Troubleshooting 280SE running issues

One of the things I noticed right away when I got my new 280SE was that it wasn’t running well.    It was very hard to start and keep running.   Eventually, if I warmed it up a bit it would idle, but giving it much throttle would have it stutter and die.   I was able to get it off the tow truck and into the warehouse, but not easily.

Getting it to run properly is my first priority as it will also confirm that the drivetrain is good.   My first attempt was to check the spark plugs.   I thought that perhaps they may have been all fouled up from getting on and off the tow truck and at the auction site.   The nice thing about the M110 is that they are so easy and accessible.  Given that, it made sense just to change them.   This would allow me to see if it made any difference and check the condition of the existing plugs.

To more easily get at the plugs I removed the air cleaner assembly.  I was very happy to see a near new Mann filter inside. The breather hose was also nice and supple.  Another sign that the car has been properly taken care of.   I changed out all the plugs and they looked pretty good.   This ruled out fouled plugs or the car running really really rich.

I took the car for another drive around my warehouse unit carpark.   There really wasn’t any change.    It felt like the car was not getting enough fuel.   I noticed the gauge was sitting between 1/4 and empty.   Old car gauges are not always the most accurate.   I thought the easiest thing to do would be to make sure there was actually fuel in the tank.   Perhaps my 280SE running issues was simply a lack of fuel?

I bought 25 litres of fuel and the car started running better.  It took a bit of time before this happened.  It was also able to cold start far more easily.  There could of course be other issues, but there were two possible issues I could do some simple tests to rule out.   Firstly, that the fuel strainer or swirl pot at the bottom of the tank was clogged.   If this was the case, raising the fuel level made it easier for the pump to suck more fuel and hence allow the car to run better.   Alternatively, the fuel in the tank was so old or contaminated that putting in 25 litres of fresh fuel allowed the car to run much better.

The test for both of these would be to drain the tank, and inspect the strainer.    Before I did this, I thought it was probably worth trying to look into the tank to see its general condition.   I borrowed a 48mm hex socket from a friend to remove the tank sender.   This would allow to have a bit of a look inside the tank and also see how dirty the sender unit was.   If the tank and sender unit was obviously filthy, then it would make sense to remove the entire tank and have it cleaned.   Some information on the internet says you can get the sender out with a pipe wrench or similar.  I found I could not get a good grip on it.   The right 48mm socket had it out in seconds.

The good news is that the tank looked very good.   Unlike the flat tank of the W111, the W116 tank is much taller, so it is harder to see into it from the sender hole.   You can’t see the swirl pot for example.   From what I could see, there was no reason to remove the tank.   The sender unit was quite clean, and there was minimal debris coming out of it.

My next step will be to drain the fuel, inspect the strainer and change the filter.    Even if this does not fix the problem, they are service items and it makes sense to be able to rule them out before starting to look at the fuel injection and the engine itself.   I am hopeful that it is one of these things that is causing my 280SE running issues.   If not, I’ll move on to things like fuel pressure tests.

I would like to get this sorted out by mid January so I can sell the 300SE and have a bit of time to drive this car before I hopefully sell it to buy the 450SE.