When I purchased my DB9 a few years ago occasionally if I just sat still in the car while it was idling I could hear – no – feel in the seat of my pants – the slightest ‘lump’ in the idle. Rarely, faintly, distantly, so infrequently that I never really worried about it that much. At the time the car was 8 years old and had about 15K miles on it.
Turn the clock ahead about 2 years to spring 2016 (and about 25K miles total now) and the faint lump rather quickly progressed to a routine ‘lump’ at idle anytime in gear when stopped at a light or sitting in the garage. The lump was only at idle, and seemed to be gone (or at least was not perceivable) at speeds above 900 RPM. I started to learn more about lumpy idles (or misfires at idle) on the forums.
Apparently this is a VERY common issue on early DB9s. I can’t find the reference, but I had read that essentially once the car is ~10 years old or passing 20K miles, you’re almost sure to have this issue by then. It happens on many cars prior to age too, just check out the AMOC, Pistonheads and 6-Speed forums.
In fact this issue is mentioned as ‘typical’ by Grant Neal in his excellent book The Definitive Guide to the new Gaydon era Aston Martin – A Buyer’s Guide and Car Enthusiast’s Guide (check out my blog article on this book). In the sections “Researching and Inspecting your Aston Martin” (page 256) “With the V12 specifically, a misfire maybe caused by failed coil packs, which is a relatively common fault…” and again in the section “The Specialist View” (page 269) “On the DB9 we do also see the odd coil pack failing, causing a misfire.”
What can be causing the issue?
The issue is likely the Coil Packs. For those not familiar with these, here is a simple summary. Each cylinder has a spark plug. In the old days cars would have one ignition coil shared for the ENTIRE engine. The ignition coil generates the high voltage electrical charge that is sent to the spark plug at just the right moment to make the spark to detonate the fuel in the cylinder. On an Aston Martin DB9 (and many modern high performance engines) the design now is to have an individual coil pack for each spark plug. Twelve cylinders, twelve spark plugs, and twelve coil packs. This way the timing of the spark and energy to each plug can be controlled independently. Cool right!
The problem is with the design of the Aston Martin Coil Pack. Over time it gets ‘weak’ at idle speed, probably since the 12 volt signal level in the car dips slightly at idle (when the alternator is at its lowest speed). Or perhaps the coil packs inner workings begin to fail due to the years of extreme heat (they are stuck right into the cylinder heads with absolutely no cooling). Baked electrical components. Regardless, the story is that they begin to fail delivering the necessary spark at idle speeds, causing unburnt fuel to discharge into the hot exhaust manifold where it ignites and creates an unsettling ‘lump’ that you can feel in your pants. Sometimes called a ‘weak’ coil pack. Whatever you call it, it’s a problem. It doesn’t get better, and only gets worse. You may have just one, or perhaps multiple coil packs with some level of this issue.
How to check for yourself
So if the seat of your pants is giving you the signal that there is a lump in your idle, how do you confirm that it’s a coil pack issue and not something else? Lots of things can cause a misfire. A clogged or failed fuel injector, a wiring fault to the coil pack, failing spark plug, and probably a bunch of other things that sound super expensive to fix.
The cars Powertrain Control Module (PCM – or engine computer) monitors for misfires in realtime with sensors. It counts them up. You can use an OBDII reader to access the PCM data and actually see how many misfires have happened to a cylinder.
I did just this. I went for a 10 minute drive around the neighborhood, letting the car warm up and idled at the lights a few times to let the ‘lumps’ accumulate. When I returned home (without turning off the car – this resets the counters) I linked up my OBDII reader (learn about how to do this in my blog post) and then reviewed the OBDII On Board Monitor Test data. Check out the video below for how to retrieve the actual data. In summary I had about 10 to 40 misfires on most of the cylinders over the 10 minutes. Cylinder 4 had the least with just 2 misfires (see my footnote below why). But, Cylinder 5 had over 723 misfires! Clearly most of my problems was developing in Cylinder 5, but the rest had some varying degree of the issue.
In the video you’ll also see me use a less high tech method. I asked my Sweetie to sit in the car and put it in gear while idling and hold the brakes on. Then I went behind the car (hoping our marriage was a happy one ;>) and put my hand in front of each exhaust pipe (carefully) and ‘felt’ for the pulse in the exhaust flow. Bad-da-bing! Felt it clear as day in the left hand exhaust stream.
Does this matter?
Yes. A few misfires aren’t a big deal (but are a harbinger of things to come). Once the problem with a cylinder becomes severe, the PCM will actually shutdown the cylinder (stopping the spark and fuel flow to it) to protect the rest of the engine. Unspent fuel in the exhaust stream is BAD mojo for those really expensive catalytic convertors, so at ~1.5% misfires, the PCM turns it off and sets the MIL (malfunction indicator lamp – the check engine or idiot light) to get your attention.
My cylinder 5 was at about 1%, so nearly to the shutdown threshold. If your MIL light goes on, and you use your OBDII reader to check out the P Codes (check out my blog post on reading your P Codes) it will probably be P0300 (if its random) or P0301 through P0312 corresponding to which cylinder is having the severe issue.
What’s the cure?
Simple – change the failing coil pack(s). Not so Simple – the coil packs are buried in the cylinder heads and are a huge PITA to service. You have to remove the entire intake system from the top of the engine to get access to the coils.
There is a constant drone in the forums from people battling through all the hassles changing their coil packs and then deciding to ‘only’ change the one coil pack and leave the rest (to save money). Dude – if one is failing, the rest will fail soon – the writing is on the wall. The process takes probably a couple of DAYS for the do-it-yourselfer to dig your way into change one coil pack, and an extra half hour to just service them all. I am firmly in the camp of service them all once you’ve gone to the trouble. In fact you should change all 12 coil packs, spark plugs, fuel injector O-rings (upper and lower) and the intake manifold gaskets. Sure, you can cheap out and gamble and buy just one coil pack, leave your plugs and reuse the injector o-rings, but you need new intake manifold gasket(s) no matter what. You own a DB9 – act like it. Replace all the bits and reward yourself with a smooth running car for years to come. [Exception: If you’ve already changed them all once in very recent history, of course a one at a time approach is appropriate, but so is diagnosing more thouroughly what’s going on that killing them off repeatedly – a.k.a. dealer visit]
How do you do it?
I researched as much as I could from the forums, online articles and the official Aston Martin Workshop manual. There was good information, bad information, and missing details. I set out to document the entire process as I worked through it as methodically as I could so I can share it with you through this Blog and in video’s. Seeing someone else do it and explain it in detail makes it a lot easier for me.
Check out my companion article ‘How to Change your Coil Packs and Spark Plugs’ for the complete list of all my articles and videos. This will show you how to do it all step by step.
Now, sit back and check out this video on my lumpy idle to see more about what I was talking about above.
I looked back into my cars previous service history (check out my cars entire service history on the My Car page), and sure enough in the 2 year service performed by the DPO’s (Damn Previous Owner) dealer a couple of months before I purchased it they changed the Coil Pack on just ONE cylinder (number 4, the one with the least misfires today) but the DPO cheaped out and didn’t service the rest even though the dealer wrote on the paperwork “Using OBDII misfiring monitor on AMDS found cylinder number 4 has the hightest misfire rate, all other cylinders are showing some misfires [!]. Replaced cylinder #4 secondary ignition coild and spark plug.” F’ing DPO, shelled out $900 in labor and only replaced one coil pack. Kicking the problem down the road to the next guy (me).
Also, back when the car was new (to me) and I was just sensing the faintest hint of a misfire I thought maybe the fuel injectors needed cleaned. So I tried adding a bottle of Chevron Techron Fuel System Cleaner (which I found online for about $8) to a fuel tank of premium fuel. Drive normally for a week using up the tank of gas, and see if the issue gets any better. A dirty or clogged fuel injector can cause the same sorts of issues, so this is a simple (and cheap) thing to try. Can’t hurt to do a little injector cleaning anyways, but if you use premium high quality fuels from Shell or Chevron, it’s unlikely this will be an issue.