Two steps forwards, two steps back
The internet is a vast source of information. Some of it is even right. Not that much, though. We’ve all heard of Russian interference in referendums and elections, and of bots writing posts to stir up trouble and divide people and distract them from the real cause of their problems, but I’m not talking about anything as sinister as that. I’m talking about the 21st century equivalent of the pub bore.
He used to hold court in the public bar at the local and drone on in a loud, impossible to shut out bellow, proclaiming what was wrong, what was right, and what everyone else (not him) should do about it. Some people couldn’t bear it, others lapped it up. And those in the second group would repeat and amplify the bore’s announcements elsewhere – at work, at home, with friends. This is how nonsense becomes accepted as fact. The internet takes this to a whole new level.
It’s my own fault. I posted a question on a forum. I know, I’m big enough and ugly enough to know full well this is a stupid idea, but there you go. I was desperate, such was my frustration with the misfire on my Mercedes 500 SEC. So I went on a Mercedes forum and posted details of it, and asked – asked! – for suggestions.
In among all the people telling me the problem was the LPG system, and those who told me the engine needs totally rebuilding, I got sucked in by an internet myth which says that the timing chains on these engines stretch, throwing the timing out and causing rough running. But when you’ve been chasing a problem for this long (the car already had the misfire before its six-year storage period) you’re inclined to clutch at any kind of
straw that comes your way.
I soon had the cam covers off. Not as soon as I’d have liked, but the LPG injector rails locate on their securing bolts so the job’s a bit more of a fiddle than it might otherwise be. I removed the spark plugs and turned the engine to TDC.
The marks on the cams lined up perfectly. No timing chain stretch then. Well, it has only done 250,000 miles, so I guess those who insist this is a real phenomenon must be talking about higher mileage engines.
At this point I engaged the help of a genuine expert
rather than an internet one. Nigel Dean of Spilsby Road Garage has been working on Mercedes since he was 16, and his dad for 50 years prior to that. He knows his Merc branded onions. I asked him about timing chain stretch. ‘On a 500 V8?’ he asked, sounding puzzled. ‘Does it rattle when it is idling?’ I told him it didn’t. ‘ Well there’s nothing wrong with it then,’ he assured me. ‘Put it back together and stop worrying about it – those engines last forever.’
Before I did that though, I had a good look at the cam lobes. A couple were showing signs of wear, and I think I know why. I ran the engine with modern synthetic oil for years, and I don’t think it had enough zinc in it. Zinc is a friction reducing additive, but it clogs up catalytic converters, so oil manufacturers have reduced its levels considerably over the last 20 years or so. This is no big deal in a modern engine with roller cam followers, but in a flat tappet engine like the Mercedes', it can lead to cam and follower wear. The synthetic oil may well have helped keep the rest of the engine healthy, but the cams didn’t like it.
Luckily, I have a spare engine in the shed. It’s not really luck that means my shed is a source of lots of Merc SEC bits, it’s more down to stupidity. When I bought my car it was missing a knob – a tiny little bit of plastic that sits on a panel in the driver’s door and controls the position of the headrest on the electrically-adjustable seats. I
idly searched online one day for the knob, and came across a bloke breaking a car for spares. He wanted a fiver just for the knob, which I thought was far too much, so I paid him £1000 for the entire car. It’s easy to justify this kind of behaviour in your head – all those parts would add up to way more than a grand if I bought them individually. My wife would argue that having to build a second shed just to house them all negated this argument somewhat, as does the fact that they’ve all sat there for 10 years, unneeded. Until now!
I removed the cams from the donor engine (it’s only done 100k miles) and spent a happy day fitting them to my car. It was a fiddle to get to some of the bolts holding the alloy cam towers right at the back of the engine bay, and I managed to break two 7mm allen key sockets as they were incredibly tight. In the end I went to Halfords and spent £8 on one of their lifetime guarantee bits, which undid the bolts and survived the task.
I used cable ties to keep the duplex timing chain attached to the cam wheels while the cams were out, because if it falls into the sump you may as well just lock the garage door and put the whole house up for sale – Merc included. I had a bit of a head scratcher when one cam tower just refused to sit properly on its cylinder head, and no amount of cajoling, easing all the bolts slowly tighter and swearing would make it play ball. So I reluctantly took the whole
cam assembly off again and spotted that a locating dowel had stayed on the head when I removed the old cam, and was now clashing with one sitting in the replacement cam tower. Out with that, and on it went.
I turned the engine twice with a socket on the crank
pulley to check that all the timing marks still lined up, then replaced the plugs and cam covers and fired it up. It runs just as it did before, with no noise from the cams and a stubborn misfire at idle. I’m off to the pub. Perhaps someone there will know the answer.