Banishing the misfire at long last
The laws of physics that govern electricity are reasonably straightforward when we’re talking about a 12 volt DC system. Straightforward that is, until you install that system on a car, at which point all the good work of Messrs Ohm, Faraday and Tesla goes out the window and the laws of the automotive electrical jungle apply instead. Becoming an auto-electrician is surely not a career choice, it’s a noncustodial sentence. It’s a bit like when you get caught doing 80mph on the motorway by a camera and get the choice of going on a ‘speed awareness course’ or taking the three points. Believe me, after you’ve talked yourself blue trying to convince the course facilitator that doing 80mph on a clear sunny day is far less dangerous than going at 70mph 10 feet from the car in front of you, you’ll give up and beg for the licence endorsement.
Anyway, my Merc had a misfire. It started just a few short years ago – seven, roughly – and resulted in me chasing the cause of it for a year, failing to find it, then stuffing the car into a lock-up and pretending it didn’t exist for six years. Then a friend of mine persuaded me to rescue it, and I’ve spent the last six months again chasing the cause of the misfire. I was never really sure whether the misfire only affected the car at idle or whether it was still there at higher engine speeds but was getting masked by the V8’s numerous combustion events. More recently, I’d become convinced that the former was the case.
To recap the process I’ve been through in pinning down the cause of this problem: it’s the same on petrol or LPG, so clearly an ignition fault. I’ve replaced the crank sensor, the cam sensor, the coil packs, ignition leads and spark plugs. The ECU that runs the ignition system, a Canems aftermarket unit, has been bench tested
and declared fit and healthy. I even replaced both camshafts after one bloke somewhere once said these engines suffer from timing chain stretch, and then several thousand people repeated this nonsense on the internet. While checking the chain I noticed some very slight cam wear, so decided that as I had a spare pair in the shed – attached to a spare engine – I may as well fit them. (The truth about the timing chain stretch is that early 3.8-litre versions of the Mercedes M116 engine had a single row timing chain that could stretch. Mine has a duplex chain that doesn’t.)
Having done all this fiddling, my engine still misfired at idle. In fact it ran pretty much exactly the same as it did six years ago. Was I disheartened? You bet I was. I had no idea what to do next – none. I’m generally pretty determined once I get my teeth into something, but I was genuinely at a loss as to where to go with the 500 SEC, apart from, perhaps, back to the lock-up.
Then one day, I was idly reading through the Canems ECU installation manual. My ECU was installed by Canems themselves, but reading through the manual, it clearly stated that the earths for the coil packs should be connected directly to the cylinder head(s). I hadn’t remembered seeing any earths on the heads, so I had a poke about and realised that Canems hadn’t followed their own manual, but had instead earthed the coil packs at the battery negative terminal. To be fair I couldn’t see why this might make any difference electrically, as the engine is clearly well earthed judging by the enthusiasm with which the starter motor spins it. All
the same, I thought I’d have a look for earth leads connected between the heads themselves and the chassis.
I finally found one, buried deep down behind the power steering pump, and decided to check its condition. This involved removing the LPG system’s evaporator so I could get to the mounting bolts. When I finally got it off, it looked nasty. Both ends were covered in green copper oxide, and one part of the lead itself was as well. I bought a new earth strap for a fiver, and having cleaned up both the engine and chassis contact points, I refitted it.
Then I tried the engine. Misfire, what misfire? It seems the good people at Canems were right to insist you should earth the coil packs directly at the cylinder heads. If only they’d followed their own advice, I might have avoided several years of frustration and expense.