2009 Jeep Patriot Intermittent Misfires FIXED!

Hi all,

I am hoping for a little more direction on this frustrating vehicle. It should be a simple diagnosis but obviously this vehicle has other ideas.

2009 Jeep Patriot 2.4.

Vehicle came in with a P0300. Bad hesitation under light or full throttle. Instant Misfire counts on #4 (over 100) using a Snap On Verus data PID 'Which Cylinder is Misfiring'. There were also several on misfire counts #3. These misfires could be duplicated in bay while brake torquing and while driving.

The Fuel trims were within 10% during misfires so i am not expecting a fuel delivery issue. Pulled plugs and the gaps were at 80 thou and the spec is 43 thou.

I scoped the #4 coil while brake torquing in the bay and it had a weird waveform. I up sold plugs as they were worn out (replaced with NGK's OEM replacement according to Alldata) and we up sold 2 coils for #3 and #4 cylinders.

The vehicle was test driven by me and another tech and it was running great.

The customer picks the Jeep up after a week. 30 miles away they call back complaining of a flashing SES. So obviously they bring the Jeep back.

Now we have a code for P0304. So, i swap #4 coil with #1 as a quick swap-tronics check in the hopes we had a faulty aftermarket coil. This is where it starts to get interesting....

I test drive the Jeep while looking at misfire counters. It again hesitates but now we have severe misfires on #3 and flags a P0303 (#3 and#4 have new coils). I swap coil #3 with coil #2. Test drive and still counts misfires on #3. So again i swap coil #3 with coil #1. Test drive and for the life of me I can not get the engine to misfire any more.

Before i left work Friday, i had another tech brake torque while i manipulated the injector and ignition wiring harness. No misfire.

It feels like a ignition misfire. I tried to recreate the freeze frame data for the #4 misfire. It happens above 184F.

We can rule out fuel as the trims are always within 10%. We can rule out a mechanical issue as it would be always consistent (same with an injector issue). I would assume if it was a wiring issue to the coils, it would flag a ignition coil circuit code. I thought a potential excessive AC ripple from the Alt but this would be effecting the VSS and speedometer.

I am trying to get a game plan together and some fresh ideas so any input would be great.

Hopefully the images work that i attach.

Hey sir, thanks for including your testing up to this point, absolutely love it.

I'm wondering if the 'Which Cylinder Is Misfiring' Functional Test is lying to you? I question it because of this:

Now we have a code for P0304. So, i swap #4 coil with #1 as a quick swap-tronics check in the hopes we had a faulty aftermarket coil. This is where it starts to get interesting....

I test drive the Jeep while looking at misfire counters. It again hesitates but now we have severe misfires on #3 and flags a P0303 (#3 and#4 have new coils). I swap coil #3 with coil #2. Test drive and still counts misfires on #3. So again i swap coil #3 with coil #1. Test drive and for the life of me I can not get the engine to misfire any more.

I kept track of your swaps on a sheet of paper. I labeled the cylinder positions #1-4, and the coils A-



If I did this correctly, then I think coil 'D' is suspect. Whenever it moved, the misfire changed. Maybe not to the cylinder that D was on at the time, but coil D was always involved in a misfire count change. That one was originally on #4, and one of your new ones. Did I track that right? :lol:

Dunno if that'll help or not, but my money is on coil D at this point. Anyway, if this were my headache, I'd probably start out by manually checking spark strength out of all coils with a test light. Scoping the coil primary might be good, too, but I feel like the spark test will expose a weak coil faster.

If you can get it to miss again, then it might be good to watch your coil current ramps on the scope during the miss. The coils won't look the same, of course, due to the two new ones in the mix. BUT, if you can get it to miss and still see consistently strong ramps, then that'll clear up any concerns you might have about wiring.

Yes, you were right on keeping track. I also just did the same with some paper. Hopefully tomorrow it will misfire or be magically fixed. What is your thoughts on the current ramp on the waveform i attached in the original post? I have never seen anything like that after the current from the coil was discharged. That capture was taken on the initial P0300. It was very consistent.

Also the firing order is 1-3-4-2 which could be a clue of the faulty coil. I have seen other vehicles register misfires for the wrong cylinders before.

DaleB wrote: Yes, you were right on keeping track. I also just did the same with some paper. Hopefully tomorrow it will misfire or be magically fixed. What is your thoughts on the current ramp on the waveform i attached in the original post? I have never seen anything like that after the current from the coil was discharged. This capture was taken on the initial P0300. It was very consistent.

I did see that! Sorry that I forgot to comment on that one. Well spotted.

That looks like a textbook open/high resistance secondary circuit to me. Paul has some great examples of this in Section 22 of his book.

With the firing order too, that D coil was always a factor which seemed to follow the firing order if that makes sense?

DaleB wrote: With the firing order too, that D coil was always a factor which seemed to follow the firing order if that makes sense?

Exactly what I was thinking. Why it quit misfiring on the last swap-a-round... I got nothin'. :lol:

Well i thought id update the thread.

The vehicle has been gone for over 2 weeks now with no issues. So we can call this one a fix! It was a bad new coil out of the box, coil D as shown in the pictures above.

DaleB wrote: Well i thought id update the thread.

The vehicle has been gone for over 2 weeks now with no issues. So we can call this one a fix! It was a bad new coil out of the box, coil D as shown in the pictures above.

Awesome! :woohoo: It's always the brand new parts that can give you the most trouble.

What brand was the coil, if I may ask?

It was a Napa boxed coil. The only time we seem to use OEM coils are on all the European vehicles we service.

DaleB wrote: It was a Napa boxed coil. The only time we seem to use OEM coils are on all the European vehicles we service.

Nah that's cool. We rarely use OE coils, either. Parts are such a problem these days, always good to know which ones work in which applications.

Thanks for updating this one, sir! I've been curious how this one turned out.

This worked on mine after moving them all around to find the bad one Thanks guys >>>>>>>07 jeep patriot
I also started with what my ck engine light read ...

GirlsCanDoIt2 wrote: This worked on mine after moving them all around to find the bad one Thanks guys >>>>>>>07 jeep patriot
I also started with what my ck engine light read ...

No problem! Thanks for using the search function. Super glad to hear this thread was useful for you.