At at 155k, the coils on my 5.4L triton started going bad, and I've been replacing them one by one, using OBD II Mode $06 to look at misfire counts. The current troublemaker however is being real stubborn. It misfires a lot, but never continuously.

Now, From what I understand, Mode $06 pulls the data from the last monitor test, not from continuous testing. So my process has been to drive around with the scan tool connected, force as much misfiring as possible by accelerating lightly up a hill, then quick clear the codes while the misfiring is going on. This forces the misfire monitor so reset. Then I can go over to mode $06, and look at the misfire counts which, if I'm lucky will show a number of counts on one or more cylinders. Done this several times, and it's worked.

I originally thought just checking the misfire counts in mode $06 any time would show something because I thought the misfire monitor is always checking for misfires. But that doesn't appear to be the case. Unless I clear the codes, and force a reset while actively misfiring, I don't get any new misfire information from mode 06, despite the fact that it may have been missing intermittently for months.

So my question is, do I have it right that the misfire data you get from Mode 06 is only that from the last code/monitor reset? Because I've been driving around for several days now with the computer hooked up, trying to catch the thing missing, and have got nothing so far.

 

What model year?

If it's the 97 Expy in your profile, the following applies:

The misfire monitor is continuous, but, before it will run, the PCM must learn its profile correction factors after a KAM reset.

If you don't have it, get a copy of the applicable Ford OBDII manual available at www.motorcraftservice.com

 

Your statements about Mode 06 testing are different from my understanding of how Mode 06 testing is done. Check out this article for a good description of how to use Mode 06 to test for Ford misfires that do not set a code. http://www.motor.com/magazine/pdfs/072011_09.pdf

 

Does your scanner have a trap function?
If yes, use it.
It will tell you the cylinder doing the dirty deed.
The reason you won't see a code is because the fault is not a hard fault but dependent on the EGR being open.
When this conditoin exists the mixture is very lean such that one coil has low output and cannot fire that cylinder reliably.
It is detected by the crank sensor seeing the rotation time of the crank for that cylinder is to slow as compaired to the normal average for all the cylinders.
Sometimes mode 6 helps, sometime it dosen't.
Good luck.

 

Yes, it's a 97 Expy, 5.4L. I'm using Alex Peper's "Car Code" scan tool. I have the full factory service manuals, vol 1 and 2, and a Ford Motor Co doc "1997 MY OBD System Operation Summary for Gasoline Engines". Is this the Ford OBD II manual you speak of?

Agree, no codes are set on an intermittent misfire. A DTC is only set on a complete coil failure, had that once. MIL flashed as expected. Code pointed to bad cylinder, no problem.

I agree that the way mode 06 on misfires seems to work with my vehicle & scan tool is not what I'd expect. Also read that the crank sensor re-learn has to be complete for misfire monitor to work. I'll make sure I do that.

Thanks for the link to that article, will read it.

I think #7, 8 and 3 are the only coils that are originals thus far, but not ruling out one of the aftermarket ones are bad. I could try swapping the suspects out with a known good, but guessing never works for me, I'd rather know which one is bad before going under the hood.

 

Also read that fuel tank can't be less than 15% full or misfire monitor won't run. Was very close to that last time I tried.