2005-2009 Mustang S197 Discussion & News

Cylinder 6 Misfire, is this explanation plausible?

Old 5/17/17, 05:40 PM
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Unhappy Cylinder 6 Misfire, is this explanation plausible?

Hey guys, today I went to go start my car and noticed the CEL was on. First time I've had the light so my heart sunk. My car is dyno tuned so I don't have a tuner or scanner, so I headed towards advance. I kept hoping it was just an o2 sensor.

I got to the shop and they scanned the codes, they didn't print it out but there was one for the fan relays (hi and low) which is weird because the fan comes on. So **** that one, I don't care. Another was for something causing a forced shut down. That has happened once or twice, but I just figured it was a little blow by from the supercharger gradually building up on the TB and was planning on cleaning that in the near future. So that's not what I'm worrying about right now. The main thing is Cylinder 6 Misfire.

Now I know misfires typically aren't too hard to fix. I really don't want to get into changing spark plugs because of the reputation these cars have. I bought the car with the blower on it so I'm assuming the plugs have been changed but not 100% sure. There's only 51k on the car so I don't see why coils would be bad but it's an option. That's money though, I don't have a lot of it.

So this is what I'm thinking it is. A few months ago, my car was running a bit lean so I took it to my tuner and we corrected it. Obviously tunes don't change, but I figured maybe I hit the Boost A Pump ****, I don't know how sensitive they are. All was well for a while. WOT is around 11.1-11.3:1. Recently, WOT has been seeing about 13.2:1. I'm thinking the injector is getting clogged and just like before, is starting to get TOO clogged for the amount of fuel it's set to be pushing and is causing the over all reading to be a bit leaner. What really backs up this theory is the fact that my Wideband sensor is on the same header as the #6 Cylinder.

So if this theory sounds logical to anyone else (or sounds stupid), let me know what you think please. If you could recommend somewhere to send my injectors to be cleaned, that would be awesome too.
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Old 5/20/17, 04:31 PM
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My thoughts:
How does the computer know you have a misfire? By noting whether the circuit gets completed. This happens when the electricity passes from the electrode to the strap making and arc or spark. If this spark does not occur the circuit doesn't get completed and the computer goes ding ding ding ding. I'm not sure that it can sense if the gases in the cylinder got ignited or not, just if there was a spark or not. With that thought I would think a misfire code would relate to a spark/ignition issue as opposed to a fuel issue. BUT, I could be completely wrong.

Swap the coils around and see if the code follows it. If it stays with cylinder 6 then swap the injector positions and then see if it follows. if it still stays at 6 then check/swap the plugs and see if the code follows. This would be the cheapest approach to diagnosing it at this point. Nothing needed unless you have issues with the plugs. But I think you should deal with the plugs sooner or later. Do you even know what kind of plugs are in there? Maybe you already have 1-piece plugs in there and don't know it. Either way I would stay off the throttle until you start running a better AF ratio. At least don't be flooring it with a load on the motor.

And for the future if you stay with a two piece plug you should pull them every once in a while to check them. Just a good habit to get into and you can tell a lot from reading spark plugs. Even if you switch to a 1-piece Brisk. Either way the checking and cleaning of plugs regularly will prevent you from ever having to deal with the carbon build up that jams these things in the heads and breaks them off. I pull my plugs 2-3 times a year, part of the old dog in me.

Last edited by 07 Boss; 5/20/17 at 04:35 PM.
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Old 5/20/17, 05:56 PM
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Thanks for the awesome reply. There is a very very small amount of metal shavings in the oil. You need a magnifying glass and an LED to see them, but they're there. That's why I don't go ahead and do the whole swapping coils, injectors, and plugs thing. I was planning on draining the oil and sending it in for analysis and then doing a few flushes with new sets of oil and then finishing with a slightly heavier weight to leave in there (5w-30?). I may just go ahead and do that sooner though so I can check the different possibilities.
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Old 5/21/17, 04:03 PM
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actually, they *might* read misfire by current, but if so, im not sure...i do know from reading up on it, they detect misfire by torsional windup in the crankshaft- the crant trigger wheel is read so amazingly fast it can actually read the amount of twist imparted due to time from tooth-to-tooth... ive got a pdf of how it works somewhere, pretty cool stuff. i ran into it after a battery swap, highway coast down and a misfiring engine that wouldnt run after coasting down from 65 mph... the ford literature explains how the pcm calibrates itself for machining variation in the starwheel, by reading the timing when coasting down in gear from 65-45 at zero throttle, and abs sensors are read to kinda verify condition of road being bumpy or not... in my case, it took a bad read on a bumpy off ramp and would barely run. limped home, did some reading, was easy- just unhook battery to erase the learned part of it, do the highway coast down on a smooth road, perfectly fine ever since
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