One of the first things I did was change the injectors.   The stutter when accelerating and hard starting when hot felt like a symptom of poor injectors.   I also didn’t want the situation of a leaky injector washing down my cylinder walls.   While not as cheap as the K Jet injectors, the MFI injectors are still available from Bosch and are not exorbitant.   The same cannot be said for the special ones used on the 6.3!  I didn’t do that job myself as it requires a special tool I do not have.     I had this job done at the same time as having the valves adjusted.

The next is a solenoid on the transmission that was impacting idle in gear.   The idle was too low even when adjusting with the idle air screw.    I’m not exactly sure what was wrong here, but it did fix my low idle problem.

The car also had a weak spark, so I had the coil and points changed.    When that was done, it was pretty apparent that the cap and rotor needed changing too, which is the part of the job I did myself.   This should have been so routine I would hardly need to mention it here, but there were a few things that I discovered along the way.

On the 80s cars, the distributor caps are marked with the cylinder numbers which makes installing them dead easy.   The older caps are not so marked.   This isn’t a problem if you’re just replacing the cap, as you simply hold it with the same orientation and move the leads over.   My problem was that in my haste I dropped the old cap and lost my orientation.   So much so that when I attempted to start the engine, it was extremely unhappy and didn’t start at all.  Obviously I had the order wrong.

This shouldn’t be a problem though.   If you can identify #1, you can start there and go around the cap with the firing order of the engine.   This is 1,5,3,6,2,4.   It wasn’t obviously apparent which one was #1.   In the end I discovered two ways of doing it.

The first is on the distributor there is a very fine line marked on the top of the outer rim where #1 should be.    You should be able to feel it with your fingernail.   From there, I noticed that the cap is actually marked.   There is a subtle raised section next to where the lead goes into #1.    In the photo below I have marked it with white paint.

The first time I did this, I just pulled on each lead to try and trace to each cylinder.   This is the lazy way to do it, as in the end I was wrong and the car still didn’t run.   Instead, I ended up doing it properly by taking off the leads and numbering each one, and also numbering the cap.    Perhaps overkill, but I ended up with a running engine this way.

The next step was to replace the plugs.   I had found these all fouled up on the 450SLC after I replaced those injectors.   They were even worse on the 250SE.    At least these plugs are much easier to swap out.  There are only six and access is very easy.

After replacing the plugs I took the car for a test drive.  It is a lot better.   Still not perfect but now all these components are fixed, it is probably worth having it tuned again.   It may also benefit from some good drives to clear everything out.    I’m planning on taking the car on a longer drive next week and will probably also take it on the next night drive.    At least for now, I seem to have no more M129 rough running.

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I find that whenever I buy one of these older cars, chances are the distributor cap needs changing.   It is not generally an expensive part on the mainstream engines, and can make a big difference to the smooth and reliable running of the engine.  An old and pitted cap is going to produce a weak, inconsistent spark.    I normally just purchase a cap and rotor when I buy the car.  On the off chance it doesn’t need replacing it is good to have a spare.

My 86 300SE is no exception.   The cap wasn’t in terrible shape but it was clearly worn and ready for replacement.  The contacts were worn and so was the rotor.

The M103 distributor cap differs from the other cars because of how it connects to the camshaft.  To remove the cap, there is a cover for the plug wires that snaps off.   Next, three 5mm allen bolts hold the cap to the engine.   Two of them are easy to see but the third one is underneath the cap and only just visible from the left hand side.   I found it easiest to remove it with an allen socket and a small extension.   Using a 1/4″ ratchet made it easier to clear the Serpentine belt.   I used an adapter for my 3/8″ socket into the smaller ratchet for this purpose.

It is easier to remove the plug wires first.  I found there wasn’t clearance to remove the cap until the plug wires were removed anyway – the fan gets in the way.    The actual distributor cap has a black cover that provides the mounting points for the plastic cover and splash protection for the ignition wires.   On the surface, mine looked ok but on closer inspection I would have preferred to change it.   It has a couple of small cracks in it.    In the picture above, the old M103 distributor cap on the right still has the cover attached.

Once the cap is removed, the rotor requires 3x3mm allen bolts to remove it.   In my case, one of the bolts was rounded off.   It was quick easy to remove with vice grips, at the expense of breaking the old rotor.

I gave the inner plate as good a clean as I could.   Ideally I would have replaced this too.  As can be seen in the picture above it has seen better days.

In total, the job took about an hour to do, including the rounded off bolt.   I had not changed an M103 distributor cap before, so it would likely be faster next time.      Note, that the M104 engine is very similar.   The M104 is basically a DOHC version of the M103.  More power, but also even more prone to eating head gaskets.

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When the car is idling, the throttle is mostly closed resulting in high vacuum.   The high vacuum causes oil in the heads to pool around the valve stems.   When the car accelerates, the oil gets sucked down past the old, hard valve stem seal, creating a cloud of smoke behind the car.

I had the valve stem seals replaced at the same time as the timing chain and guides.    Driving the car a couple of times since has shown there is no longer any smoke at takeoff (or at any time).  I took the car for a night drive to really be sure – it is very easy to see the smoke in the headlights of the car behind.

At the same time, the distributor cap and rotor was replaced.   The cap was extremely pitted – I’m surprised it ran as well as it did.    With the new chain, replaced seals and distributor cap and rotor the car runs really well.

The next thing was to look at the paint.   The paint was flat and felt rough to the touch.   This hasn’t changed after a couple of washes.    A couple of hours with a clay bar has made a huge difference.   I’ve used clay bars on a few cars, some that looked a lot worse.   But none of them had the level of containments that I got off the paint from this 560SEC.   After a couple of hours with bar, it was a brown colour and the paint was quite smooth.

It is very hard to see from the photo, but the paint is much shinier than before.   I will probably get a fresh clay bar to finish the job and then use it on one of the other cars that is not so bad.

The 560SEC is responding well to some maintenance and general care.   I’ve still got a fair few things to do to the car, but none that will prevent me from using it.

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A quick test of the new coil had the car roaring back to life, so I swapped the new coil in properly.   This job can be done with the tools found in the Mercedes tool roll (screwdriver, 8mm and 10mm spanner), which helped as the car was not near my tools.

I also replaced my distributor cap and rotor while I was there, as the united fitted to the car looked rather worn out. Both ballast resistors seemed fine, so I left them alone.  I went with Bosch parts.    They have probably been on the car for years.

The end result is a car that starts more easily, has fixed some of the poor running and hesitation I was having with the car.   More importantly, it runs! At some point it might also make sense to replace the leads as they seem fine, but a bit old.

Total time was less than one hour and the car is transformed.   The last few months I had not been enjoying driving the car as much.   Now the car is back to normal it reminds me what a nice car do drive it is.   I should have realized something was amiss.

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