Hello all,

New to the forum, been lurking the last several months, and need some help now.

First, a bit of background about me: I am a master mechanic / diag tech by trade, and specialize in classic Euro and Japanese cars. I have access to numerous scanners/diag tools at my day job, but sadly not AMDS.

Now for my issue:

I own a 2008 DB9, that I purchased with 16k miles. Within the first 850 miles, I was getting the Emissions warning with P0301, P0303, P0316, and occasionally P0309 and P0312. This was only happening on cold days (sub 40F) I cleared the codes, and reset my misfire corrections once spring hit, and it was smooth sailing for the next 4000 miles without a single hiccup until about 3 weeks ago. I had a couple instances of lumpy idle, and noticed the car seemed intermittently down on power accelerating at highway speeds, and audible misfires under low speed acceleration in town. No CEL or emissions system warning.

I took the car to work, and plugged in to my Autel MaxiSys to review whatever data I could. I determined via data that the car was misfiring on 1,3,9, and 12. So the following weekend, I replaced plugs, coil packs, PCVs, checked injectors and replaced o-rings, etc. and reset misfire corrections. As of that Saturday/Sunday, all was well, and the car ran absolutely flawless.

However, the issues came back on my lunch break that Monday no lumpy idle, but the audible misfires and hesitation returned. I took a look at the data again, and same exact cylinders misfiring as before.

I have also noticed a couple odd phenomena with the misfires, after replacing the parts. On first start from cold, the issues are much worse. However, if I shut the car off for a few minutes after driving at operating temp and then restart, the misfires are much less or nonexistent. I have also noticed that the misfires are far more prominent on hotter days (above 80F). The other day we had a nice 65F day, and the car ran absolutely fantastic all day.

Is there anything here I could be missing? When I look over the data, everything appears to be okay (less the misfires).

Any input is greatly appreciated, as I’m a bit stumped at this moment, and need this thing up to par ASAP.

 

I've seen people here struggle with chasing misfires thinking it was ignition related but it was fuel. Either the fuel pressure regulator or one of the many magical emissions related devices in the fuel tank.

However, something to consider if you're using Chinese ignition coils. I know plenty of people use them but I've had bad luck with them. One failure after 20 miles and another after 800 miles so I switched to using the genuine coils and haven't had any issues since.

Since you're misfiring on the same cylinders as before it can't be a coincidence.

Have you read the diagnostics book? Misfire is page 100.

 

I am using Chinese coils, against my better judgment. But given the same cylinders are misfiring as before with the OE coils, I tend to think the issue wasn’t coil related at all.

I have read the book, and that’s kind of why I’m stumped. I haven’t measured the pulse width or flow rate of the injectors, but the resistances were perfectly in spec.

I plan to go for a drive and monitor data on my break Monday. Just trying to rule out whatever I can, as the ‘typical’ culprits don’t seem to be the cause of my issue

 

Coils, injectors, MAF, fuel, vacuum leaks, and O2 sensors play heavily into misfires on the AM V12. I *think* I've finally got my misfires sorted after sorting *all* of those things. If you've already done coils and injectors, the next thing I would look at is fuel. The pumps (there are two) can bypass fuel through an internal regulator that can fail. There are a few threads on here about this issue. Monitor fuel pressure at idle and under load to make sure you're seeing stable pressure with no dropouts especially after key-off. Get yourself a scan tool that can view both ECUs of the car. Watch all eights o2 sensors of all four banks. Each half of the engine is two banks. If any don't look right, just bite the bullet and replace them all. Don't believe anyone that says the Ford DY1401 sensor will work in any position, they wont. I replaced all of mine and the car ran like garbage. Rich and incredibly rough as soon as the ECU transitioned from open to closed loop until it was fully warmed up. The ECUs complained of stuck lean conditions while these were installed. I replaced all eight again with the OEM sensors from AMBits at significant cost, and now it runs better than it has in a long, long time. At the end of the day, the V12 is a ford engine with more cylinders. No reason to get mystified by it.

 

Just a thought....What condition is your exhaust components? Cat failure?

 

Cats inspected about 1k miles ago, and looked great

 

Misfires are a "back to basics" discussion.

IC engines need - AIR - FUEL - IGNITION in specific proportions and timing. Any anomalies in any one or more of these can be registered as a misfire.

As stated earlier, so many components contribute directly to the operation of fuel mixture (stoichiometric for gasoline is 14.7:1), ignition source and timing of that ignition. You can focus your diagnosis based on what the EMS is telling you in codes. If you're getting misfires such as P0300s, then some components become suspect over others. With specific misfires such as P0301 - P0312, then you can eliminate a group of suspects that cause general misfiring.

Components such as MAFS, O2 sensors, fuel pumps (fuel pressure) usually cause misfires over all (or most) cylinders (or all cylinders on one bank), Individual misfires require a specific look at the contributors/controllers of Air, Fuel, Ignition at that specific cylinder. A misfire on an individual cylinder can be caused by - ignition coil, spark plug, fuel injector as primary suspects.

Remember too, how misfires are measured by the EMS. Misfires are identified by the crank position sensor. On a V8 a cylinder is on power stroke every 90*, a V12 every 60*. Every power stroke causes a rotation acceleration that the CPS sees and the ECU expects that acceleration to take a known amount of time. If the acceleration doesn't occur, at one of the 90* or 60* segments (takes longer time to get to the next segment) the ECU records a misfire at P03xx cylinder.

I've seen multiple or random misfires caused by a bad CPS.

 

I guess the thing that has me scratching my head, is the lack of codes (it is below the threshold to trigger a CEL), and the level of intermittence. One moment the car is running great, no noticeable signs of misfires. The next (typically a different drive cycle), it will run/drive very poor, with audible misfires and severe hesitation. And vice versa.

The data shows higher misfire counts on 9,3,12,1 (in order of severity) but does show misfires on other cylinders as well, though I tend to attribute the much lower counts to be caused by the speed up/down from a larger/actual misfire on the previous cylinder. With the same cylinders showing higher counts as before, I that points to an injector issue in my mind.

However, I circle back to the part where it’s an intermittent issue. I wouldn’t expect that a coil/plug/injector, or even maf/cps to only cause the problem sometimes.

The only variables I’m seeing that *might* be a contributing factor is ambient air temp and engine heat soak (or lack thereof), which point me in the direction of a fueling issue caused by maf (as IAT is integral) or evap system (increased pressure in tank caused by heat, possibly pushing fuel through the evap lines???) but I’ve yet to gather data or physical evidence to support that theory.

 

IAT reaches a concerning point at about 86*- 90* ambient air temp and above. At that point the ecu begins reducing timing slightly. If the IAT wasn't providing a signal to the ECU that would produce a CEL. Timing is selected from an adjustment table so the ECU does what it does. You shouldn't be able to "feel" heat soak while the ECU isn't saying anything.

On the fuel pressure, I'd say no BC of the purge valves. Their job is a gate to any fuel from excess pressure in the tank, and when the ECU commands the purge valves cycle, it is adjusting fueling to accommodate that.

This is a table from a tune that shows where the ECU begins to pull timing when ambient air temps reach a critical point and where cylinder air mass requires it. This is not the actual spark table, just an adjunct table the ECU uses to adjust what timing is on the spark table at the same given point.

 

If i was in your shoes i would send all of the fuel injectors out to be professionally reconditioned, and seeing the results of the flow rates recorded for pre and post work for each cylinder might tell you what you need to know. If not, well at least they are refreshed and up to par with the other new parts you just replaced.

 

I have yet to check any data, but I took the car out today just to see how it would behave. Leaving my house this morning, it had 1 little stumble then was perfectly smooth the rest of the day. Multiple key offs in the morning, and 2 30+ mile drive cycles.

I’ll be having a more comprehensive look at data tomorrow. Unless I come to a conclusion by the end of the week, I’ll be pulling my injectors and having them flow tested this next weekend.

Thanks everyone for your input thus far, this has me baffled due to how random/intermittent the issue seems to be.

Will keep the thread updated with any/all findings.

 

Had a chance to review some data today.

Fuel Pressure on both banks is stable, staying almost exactly 40psi at idle, and sustained 2500rpm.

Misfire data showed the following for my last trip (8 miles, ~20 minute drive to work)1-30

2-5

3-21

4-10

5-8

6-8

7-55

8-17

9-74 (5 ewma previous)

10-21

11-5

12-13
This no longer correlates directly to before I changed anything, though 1,3,9 were the largest problems before. However, 7 had not shown any misfires at all, and 12 was more problematic on all previous checks.

I also closely reviewed MAF data.

Bank 1 MAF initially ranged from 1.17V at idle, to 1.22V. However, after a blip of the throttle, voltage was very erratic. Ranging from 0.82V-1.72V, and it never stabilized after idling for several more minutes. This certainly seems to be a problem, but I’m not sure if it’s THE problem, as the higher misfire counts are shown on bank 2

Bank 2 MAF ranged from 1.27V at idle, to 1.32V and remained stable after throttle input.

I believe I’m onto something finally, though not convinced it’s the 100% cure.

I was able to get a contact for a local shop that has the means to flow test and clean/rebuild my injectors if necessary.

 

Aston1936 has a YouTube channel and he may have covered this problem. You can email Steve and he is a great guy. It seems there is nothing he doesn’t know about a DB9. Also Mike from Bamford Rose would have some idea because he was one of the engineers that developed the V12 when he worked for AM.
I can’t believe this would make any difference but cleaning the mass flow sensors and new air filters might help. Again this more than likely not make any difference but a new battery might help.

Good luck.

 

Since each cylinder bank is essentially a separate system it would seem you either have separate problems for each bank or there is something common to both banks causing a problem. Somebody mentioned CPS and that would be common to both banks. Seems like that would be pretty straightforward to check or replace.

Maybe focus on just one bank and see if you can figure out what is going on. I'd be leary of the chinese made coils. Could try swapping the originals back into one bank and see what happens. Or get a set of 6 new OEM ones for one bank.

 

Small update:

I’ve had a chance to review more data, and do plenty of driving. I was able to replicate the hard stumble on acceleration/downshift while logging data.

Really nothing conclusive was found.

However, this morning, I put it to the floor at highway speed, and it misfired bad enough to trip a CEL, and set the emissions and drive slow service urgent warnings. I let off, and after cruising less than 15 seconds, the light and warnings shut off. The car drove normal and smooth the rest of the way to work.

Checked codes and data as soon as I got to work. P0309 (no surprise, based on previous misfire counts)



The plan now is to swap cylinder 9 coil to a known good OEM this weekend. I am also going to swap cylinder 9 injector with cylinder 10, just to hopefully rule injectors entirely.


Unrelated to the misfires, but possibly related to drivability, I did review some transmission data as well.
I noticed something I thought was strange. Solenoid 4 in my transmission was constantly flickering between active and inactive. Key on, engine off, as well as engine in gear. That doesn’t exactly seem ‘normal’ to me, but my experience is predominantly with manuals… can anyone confirm that this is or is not normal? No codes in the TCM, but a solenoid cycling under all scenarios doesn’t sit quite right with me