DB9 Misfire Monitor

DB9 Misfire Monitor

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Discussion

NoGrip

Original Poster:

31 posts

19 months

Sunday 6th October 2019
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I inherited my Father's 05 DB9 last year with only 16500 miles on the clock. The engine had a terrible misfire / hesitation which I reckoned was failing coils. Reading on this forum, I suspected the misfire monitor hadn't been enabled, so I took her out and managed to enable on the second rundown using a Foxwell NT520pro. This let the dash light up with the emmisions warning and let me see that three cylinders had a large number of misfires.

Roll on a few months, with 12 new Beru coil packs, NGK plugs, PCV valves & air filters, the engine is running very well. However, I can feel some popping at idle from the N/S exhaust, but no matter what I have tried I cannot get the misfire monitor to enable again. I have disconnected the battery twice and then cleared all fault codes, but still after about six attempts at the rundown sequence, the misfire monitor will still not enable. The rundown sequence that I am using is:

No aircon, infact whole air system set to off.
6th gear in Drive
Not in Sport mode
Cruise at 70+ and dropping to 20mph (no brakes / straight line)
Repeat another 4 times

Is there something else that I could be overlooking? Or do I just ignore, drive on and enjoy the experience smile. Any help much appreciated!

Ken Figenus

4,779 posts

72 months

Sunday 6th October 2019
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Misfire fuel hitting the manifold cats could be a big worry so I would bite the bullet and take it to an indie that has the AMDS system to diagnose.

NoGrip

Original Poster:

31 posts

19 months

Sunday 6th October 2019
quotequote all
Hi Ken, I totally agree about the cats and that's what has me wound up.

I'm feeling a bit confused (maybe because I was up in the middle of the night trying to relearn the parameters) but I have just been searching through the OBDII reader and found a parameter '36-1 profile learned' and it says YES. A quick Google of that says that this is the misfire profile, but it is definitely not the same 'misfire corrections' parameter that we watched change to 'enabled' during the first rundown sequence which immediately activated the emmision warnings for the first time ever.

I can now see the misfires per cylinder and #10 stands out with a high misfire rate. It is worse when cold but still only ~0.6% misfire dropping to the 0.0X% range when at normal temperature. Hopefully if it does go above the 1.4% threshold it will light the dash up. No biggy stripping the engine down again but I need to figure out which component is at fault.

Ken Figenus

4,779 posts

72 months

Sunday 6th October 2019
quotequote all
I have a Foxwell too - good to know it is so capable as I'm around the scheduled time for plugs and coils, but she runs sweet as a nut so no need yet to do plugs at 50,000 miles - 20,000 left in 'em!

NoGrip

Original Poster:

31 posts

19 months

Sunday 6th October 2019
quotequote all
The thing that threw me this time was I was looking at the parameter 'Misfire Monitor' which was stuck at 'Not Enabled'. I'm 95% sure that this was the one that changed to 'Enabled' during the pre-coil change but now will not change to 'Enabled'. Very possibly during the coast down procedures the '36-1 profile learned' parameter had updated without me knowing and let me read the misfire counts. The '36-1' just didn't register as being the one banghead

NoGrip

Original Poster:

31 posts

19 months

Monday 7th October 2019
quotequote all
I have spent today going through the OBDII reader and hopefully this will be a useful summary for whoever may need it.

Firstly, using the Aston Martin software on the Foxwell NT520pro, screen by screen, to check that the Misfire Corrections have been learned using the 'coast down' procedure.






This is where I appeared to go wrong. Watching for the 'Misfire Monitor' to become enabled. It never did!





Then I found this parameter!




Success!




Now the misfire readings are unlocked smile


Checking the misfire readings using the generic OBDII read function:









$80 Test / TOTAL engine misfire and catalyst damage rate (updated every 200 revs?)



$81 Test / TOTAL engine misfire and emission threshold rate (updated every 1000 revs?)



$82 Test / HIGHEST engine catalyst damage misfire and catalyst damage misfire rate



$83 Test / HIGHEST engine emission threshold misfire and emission threshold misfire rate

I can't explain why this is zero, unless is it calculated using a different sample rateconfused



$84 Test / Inferred catalyst mid-bed temperature



You can also look at individual cylinders. #9 for example:



EWMA - Misfire counts (average) for last 10 drive cycles. 0.00% in this case (sorry, no screen shot!)


Misfire Counts Last / Misfires on this cylinder during last drive cycle (3 miles)




$80 Test / Cylinder misfire rate and catalyst damage rate



$81 Test / Cylinder misfire rate and emission threshold rate



So the new coils and plugs appear to be doing the trick and are well under the misfire thresholds. However, with these readings I can still feel some popping from the exhausts at idle.

Hopefully I have pulled the information together correctly for this, but please do tell me if I have anything wrong.

Gavin

gaussianmist

37 posts

24 months

Monday 7th October 2019
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You might want to communicate these findings to Aston1936 who is the DB9 and Foxwell maven.