A persistent misfire on the Mercedes 500 SEC
Recommissioning is a very grand word often used by people who read magazines with posher cars on the cover than Classics Monthly. That might sound a little rich seeing as how the car in question is a Mercedes, but I paid just £2500 for it 10 years ago, so I’m sticking with ‘fixing.’ It needs fixing because I neglected it. For six years it sat mouldering in a dank garage while I tried to pretend it didn’t exist – all at once guilty about leaving it there, too scared to face what the leaky lock-up had done to it and yet desperate to rescue and revive the car.
Eventually my hand was forced by a friend who insisted on helping me extract it from its gloomy tomb and installing it in the garage I built to accommodate it a few years ago (see the September issue for the full story of this). And now it’s sharing space with my 1977 Yamaha XS650 and Bling the cat, who sleeps in here because she’s 18 and too old to rough it outside at night.
So what have I discovered since the Mercedes' return? Well, it’s survived remarkably well. I don’t really understand why it hasn’t got any rustier, but having scrabbled about underneath I’m fairly sure it hasn’t. The front offside brake caliper that was sticking when I laid it up is still doing just that, the steel fittings on the front brake hoses have rusted so it’ll need a new pair of those, and, er, that’s about it. Of course the MoT man may disagree, but there’s only one way to find out.
I’ve spent some time cleaning the interior and treating the leather with hide food, and it looks fine.
There’s a small rust patch at the front of the rear nearside wheelarch, which was there when the car went into hiding. I have a wheelarch repair panel, which I’ll only need a small portion of to fix this, and most of the lacquer on the roof is flaking off. Which is odd really, because lacquer normally suffers as a result of sun damage, and that's the one thing this 500 SEC hasn’t seen in a long time.
So, all things considered the car’s in pretty good shape. But although Mercs from this period were built without regard to cost (this was £75,000 in 1987, which is the equivalent of more than £200k these days) it still hasn’t managed to fix the misfire during its six-year layup. That's annoying really. I mean, what the hell was it doing for all that time, apart from pushing all the lacquer off its roof?
This is a relatively simple car by Mercedes standards. It has fuel injection, but it’s only Bosch KE-Jet, a largely mechanical system with a big flap that sits in the airflow detecting how wide the throttle is open. There are some extra electronic gadgets on it (that’s what the E is for in KE-Jet) but the system remains fundamentally mechanical, and the injectors are simply jets which constantly squirt fuel onto the back of the inlet valves. There’s no electrical connection to them, which ironically was a big pain when I wanted to fit LPG to the car soon after buying it.
LPG, or Autogas, seems to be pretty Marmite amongst car enthusiasts. Some are big fans, others hate the stuff. In amongst all this there’s the inevitable mix of nonsense talked by people who just don’t know that they don’t know something. For instance, a long time ago I had a 1971 Mercedes 280SE, to which I fitted an LPG system too. I was told quite forcibly by several people that the engine would blow up. I asked them what led them to this disastrous conclusion, but ironically people who like to present strong opinions as facts are rarely very good with real facts, and tend to disappear back into the woodwork when asked for some.
Of course, the 280 didn’t blow up, and I used it as my everyday car for several years, regularly doing a 600 mile round trip to Devon to see my folks. So I’m a big fan of LPG, but it’s fair to say that it does require some care. For a start, your ignition system needs to be in good order. If you have a misfire on petrol, it’ll be worse on gas. And you don’t want to be running out of gas either, because if the tank gets really low there’s a good chance of a backfire, which at best can blow off your air filter, or at worst can damage the K-Jet system, which is what happened on a 380 SE saloon I had with an LPG system which used a mixer ring bolted to the top of it.
So when it came to LPG on the 500 SEC, I wanted to use an injection system, partly because they’re more fuel efficient, but also because the chance of backfires is greatly reduced as the gas is squirted in close to the valve rather than filling the entire
KE-Jet metering head. But gas injection systems rely on piggy-backing the electronic petrol injectors on a modern car to tell then when to fire. The SEC doesn’t have any, so I needed a different solution.
This came in the form of a standalone ECU made by Canems. I had to fit a toothed wheel to the crank so it knows what’s going on, and inside the distributor is the sensor from a VW Golf Mk3. These usually have four segments on them, but in this installation three are removed, so the single remaining one acts as a cam phase sensor. In addition there is a throttle potentiometer so the ECU knows what that’s doing too. Then there’s an inlet air temperature sensor, and an engine temperature sensor. With these five pieces of information fed into it, the ECU knows exactly how much LPG the engine needs, and fires the gas injectors accordingly.
Back to that misfire. Those in the ‘leave the LPG in the cupboard along with that disgusting yeast based sandwich spread’ camp will now be lining up to say that LPG systems always misfire, but the misfire is the same on petrol. As ever, 90% of fuel problems are ignition based. All I’ve got to do now is find this one.