Question about CEL codes and how they're triggered

Modoc Guy

Modoc Guy

Jack of all trades, master of none
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I’ve been chasing random misfire codes for a few months now. Since the Jeep’s new to me, I had to use common sense, found a couple vacuum leaks, and forum knowledge, clean the idle valve, run the right spark plugs, clean the throttle body, and that’s all helped.
But I still get random misfire codes.

Now my question is, does anyone know what the computer monitors for it to know that the engine is misfiring?
Feedback from the coil pack, a rich mixture from the O2 sensors, what thing, or things are telling it a misfire is occurring?

It’s not a hard code anymore, but just an occasional flashing light either after semi hard acceleration, or freeway speeds.

So I think if I understand what it’s seeing, I might better understand what the cause is.

I was guessing maybe a dying coil pack or an injector or two is acting funky.

Any thoughts?
 
I believe it is sensing changes in crankshaft speed at the crankshaft position sensor. If the timing is slower than expected at a certain pass of the sensor, it can map that to a cylinder that should have just fired, but didn't.

Occasional misfires can often develop into major ones that will put your engine into limp mode, so it's best to try and diagnose it before it gets to that point.

What specific code are you getting? If it's consistently one or two cylinders, it will tell you which ones.
 
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Before I did the work I outlined, it threw misfire codes on various cylinders. Hard codes, ones I had to clear. No one cylinder regularly.
As I fixed/cleaned and replaced the TPI, the hard misfire codes stopped.

Now, it will flash the CEL occasionally, either at freeway speeds, 2600-2700 rpm’s, or after a medium-hard acceleration.
And not consistently either. I just had it on two trips of 260 miles and 325 miles respectively, and no issues.

I drove to town yesterday, a 20 mile round trip, and it flashed the CEL at me both ways. Briefly, maybe for half a minute each. And when it does it, it won’t hold a code.
It’ll read done when I check.

I’ve been slowly replacing sensors, TPI was first, MAP is next, to try and fix its surges when accelerating. I’ve also got a heat shielding kit for the injectors to make sure it’s not a heat soak issue, but I’m just throwing parts at it more or less.

I’ve thought about doing the CPS, and even the OPDA too, but that’s just expensive guess work. I was hoping others on the forum might have had similar problems and could help diagnosing the real problem.

BTW, I have a Viper coil kit to replace the coil. Don’t know if the coil’s bad but it couldn’t hurt.

Thanks for your reply Steel City 06! Always like learning new Jeep info.
 
My recent misfire ended up being the catalytic converters. One of the pre-cats self-gutted and plugged the 3rd cat, causing misfires on all cylinders, especially at high throttle and high RPM.

I also was having issues with carbon fouling, but since switching to exclusively Top Tier gas brands, I haven't had any more carbon issues. Before I was just using whatever was cheapest.
 
Steel City 06 said:
My recent misfire ended up being the catalytic converters. One of the pre-cats self-gutted and plugged the 3rd cat, causing misfires on all cylinders, especially at high throttle and high RPM.

I also was having issues with carbon fouling, but since switching to exclusively Top Tier gas brands, I haven't had any more carbon issues. Before I was just using whatever was cheapest.
Click to expand...
Hadn’t considered the cats. I’ll have to think about that.

Wonder if the exhaust is still available from Mopar?
 
dbbd1 said:
Have you checked with a code reader for "pending" codes?
Click to expand...
Unfortunately I don’t have a code reader yet. I used the ignition key trick.

But you bring up a good point, being able to check the computer would be helpful if it shows more info than just the key trick tells me.
I didn’t know about “pending” codes!

I’m shopping for at least a reader, maybe a reader/programmer that would do my TJ and my 06 Dodge Ram diesel.
Guess I better get on with buying something!
 
This is the one I have and then use with OBD Fusion app (on my iPhone) but there is also the torque app (which I tried on an android and did not like as much). But there are a bunch out there. Caution- I have a Cheapo ($50) handheld code reader that missed a code that my $100 reader saw. The dongles do seem to do the trick though.

https://www.amazon.com/gp/product/B073XKQQQW/?tag=wranglerorg-20
 
And, I prefer BT ones instead of wifi units. The wifi units always seem to have connection issues, at least for me.
 
OK, finally got a code reader. Also, an OEM crankshaft sensor in hopes that it was the issue as Steel City 06 had mentioned.

I ran the code reader and found everything good except a pending P0300 for random misfires.I cleared that pending code, then disconnected the battery in preparation for changing the crank sensor.

The next morning I started swapping the sensor out. It looked super simple, and it is, providing you’re a contortionist! 😉

Thankfully, nothing was stuck or broken. And after a couple hours and a few swear words, it was in. I did notice the sensor I pulled out was not OEM, maybe that was the issue.

I’ve driven it to town twice now and so far, no flashing CEL yet!

Thanks again Steel City 06 for the lead on crank sensor!

I plan to continue working at replacing and upgrading stuff since it seems like the PO did little for maintenance and repairs.

There’s nothing more rewarding than having everything in tip top running condition!😸
 
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Most check engine lights are just emissions related and nothing that will hurt the engine but a flashing check engine light means it is a problem that can ruin your cat(s) if you keep driving. So if it does come back then I would definitely get it fixed before driving it more because cats are expensive.
 
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