-on" 12V at the coil = good. Then, I checked for coil triggering (noid light) with the engine running and found NONE (with a good signal at all other coils).
For shíts and giggles, I also checked for 12V at the injector (good) and for triggering = NONE, altho a good signal @ other injs). Seeing evidence of past rat/rodent infestation, I ASSumed wires had been chewed. Fortunately, I found a link to the factory service manual, which included a wiring schematic with the ECM pinouts (orange/black harness connector, pin 9 = coil control AND pin 11 = inj control. So, I tested both wires for continuity between the ECM plug and the coil & inj connectors, and (drum roll)...... both wires are good.
Soooo......... I'm faced with the possibility that the bad coil (zero ohms = shorted) failed the coil driver in the ECM ~ BUT, why is there no injector signal? I ASSume that the coil and inj drivers are separate, but..... DUNNO! Is there a possibility that either the cam or crank sensor isn't providing a "signal" for JUST the #2 cyl position/sequence? Seems unlikely. If an ECM is needed....... does it need to be programmed for this specific vehicle? <shrug>
Whaddaya all think? SORRY for the dreadfully long post, and..... TIA for the anticipated help.
I initially suspected either the sp/plug or the coil. I swapped the plug with another cyl (#1), and the misfire remained with cyl 2. I then installed a new coil in cyl 2..... to no avail (still misfired). Next, I checked for "key-on" 12V at the coil = good. Then, I checked for coil triggering (noid light) with the engine running and found NONE (with a good signal at all other coils).
For shíts and giggles, I also checked for 12V at the injector (good) and for triggering = NONE, altho a good signal @ other injs). Seeing evidence of past rat/rodent infestation, I ASSumed wires had been chewed. Fortunately, I found a link to the factory service manual, which included a wiring schematic with the ECM pinouts (orange/black harness connector, pin 9 = coil control AND pin 11 = inj control. So, I tested both wires for continuity between the ECM plug and the coil & inj connectors, and (drum roll)...... both wires are good.
Soooo......... I'm faced with the possibility that the bad coil (zero ohms = shorted) failed the coil driver in the ECM ~ BUT, why is there no injector signal? I ASSume that the coil and inj drivers are separate, but..... DUNNO! Is there a possibility that either the cam or crank sensor isn't providing a "signal" for JUST the #2 cyl position/sequence? Seems unlikely. If an ECM is needed....... does it need to be programmed for this specific vehicle? <shrug>
Whaddaya all think? SORRY for the dreadfully long post, and..... TIA for the anticipated help.
