Lean misfire – Have you checked the coils?

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Fat Freddy
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Lean misfire – Have you checked the coils?

Post by Fat Freddy » Mon Jun 24, 2013 4:15 am

I was presented with this 1991 6cyl E36 325i BMW which had an ongoing problem for the last two years and it seemed unrepairable. The complaint was lack of power, hesitation and the occasional cutting out on acceleration once warm.
First up was the test drive to confirm the fault. Sure enough, once warm, as I accelerated it bucked badly. A massive flat spot that was hard to drive through. This was also accompanied by what sounded like a lean backfire.
To me it appeared a definite lean fuel issue. And having six individual coils, the ignition system seemed an unlikely candidate.

Could this be easy pickings? A faulty MAF sensor was my first thoughts. But how could it be missed by so many? A quick scan revealed no codes to be logged so out with the Picoscope. My first choices to scope and the easiest to access, was the MAF, TPS and O2 sensor. This would help prove or disprove the MAF sensor to be at fault.

The results were not as expected. The MAF voltage did not appear to be unusually low but it was defiantly going lean on acceleration. I also noted at other times it appeared to stay in closed loop during WOT.
I.gif
MAF. TPS and Oxygen sensor
Ok, time to check fuel delivery. A quick volume and pressure test again gave no cause for concern.
Next thought was to do the same scope test but this time access the injector loom so I could observe the pulse width to confirm whether the ECU was trying to add fuel or not.
Bingo! Now I knew why I had a lean miss and flat spot. The injection was being cut. Totally. A quick set up change proved it was not losing its power supply to the injectors.
II.gif
MAF Oxygen sensor and Injector volts and Amps
I next decided to check to see what was happening to the ignition. That also had problems, it was either totally missing or very erratic along with some double stacking on the coils.
This had to be some poor output from a crank or cam sensor. Nope, perfect.
III.gif
Cam Crank Coil and Injector Amps
Perhaps the ECU? So I pulled out the ECU and checked the board. It did have evidence of water damage, but it was very minimal. Could this be the cause of all these symptoms? It was at this point the customer called to get the verdict. I told him what I had found and at the moment my thoughts were the ECU, but I wanted a bit more time to think about all the evidence. He then told me that BMW had also noticed the water damage and wanted to replace the ECU for $3000 but no guarantees’ of a fix. Also a $800 injector clean had been carried out and a substitute new MAF sensor had also been tried. All resulting in no improvement.
I then went back over my captures to look for some sort of a clue. Occasionally the poor ignition control seemed to have an odd regularity to it. I decided to check which coils were actually apart of this irregular regularity. Out of the front three coils, cylinder one followed by two always fired. Along with another two coils from the rear, this was the area of coil stacking. A clue perhaps.
Number two along with one of the rear coils seemed to fire last in this regular area of poor ignition control. So this time I connected up to the primary of number two to see if it revealed anything else.
IV.gif
TPS Injector and Coil Amps Number 2 Coil primary
Immediately, I could see a problem. When the ECU shut down injection and ignition it was always preceding number two coil firing. Jackpot! I also proved this further by disconnecting number two coil. The vehicle now ran as expected apart from the single cylinder misfire.

The coil was replaced and the fault fixed.

Altough only a coil was the fix. It certainly did not behave as a simple bad coil problem. Six other places had a crack at this and he had been charged a total of $1900 for no fix.
The car was handed back over to a very happy customer.


Try finding that without a Picoscope.

Valhalla
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Re: Lean misfire – Have you checked the coils?

Post by Valhalla » Mon Jun 24, 2013 10:41 pm

I'm impressed. I would have never thought of checking for coil failure with a lean misfire - quite the opposite. It would seem a bit premature of BMW to shut a cylinder down so fast on a coil diagnostic, you would have though a few cycles of injection would not have killed the catalyst outright, and it would have given the coil a fighting chance of redeeming itself. Much better if they had logged an intermittent misfire on cylinder 2....... :roll:

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Fat Freddy
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Re: Lean misfire – Have you checked the coils?

Post by Fat Freddy » Tue Jun 25, 2013 1:09 am

Technically it wasn’t a lean misfire but a complete shutdown. But the symptoms did a very good job of imitating it. Hence the new MAF and injector clean.
The big dumps of fuel it got when injection started again probably helped to give the lean sounding back fire.

My thoughts are the coil was sending enough back to the ECU to reset it which may of attributed to the lack of codes. Once it came back on line it was trying to re sync it’s self.
You can see that in image two where it’s stacking the coils but when number two fires (shown by it's relation to the cam) a bad shot, its down again.
Just my thoughts.

I did also find out why possibly it was staying in closed loop during WOT. A performance chip was fitted. Although that doesn’t make total sense either.

FF

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Re: Lean misfire – Have you checked the coils?

Post by AuthorMike » Wed Jun 26, 2013 2:46 pm

Great Case Study. Good entry for the August Competition!
Mike Valentine
Technical Author @ Pico Technology UK

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Re: Lean misfire – Have you checked the coils?

Post by edagrade » Sat Aug 31, 2013 4:17 am

Great find and write up - thank you for sharing :)

For such driveability issues, looking at components individually without seeing them dynamically along with related components often provides little if any diagnostic value.
Your technique of sampling as much related information as possible enabled you to observe irregularities and then home in on them. Great skills :!:

Similar symptoms along with other weird driveability issues are common on B series 4.0L Ford Falcons - the cause often is bad coils.

Eddie Worsnop
A Grade Mechanical Services

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Fat Freddy
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Re: Lean misfire – Have you checked the coils?

Post by Fat Freddy » Wed Sep 04, 2013 11:12 am

Thanks guys for taking the time to read it. Defiantly a tricky one.
Ironically over the two years this guy had the fault, two coils had been replaced. Just the wrong ones.

Anyway through a PM I have been asked by a Nerd for the psd data files so it can be discussed on the Autonerds forum.
I think since Pico have generously run the competition the files should be posted here for any discussion.

After looking back over the files (last two mainly) my ECU reset theory looks plausible. Once it starts to play up, all coils are fired every revolution. Once they return to the correct order the injectors are turned back on. Again, just a theory but other thoughts welcome.

FF
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xandro
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Re: Lean misfire – Have you checked the coils?

Post by xandro » Sun Mar 27, 2016 2:16 am

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Fat Freddy
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Re: Lean misfire – Have you checked the coils?

Post by Fat Freddy » Wed Mar 30, 2016 10:08 am

here friend I got a defective handa civic 1.7 16v so strong faulting at a time standard coil and other bad cop and the vehicle was horrible eat testing by ckp guns cmp probe o2 until you reach the ignition coil sec cil measuring at secondary, I had Alimentaçao pulse of central and absence of inductive shooting and burning time. I changed the coil with a new and resolved defect, look there in the picture lack of inductive kick the absence of clearly visible burn the fault line here in Brazil still are few repairmen who work using the scop for accurate diagnosis, this defect I'm showing to friends for many and a very difficult defect but for us to study the scop waveforms ends up being easy to solve defects in eletroinjetores coils ecu with supply problems posit missing ground engine steps and many other items in the vehicle that where without study and use of scop it would be much more difficult to repair, I hope here be able to share and learn but with you, hug to all forum and the opportunity of being here with you
Thank you and welcome xandro :D

It's very hard in today's world to make an accurate diagnosis with out the scope. No scope often leads to parts being fitted incorrectly in the hope of getting lucky.
The other part is having the willingness to learn how to read what you see, which you have done here.
Anyway thanks for posting.

Cheers
FF

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Re: Lean misfire – Have you checked the coils?

Post by RYM6746 » Mon May 08, 2017 6:53 pm

Sounds like this case study matches your description.
https://youtu.be/kDcwbNZuLRw
https://youtu.be/wDpLuSfNlvo

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