I have had a 1990 Mercedes 190E 2.0L (111k miles) from new. It has been totally reliable, except for one problem, an intermittent misfire, which it's had off and on for the past two years.
Neither the main MB agent nor my local garage have been able to fix it, mainly because it works perfectly when they have it!
The misfire only occurs when the engine is under load, accelerating up a hill, in any gear, when the revs exceed 2000RPM. The misfire is so severe I feel sure it involves at least 2 or more cylinders. At all other times it runs perfectly. (even when in a period of misfire, I can rev it to 6000RPM in neutral without problem, but as soon as it's back under load, it misfires again - it's the loading causing the problem)
The garages have replaced the distributor cap and rotor arm, leads and plugs over the period, all to no avail. The car will run perfectly for 2000 or 3000 miles, then suddenly develop the misfire, especially pulling away from a junction. It lasts any where from 100yd to 30 miles, then disappears and runs fine again, until the next time.
The fault is reducing my confidence in what is part of our family and my daily workhorse. (it does a 60 mile round trip to work most days). It has TSZ ingition and uses no oil between services.
I'd be very grateful for any views you might have either as to the possible cause of the problem or where I should take it to get it diagnosed properly.
I would check by substitution the Hall effect sensor in the distributor. If your car has vac controlled advance, the leads of this are stressed each time it operates. This might explain why you only see this effect under load. The device is a Siemens HZK101, possibly packaged as a special by Bosch. Shop around and you can get one of these quite cheaply.
Actually, thinking back a couple of years, I have seen one or two Mercs with tracking on top of the coil (on the plastic 'nose' of the coil). They usually have those clip-around plastic shields and sometimes muck can build up on top of the coil.
Another thing to watch is that IME the aftermarket king leads sold by GSF and ECP are actually a little too long. They can end up resting against other components and tracking, if not carefully routed.
Thanks for your responses. The king lead has been replaced, but not the coil. I like your Hall effect sensor suggestion though, as it was in the area I susspected. The coil top has been extensively cleaned and there are no signs of tracking.
As an installed engine is loaded (torque sourced from crankshaft) the body of the engine will twist in the opposite direction on its flexible mounts - Newton's third Law. If the king lead connects the centre of the distributor (on the engine) to the coil (on the body), this lead will be stressed as torque is applied and removed.
If I had recalled N3 L correctly I would have got there!
I was looking for something much more subtle, eg changes in the conductivity of the compressed mixture prior to ignition, resulting in a higher voltage gradient (ie electric field stress) appearing on the insulation of the lead. Leading to insulation breakdown and arcing.
I guess you've been to MB Bath (formerly John Tallis), which is in the MB Club Good Garage Guide and is one of the few main dealers to be included. The nearest alternatives in the guide are Autoclassico (Henfield, 0117 956 9115), The Gibb Garage (Castle Combe, 01249 782263), and Robin Lamb (Gloucester, 01452 500808).