When my warm up regulator returned, I refitted it to the car.   Even without running the engine, the control pressure was much better.   Actually it was now too low.    I still had the fluctuation though.   That got me thinking that while the fuel tank was clean, filter clean, pump ok and so on, I had not ruled out the small opening in the bottom of the swirl pot being clogged.   A quick test could rule that out.  Adding another 25 liters of fuel would bring the level above the swirl pot.

Adding that extra fuel immediately fixed the pressure going lower and the noise from the fuel pump.   It was pretty obvious that this part of my issue was caused by the small hole in the bottom of the swirl pot being clogged.   This is a common problem in this era of Mercedes-Benz when they have sat for a while not being used.

I don’t have a non-copyright photo of the swirl pot to illustrate.  However, the function of the swirl pot is to prevent fuel starvation when the tank is below 25% and the car is being driven up or down a hill, or hard cornering.   Without the swirl pot, the fuel would slosh around in the bottom of the tank and the intake would suck in air.   Other cars use various baffles or a sump to achieve the same result.

The swirl pot looks like a little plastic flower pot in the bottom of the tank.   The fuel strainer sticks up into the centre of the pot.   When the tank is above about 25-35% capacity,  the swirl pot is always full because fuel enters from the top.   Once the tank starts to empty, the return line is angled so that it causes the fuel to swirl around the edges of the pot and suck in petrol from a small hole via a venturi effect.   This keeps the pot full, even with a comparatively empty tank.   I had this exact problem on my old 250SE coupe.   I ended up dropping the tank and cleaning the hole via long pipe cleaners.   The shape of the W111 tank makes it easier to see the swirl pot via the holes for the sender and strainer.

Right now I am going to ‘solve’ this problem by keeping the tank above 30%.   I may come back and look at this once everything else is sorted out.   To fix the issue properly I would need to remove the tank.

Running a static test (engine not running), my control pressure was 0.5 bar.   According to the manual, it should be 1.0 to 1.4 at 15c.   Starting the engine had it up to 1.0 bar, just in spec.   Plugging the WUR back into the electrical system, control pressure climbed to 3.8 bar.  This was at the high end of spec when warm.   Pulling the vacuum line also no longer stalled the engine.

At that point I wasn’t able to take a test drive of the car, as the 280SE was behind two other cars.   Today I went back and moved those other cars out of the way and took the 280SE for a test drive.   It was much better.    I wasn’t getting bad hesitation, at one point I got some very minor hesitation, but overall the car drove quite well.   This had me feeling confident enough to remove the gauge set and re-install the air cleaner – which I will do on a cold engine.   It feels like I am starting to get somewhere with this car.   Assuming its running well. my next step is to sort out the steering coupling and muffler.   There is a 116 event coming up I would like to take the car if possible.

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As I own two KE Jetronic cars and one K Jetronic, It made sense to purchase some fuel pressure gauges.   They were about $100 USD.  The test gauges plug in to the fuel distributor in two places.   The first plug (without the valve) goes into the test port on the side of the fuel distributor.   The second plug goes into the outlet for the cold start valve.     The gauge set comes with the right fittings to rig it up.

Immediately I was able to rule out the fuel pumps and fuel pressure regulator as my starting fuel pressure was good.   The test calls for two readings.   the first one is from the test port and the valve closed.    The second is with the valve open to the cold start fitting.   The ideal readings are 6.0 bar with the valve closed and 6.4 bar with the valve open.   What is more important than the actual values is the 0.4 bar difference.

As can be seen from the picture above, I had an 0.3 bar difference.   This meant the car was too lean – which is what I had observed on the dyno.   Most KE Jetronic cars have an oxygen sensor that controls the operation of the electro-hydraulic actuator (EHA).  The 560 ECE does not have an oxygen sensor.  In this case, the EHA can be adjusted manually.   It is the small back control unit on the right hand side of engine.

Once removed, the slotted screw is removed to allow for adjustment.  A small allen key is then used to adjust the EHA.  In my case, I adjusted it in about one third of a turn.  The EHA is then replaced, carefully to avoid disturbing the o-rings.

With the EHA adjusted and re-fitted, I re-ran the test.  This time, I saw 5.9 bar on the test port with a bit over 6.3 valve open.  Success! I now have the pressures in specification.  I’m happy to leave that bit extra over 6.3 – I would rather be ever so slightly rich than too lean.

These test were all done at idle.  Next thing was to check the pressures at some higher RPM levels.   I asked a neighbor to hold the car at 1,000, then 2,000 RPM.  The fuel pressures stayed steady at these increased revs.

I also ran another test from a member of BenzWorld.   Holding the revs at 2,000, I pressed on the air flow meter plate.  This tricks the car into adding more fuel.  If the engine is lean, the revs will increase, but if it is not, the car will bog down from getting too much fuel.   In my case, the car bogged down.  This was another sign the pressures were at the right level.

The final test was to check the readings of the potentiometer for the air flow meter.  This is on the other side to the EHA.  Correct spec is 0.7 volts at idle between pins 2-3.   My car was at 0.9, so very close even if not perfect.  The voltage also increased as I pushed down on the air flow meter pressure plate.

After these tests, all evidence points to having fixed the lean running condition of my car.  I don’t have a dyno to test it on, but the car is now adjusted back to the correct specification.    I found the KE Jetronic EHA adjustment to be fairly straightfoward, with the pressure gauges.

Update, 26/11/19:  While the car was in for an A/C regas,  I asked to have it quickly tested on the exhaust gas machine.   Air/Fuel was 14.8, so this adjustment was perfect.  

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