DB9 Misfire Monitor

[NoGrip]

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 . Any help much appreciated!

[NoGrip]

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.

[NoGrip]

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

[NoGrip]

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

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 rate

$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

[NoGrip]

Hi Paul, good to know thank you. I found I was forgetting how to navigate my way to the misfire counts after a while and I often refer back to this myself

Since I did this I have been hearing more talk of the damaging Type-A and less damaging Type-B misfires. Does anyone know if I am right in suggesting that Type-A is represented by the $80 test and that Type-B is the $81 test?

After plugs & coils I was getting some misfires but every time I checked they were on random cylinders. I will be running further tests in the coming weeks to see if I can distinguish if they are Type A or Bs and then decide if I am going to do a primary decat.

[NoGrip]

paulrog1 said:

He told me the next time I replace the sparkplugs fit a set of 2010 DBS plugs as the car will have some extra performance, around 25bhp which those sparkplugs are better suited for.

Good info for the decat!

[NoGrip]

I’m back to address the misfires on the DB9: New coils, plugs & PCV valves fitted, but low-level idle misfires remain.

I connected the Foxwell NT520 and looked at the fuel trims at various rpms. Seem fine above idle, but at idle the long term fuel trim on the LH bank is +7% and the RH bank +5%. I did a smoke test on the vacuum side and all good, but before the throttle bodies I discovered an air leak around the LH MAF. The MAF was loose and I discovered the plastic threads of the ducting had been pulled out. Putting the MAFs under the microscope, the RH one has contamination which coincides with slightly more pull-over of oil from the PCV system on that side.

So I have removed the ducting and air boxes for cleaning. 2 x throttle bodies also cleaned. Two new MAFS and air filters are on order, so I will let you know the results once fitted.