How hard is it to change the head gasket yourself on a 1987 Mercedes 190e?

It's a careful d.i.y. on a 1988 car with 2.0 litre, and takes a day at least. You will need a torque wrench. Anything cheaper than a Draper is probably junk (buy two if you also want to torque the smaller bolts, I didn't bother.) Read Haynes beforehand: this answer is intended as a supplement to Haynes.

After taking off the air filter, take photos of the wiring layout for sensors etc.. (idea:) Put unique parts into egg carton as they are removed. A desk-sized table next to the car is useful and also where you can put the heavy head (+manifold) when you lift it out.

Tricky parts of the job:

There is an extra 'head bolt' that is m8 sized and holds down the timing chain cover extension of the head. It's not mentioned in Haynes so, after removing ten head bolts, you might wonder why the head won't lift.

Removing the pins that hold in the upper timing chain guide is a lot easier if you put together a little extractor made of 1. a bolt that screws into those pins (metric thread) and 2. a nut on the bolt, which you can turn against 3. a steel sleeve (outside the bolt.) This 'sleeve' can be a few larger nuts piled to form a small cylinder. (This idea saves removing the radiator to make room for a puller.)

Keep a magnet-on-a-stick handy in case you drop anything inside the timing chain cover.

Removing the timing chain tensioner is covered well enough in the Haynes manual but can be done easily with a large shifting spanner for the outer cap, and a gas pliers for the tensioner body (using cloth to protect the threads). Remove the alternator's upper mounting bracket from the block to make access to the tensioner a lot easier.

Mark and scratch the timing chain and camshaft sprocket well. Always preserve the chain's position: not letting it lose its position on the two lower sprockets that you cannot see. In other words don't move the crankshaft far from TDC position at any time even though that's recommended in Haynes for cleaning of the cylinder walls after scraping clean the piston tops.

To disconnect the exhaust manifold from the exhaust downpipes takes some wriggling from underneath and a selection of socket spanner extension pieces and universal hinged connectors. Reconnecting the exhaust requires paste so that is best done in an exhaust workshop. You can drive there with noisy un-pasted exhaust.

An important caution is to make things very clean before putting the new head gasket in. Any oil or water on the mating surfaces will later waste all your effort. When you lift the head to move it back into position on the new gasket, take care no oil trickles from some unnoticed passage. Water hidden in the threads of the head bolt holes (in the block) can also become squeezed up when the bolts are reinserted, so clean out those orifices with a dry towel material.

Tightening of the head bolts can be done with a six-point bit (m12). I found a twelve-point bit a week later in a local car boot sale and would have preferred to use it. The torque settings are in Haynes manual but I did not follow their instructions for tightening (re-used) head bolts (they suggest two ninety degree turns after a 55 Nm setting - certain to snap your old bolts).

Observe the first part of what Haynes says: to tighten the head bolts to 55 Nm in the prescribed pattern (from the middle two, outwards). However, it is my opinion that then adding a 90 degree turn on each bolt would be reckless and there are web forum postings that mention how this snapped head bolts.

Instead, I worked in steps that felt comfortable (following the tightening pattern, of course) and left a final torque ranging between 97 and 110 Nm. I wanted them all the same, but felt a slipping sensation in the '97 Nm' bolt. That is sometimes felt when tightening ht bolts, just before they snap. Bearing in mind that the final torque on Honda 1400 head bolts is 70 Nm, 97 - 110 seems enough.*

*after settling for the torque max of 110 Nm, I found a US web post (Automotix) stating that final torque for all Merc engines (since engine end number 712 988) is reached at 82 lbs.ft.. This pleased me because 82 lbs.ft. = 111 Nm. Pretty close! p.s. I realized later they were referring to widely-used OM352 engines (see on facebook) but my new head gasket is still okay, 5 years later.