Bentley Arnage Project - misfire

Last time we went out in Betty she drove perfectly, a real pleasure. We had been to Horncastle on the A158, a really pleasant drive through some nice little villages and some open road with far reaching views across the Lincolnshire Wolds. The weather was sunny but not too hot, the traffic was light, it was perfect. That was a week ago, so I thought it was time to take her out again, I don't like to not use her at least once a week, they are meant to be driven. Anyway, we had an awful lot of rain this week, I noticed some had got in to the car, just a bit on the passenger side, it had been torrential but still shouldn't have got in there so that's something I need to sort.

We didn't get far, almost immediately I noticed a misfire. I hoped it would go away but it didn't and a short distance later the dreaded check engine light came on. I turned around and brought her home, something was obviously badly wrong. Fortunately being a post 1996 car it is fitted with an OBD2 port - it was mandated that all cars had to have them at that date. I had never had any need to look for one before but quickly found it underneath the dash immediately below the steering column. I wasn't convinced that my super cheap reader would connect to it but it did and I was able to pull a code - P0203 misfire cylinder 3. Great, that gives me a place to start looking. I reset the code, the warning light went off but the engine was still stumbling. I took it for a drive and the light came back on, same code, it reset again and then drove perfectly all the way home. It came back again when I restarted it later.

I thought maybe the rain had got in there and caused some havoc but then I discovered the ecu is mounted in a water proof box that is tightly sealed. I looked in there anyway but it was absolutely dry and all the connectors were secure so the fault lies elsewhere. 

I have experienced this a couple of times in the past, notably on my 4 litre Jag. Both times it was a failed ignition coil - they are coil on plugs on these so every cylinder has it's own coil. There are advantages to such an arrangement but it does mean the coils are exposed to all of the heat and vibration of the engine so failures are not uncommon. I don't know how old mine are but there is every chance they are the originals. This brings us back to the Bentley tax and another whinge. I looked them up on the usual Bentley supplier's sites and was rather horrified to see a price of £260 plus the VAT each. Remember that this is a BMW M62 4.4 v8, the coils are the same, they are fed off the same Bosch ECU, albeit with a different map in it. If you search for an M62 one rather than a Bentley one they start at just 17 quid for one from Autodoc. OK, you have to pay another 7 quid postage but that gets you a brand new unit with a 1 year warranty. Obviously that's not a genuine Bosch item, if you want one of them it's 40 quid, still a fraction of the price of exactly the same thing in a Bentley box. If you want to buy a set of 8 you can get after market ones for just 90 quid - just over a tenner each or 1/26th of the price of one Bentley branded one.

I'm going to buy 2 mid priced ones so I have a 1 year warranty and a spare. I don't know for sure if this is where my problem lies but it's a starting point and even if the fault turns out to be elsewhere I will have a couple of spare coils on hand, which can only be a good thing. It will allow me to substitute the coil out without disturbing the coils on any other cylinders, which brings me to another point. I was unsure as to how the cylinders are numbered and when I did a google search I found sites that said the number 1 bank was on the left, others said it was on the right. I think the confusion comes where there are left hand and right had drive cars and some people talk about viewing the engine from the driver' seat and others talk about viewing it from the front. Well the correct answer is when viewed from the front of the car, cylinder number one is front left, cylinder 5 is front right, my failing cylinder - number 3 - is on the left, third one back. I am told that once you take the plastic cover off the cylinders are marked but I haven't got that far yet. I have my coils on order, I will get further in to it once they are here.

Slight change of plan, the weather brightened up so I thought I would go an have a look to see what I am up against, that quick looksee resulted in parts being removed, it didn't seem like a difficult job. To be fair, it wouldn't have been a difficult job on the right hand side of the engine but over on the left it's a wee bit harder thanks to an a/c pipe being right where you don't want it. There are two turbo pipes in the way as well, rather inconveniently these are bolted to the covers that go over the coil packs. Getting that cover off was a bit of a sod, in my old Range rover with the same engine without turbos you just remove two nuts and lift it off but in the Arnage you have to remove the turbo pipe and the nuts holding the middle pipe to the cover. That's a lot easier said than done, it's tight in there and pulling the mid pipe out is quite hard, it's difficult to get a grip on it. In the end I left it and just removed the two rubber supports from the cover, that gave me enough room to wiggle the cover out so I could get at the coil pack. Undo the connector by lifting the silver clip upwards, pull the connector back and it comes out easily, there are then 2 10mm nuts on the pack, which lifts out easily. On my car the packs were the originals - BMW 1 748 017, Bremi 11860, they were dated 09.01.98 so I know they are original, 28 years old and have done 64,000 miles so it's fair to say they are due for replacement. Given this information I decided a whole new set would be a good idea, it's half stripped anyway and for the sake of 90 quid I got a set with a 5 year warranty. It just seems to make sense having gone this far. While I am at it I might as well put a new set of plugs in it. I took one out, it was very stiff, I suspect they have been in there a good while, although they don't look like they have many miles on them. Given the amount of work to get a them it seems daft to put old ones back. They are original BMW NGK r1 items, they have a built in 1k resistor. The equivalent is NGK BKR6EK, though I believe they are one heat rating lower. The BMW guys reckon they are fine, loads of people on the M62 forum are running them and say they are perfect. The Bentley tax comes in to play again, they want 30 quid each, genuine NGK from an authorised dealer want 36 quid for a set of 8 delivered.

I will say that removing the spark plugs is the scariest thing I have done in a long time. They were very tight and have obviously been in there a very long time. I was terrified that one would break or strip the head, I can fully understand why people avoid this task. I soaked them all in penetrating fluid before I started, I hope I never have to do this job again. Weirdly the four on the left came out much easier than the 4 on the right, number 6 was a particular nightmare. It takes a range of tools to get them all out, expect to lose some blood and the skin off at least 4 knuckles.

So now all 8 coil packs are out, all 8 plugs have been removed and replaced loosely so no detritus gets in there, all parts are on order and should be here in 2 or 3 days. In total it has taken me about 3 hours to get this far, I don't expect replacements to take that long.

I was rather pleased though when number 3 plug came out slightly damp and smelling of unburnt fuel - it's the only one that did and strongly suggests the diagnostics were spot on and that's where my misfire is. Hopefully the plug or coil will fix it, the fact petrol is in there suggests the injector is firing ok.

So total cost so far is £126, less than half the cost of one single coil pack from Bentley and I have a full set of nice new shiny components. Hopefully I will never have to see any of them ever again :-)