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'06 Liberty 3,7. Misfire on cyl 4. Scoped for secondary waveform on its COP and got no signal (tested adjacent COPs and got a good pattern). Swapped #4 COP with one of those that checked out and, again, no secondary waveform. I then disconnected COP's power plug (which is 2 pin, BTW) and probed the red wire and got momentary battery voltage with key-on and constant battery voltage with engine running. Just for laughs, I plugged COP #4 power plug into cyl #2 COP next to it and tested for secondary again. Still nothing. I did not test #4 injector.
Is it safe to assume I have a problem with #4 COP signal wire? Either an open or a problem with driver inside PCM? I don't know if the PCM could have possibly detected a problem with COP 4 and disabled the trigger temporarily (wishful thinking).
It looks like you verified power to the coil with it disconnected, but did you back probe coil control with the engine running?
Some Chrysler cars will shut down the coil driver for a misfire, but even then there will be a few strikes on start up.
After checking coil control, you need to verify circuit integrity back to the PCM before blaming the computer.
Is it setting any codes other than p0304?
No. I didn't back probe the ignition primary as I'm honestly a little unsure on the process for testing. I've heard you can check for trigger using a test light, a meter set on frequency scale or a scope with proper attenuator but haven't actually done any of them.
Perhaps related to this problem is I've been having issues regarding faulty valve lifters I installed recently on that bank of the engine. A lot of misfire counts have been racking up under mode 6 as well as having a few rocker arms pop off on me. I rook a peek under the valve cover on cyl 4's side yesterday, but all the rockers look fine. I ordered another brand of lifters but am still using the vehicle while I'm waiting for them to arrive.
OK. I read up on how to test the primary with a scope, so I'll check for those initial strikes at startup that you mentioned in a couple days. The lifters also arrived, so I'll be doing those shortly, too.
In the meantime, I was wondering, on the Chryslers that can disable COP trigger, do they do the same with injector trigger?
Yes, I recall on a Hemi engine (Durango, Ram, Charger maybe), that had a lifter issue and misfire on cylinder #2. I expected the injector to control to be disabled, and it was. That's a very common strategy. I was surprised when the ignition control was also disabled. That was the first time I had seen that. Your generation Jeep Liberty may not disable the spark at all for a misfire, but I do expect it to disable the injector after so many catylst damaging (type A) misfires are realized by the computer.
Welp, I tested today and I'm not getting trigger on that COP, not even at startup. The trace just hangs at battery/charging voltage. I'll have to backtrack the wire to see I can find a break in it or just run a jumper, I back probed the injector's trigger wire on that cylinder and got a good pattern for about 5 seconds after startup, then it just stayed at charging voltage, which, as you mentioned, is the expected strategy
This vehicle apparently doesn't disable ignition control in response to misfires, as you figured, as I unplugged an injector on a different cylinder and monitored its ignition coil secondary waveform, Pattern was still there after about 20 seconds of testing,
Thanks very much for the help so far. I'll post an update when I get more time to look at it,
Thank you for the update.
My next test would be at the engine control module. Back probe the same wire at the computer and repeat the same test.
Battery voltage at the control wire on the computer connector and lack of activity would verify that the circuit is intact from the coil to the computer , and put one more nail in the coffin as far the coil driver is concerned.
Great suggestion. I hadn't thought of testing for power there.
Though I know it not might be common practice, is it acceptable at all to place the ground lead of scope to battery positive and use the test lead to check for ground control that way? The thought had occurred during testing, but I wasn't sure how good of an idea it was. I realize on common ground scopes, connecting things up like that would rule out taking measurements on any additional channels,
I understand why you would want to test the ground that way. However, it is not necessary. Testing from negative to negative with the circuit ON (can't understate how important it is to have the circuit completed) is voltage drop testing, and widely accepted as the most accurate way to test circuits. At least in the automotive field. If there is a poor ground, this will show as higher voltage on the scope, open ground will be battery voltage. Reversing the leads may get you mixed up. Once you get used to seeing a bad ground, you won't find the need to reverse the leads, or to "remove and clean the grounds" as part of the diagnostic process like so many are apt to do.
I probed #4 control wire at the PCM and got the same result that I did at the ignition coil, unfortunately, I'll have to look into possibly getting the PCM serviced or just get a replacement
I noticed over by #4 cylinder today evidence of a coolant leak coming from the overhead reservoir. Got me wondering if that may have played a part in frying things.
Anyway, thank you very much again for the help with diagnosing. Though I'm bummed about the bad news. it was nice to get some practical experience in using my scope finally,
