Troubleshooting stuttering and stalling in my 250SE

My 250SE has not been running well lately.      Back in April, I hit a Kangaroo and the car was repaired during May and June.    In July, the state of NSW plunged into a strict lockdown for COVID-19, preventing more than minor trips up to the shops for necessities.    It was during one of those trips that I noticed that car was stuttering and stalling.  It felt like it was not getting enough fuel.

In some ways it reminded me of a fuel delivery problem I had on my 1967 250SE Coupe.   That car had been laid up for some years and the swirl pot had become clogged up.   The swirl pot is the solution Mercedes-Benz used to ensure that the car still gets fuel during brisk cornering, and on long uphill and downhill sections.   Inside the fuel tank there is a plastic structure that looks like a flower pot.   It is open at the top, but (mostly) closed on the sides.   When the tank is quite full, the swirl pot is unnecessary and fuel is above the top rim.   Once it gets down to about 1/3 level, the swirl pot acts almost like a tank in a tank.   Without it, fuel would slosh around and the fuel pickup would not be submerged at all times.   The pickup for the fuel pump is at the bottom of the swirl pot, and the return line comes in the side, swirling fuel around inside the pot.   This draws further fuel from the rest of the tank through a small hole in the bottom of the pot.   When it is working properly, there is always plenty of fuel inside the swirl pot, ensuring that the car has a reliable fuel supply.

On my 1967 250SE, the small hole in the bottom of the pot had become partially blocked with debris.   Once driving for a while, the car would use fuel faster than the pot would refill, causing the car to stall.  On that car, it would refuse to start for a few minutes until fuel slowly entered the swirl pot.   I fixed the problem by dropping the tank and cleaning it out as best I could.

There was a difference in this situation however.   When the car would stall, I could start it right away.   I also didn’t get many stalls, mostly stuttering and hiccupping.    It did feel like fuel starvation and so worth further exploration.

The first thing I did was change the fuel filter.   The filter is quite easy to get to on this era of mechanical fuel injection engines.   It is rather like the oil filter with a removable element inside a housing next to the engine.    It only takes a few minutes to drop the housing and replace the element.   I had a spare filter on hand from my old 250SE Coupe, so I figured this would be a good place to start.   The filter was very dirty on my coupe as the debris that blocked the swirl pot was also making it up to the fuel filter.

Unlikely on my previous 250SE, this filter was pristine.   The fuel in the housing was also very clean and the old filter was so good, I could have almost put it back in.    It had been a while since the filter was changed, so I didn’t.   I also wanted to cut the filter open and make sure that there was no debris.

While changing the filter, there are a couple of gotchas, the first is the felt attachments on the filter detach once they get old.   Its pretty normal to find the top one still on the engine and the bottom one stuck in the bottom of the housing.   These should be removed.   Secondly, there is an O-ring between the housing and the flange.   Mine was not in great shape, so I replaced it.

Cutting open the filter confirmed my suspicions.  It was very clean inside, so highly unlikely to be debris in the tank causing my problem.   Changing the filter is good routine maintenance anyway, so nothing lost.  While I was under the car I checked the condition of the steering shock too, which was still good.

I did one more test to eliminate the swirl pot from the equation.   The pot can be partially seen through the opening in the tank for the sender unit.   The sender unit is very easy to remove on the W111 and derivatives (e.g. W112, W108, W109, W113 etc).    It is simply behind a plastic cover on the boot floor.   I removed mine and then let the pump run.     Everything looked really clean.  I could see fuel swirling around the pot as normal.    This is not a definitive test as I did not measure the flow rate, but given how clean the tank was that didn’t seem necessary.

While I was there I replaced the cork gasket for the sender.   Mine was disintegrating and I had a spare on hand, again from my 1967 car.   An old gasket can cause fuel smells.

My next theory was bad or off fuel.   Modern fuel seems to have a short shelf life.    My Jaguar is particularly sensitive to this.  It doesn’t stall, but it can run poorly if old fuel is still in the tank.  I took the car for a drive to get the level down below 1/4 of a tank and then filled it up with a fresh tank of 98.   I probably should have driven longer as I added 60 liters.   As the tank capacity is 80 litres there was still almost 20 liters of old fuel in there.

There was a huge difference.   The stuttering was almost gone.  I had one minor stutter during a 20 minute drive and a stall when parking the car.   the performance was transformed, and I am attributing these smaller issues to the 20 litres of old fuel still in the tank.    I will have to drive the car more to really be certain, but it is looking more and more like the reason the car was stuttering and stalling and running so badly was old fuel.