ODB-II - How does it sense a misfire?

[duke16]

How does the ODB-II system know which cylinder is misfiring? Reason I ask is because I'm getting a misfire on cylinder 8, but everything looks fine on that cylinder. So I'm wondering if when my engine was pulled/rebuilt that when it was all put back in, if something was not put back in the correct spot. What I'm getting at is, is there a sensor for each cylinder that could potentially be put on the wrong cylinder, causing the ODB-II system to tell me the wrong cylinder is misfiring?

 

[MountaineerGreen]

I can't answer your question, but do check your plug wire there- the headers tend to cook the boot or wire, its very tight in there.

 

[duke16]

I can't answer your question, but do check your plug wire there- the headers tend to cook the boot or wire, its very tight in there.

I know this all to well, I've gone through several sets of wires since having the TM headers. But this time, I've gone through each wire several times and can't find a single problem with them.

The strange thing is, I had just replaced a cooked wire on cylinder 4 and then as soon as I get my header gasket which had formed a leak from all the backfiring replaced, another misfire occurs on the other side of the engine.

 

[MountaineerGreen]

I know this all to well, I've gone through several sets of wires since having the TM headers. But this time, I've gone through each wire several times and can't find a single problem with them.

The strange thing is, I had just replaced a cooked wire on cylinder 4 and then as soon as I get my header gasket which had formed a leak from all the backfiring replaced, another misfire occurs on the other side of the engine.

I just replaced my wire set- I tried Taylors, $$$$ wires, they lasted 1/2 the time of cheap wires. To see the fault in the taylors, I took off the wires "rolled" the boot around, cracks showed in the insulating jacket or boot. I have also pulled the rubber flaps had someone put it in gear, hold the brake and load the engine, in a dark area you can see the arcing clearly. #4 and #8 seem to be my most common wires to fail.

 

[rwenzing]

How does the ODB-II system know which cylinder is misfiring?

There is a 36-1 tooth wheel on the harmonic balancer and a Crankshaft Position Sensor (CKP) that counts the teeth. "36 minus 1" means that the teeth are spaced at 10 degrees from each other (like a 36-tooth wheel) but there is one missing tooth yielding a total of 35. The PCM uses that single larger gap to recognize TDC of cylinders #1 and #6 to synch up its timing with the mechanicals during cranking.

Using the CKP, the PCM knows when each pair of cylinders are together at TDC or any other crank position. The Camshaft Position Sensor (CMP) turns at half crank RPM and enables the PCM to determine which of the cylinder pair is on a particular stroke of the 4-cycle sequence - for example, which is on the compression stroke and which is on the exhaust stroke. Using the CKP and CMP together enables the PCM to time spark and fuel events to crank and camshaft events.

Misfire is detected by computing the acceleration and deceleration of the crank from the time difference between the CKP sensor pulses that signal the passage of the teeth. Misfire on a specific cylinder causes a momentary deceleration of the crank during the 1/4 revolution (9-teeth) that correspond to that particular cylinder.

So, back to your question: The only remote possibility is that the CMP / Synchronizer is installed something like 180 degrees off. In that case, the PCM would reverse all of the cylinder pairs and misidentify cylinder #2 as cylinder #8.

Having said all that, I do agree with the others above. If you have a 5.0 with Torque Monsters, the #8 wire is the first suspect.

 

[duke16]

Excellent info.. thanks..

Unfortunately, even after swapping out the wires for 2 and 8 the problem still exists.

 

[Turdle]

Excellent info.. thanks..

Unfortunately, even after swapping out the wires for 2 and 8 the problem still exists.

Have you tried swapping the coil packs for each other?

 

[duke16]

Have you tried swapping the coil packs for each other?

Yup, misfire stayed at 8.

 

[Turdle]

Ok, next question, getting deeper--

Roller rockers?

 

[duke16]

Ok, next question, getting deeper--

Roller rockers?

Questions over my head... I know when the engine was rebuilt new rockers were installed (HyLift Johnson rockers). That's about all I know though.

By the way, compression looks fine on the cylinders I managed to get to before the battery died.

#1 135psi
#2 155psi
#3
#4 150psi (this is where I had to replace my last wire, leading me to believe I had burnt a valve... but the compression seems fine)
#5 135psi
#6 135psi
#7
#8 155psi

Also, all plugs look good and are gaped @ 0.054ish.

One additional piece of information I just got was a pending code misfire on cylinder 1. That's the second time I have gotten that. The last time it happened, the pending code for it disappeared and just left the misfire on cylinder 8. But I have consistently (no matter what I do) been getting a misfire CEL on 8.

 

[duke16]

Well looks I found the problem. Damaged spark plug. The little white tip had come loose some how and was rattling up and down. No idea how I didn't notice that until now.

 

[Turdle]

Ha ha--
seems like we all overlook the obvious at one time or another.

Phew huh?

 

[TekMan05]

Im having the same problem too. Me and my friends replaced the wires yesterday annd I believe I cooked one because of the burning smell and ROUGH idle. BUT... No CEL?? That's very strange. I still have an exhaust leak somewhere also though. This is very strange.

 

[duke16]

Im having the same problem too. Me and my friends replaced the wires yesterday annd I believe I cooked one because of the burning smell and ROUGH idle. BUT... No CEL?? That's very strange. I still have an exhaust leak somewhere also though. This is very strange.

Check to see if you have any pending codes. They won't turn the CEL on, and may come and go with each drive cycle. But they may help narrow the problem down.