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.

The post K-Jet fuel pressure test part 4 appeared first on Classic Jalopy.

Luckily I had a 420SEL parts car on hand.  The previous owner told me the fuel pumps were almost new.   They were one of the last things tried to get the car running before it was taken off the road.   I removed one of these pumps and installed it on the 280SE.   The pumps are actually the same, its just the packaging that is different, as the older pump comes with a rubber sheath.

At least starting the car on the new pump didn’t change any symptoms, but the car was quieter.   Today I went to run the fuel pressure tests again, to see if the pump change had any impact.

My first test was to test the fuel pressures without the engine running and the fuel pump relay bypassed.  The WUR electrical connection was unplugged.  I got:

• Tap Open: 4.9 bar (Control Pressure)
• Tap Closed: 5.3 bar (System Pressure)

These were basically the same values as I got last time.   However, after running these tests for a few minutes, I got the same noisy fuel pump again.   I’m wondering if there is some other blockage in the system causing this that is also impacting my pressures.  In any case the control pressure is way too high.

Last time I also did the test with the car running and got:

•  Tap Open: 5.0 bar
• Tap Closed: 5.5 bar

Then, plugging in the WUR electrical connection and running the tests, I got 4.9 bar with the valve open and it slowly grew to 5.2 bar after about 3 minutes.   Unplugging the big vacuum line from the top of the WUR caused the car to stall immediately.

I removed the WUR to take a look, and it definitely looks like a previous owner has installed a rebuilt unit.   I looked up the number in the technical data book and it is the correct unit.

It seems clear my cold control pressure is a problem.  However, what is not clear is the fuel pump noise a symptom of that, or a symptom of some other problem that is manifesting itself as cold control pressure?   A rebuilt WUR is quite expensive, so I have taken the one I have to a local Benz mechanic to have a quick look to see if there is anything obvious.

I feel like I am narrowing down the problem, but have not yet got to the root of it.

The post K-Jet fuel pressure test part 3 appeared first on Classic Jalopy.

The readings made more sense this time.   System pressure seemed within specification.    Control pressure on the other hand was too high.    The warm up regulator also did not behave as it should.    The control pressure stayed high, and when the electrical connection was plugged in, it just got higher as the engine warmed up.    Once the engine was at operating temperature, removing the vacuum line stalled the engine. Noisy Pump

I’m not going to bother posting the readings here, as I plan to re-do all these tests.    After I stalled the engine with the vacuum line test, I noticed how noisy the fuel pump had become.   The longer I ran the tests, the noisier the pump became.   In addition, as well as getting noisier, the pressure started to fluctuate and the engine started to run roughly.

Previously, I ran the fuel volume test.   The test passed, but I still think the pump is on its way out.   The more I go over this car, the more I think it has sat for a while, and these things just don’t like sitting.  I wish I had heard this before I changed the hose and filter, would have been so much simpler to do it at the same time.

I don’t think changing the pump is going to be the fix for my issues.    It looks like I have a high control pressure issue even with a dying pump. The tests still failed when the pump wasn’t noisy and was passing the volume test.  The gauge was also pretty steady at that time.  However, It seems silly to go and spend a lot of money on a rebuilt WUR before fixing the obvious stuff and running the tests again.

I have a couple of spare fuel pumps from my 420SEL parts car.   According the previous owner, they were replaced just before the car went off the road.   I’ll try one of these and re-test.

The post K-Jet fuel pressure test part 2 appeared first on Classic Jalopy.

However, when I needed to start the car and move it out of the way it was back to the same stuttering and lack of power.   I figured that perhaps my issues were something to do with the cold start system.    The next step was to run a K-Jet fuel pressure test to isolate the problem.    I’ve run these tests on the KE-Jetronic system, but regular K-Jet is a little different.    I found three resources that helped me here.   This page,  which gave really easy to read instructions was a good starting point.   I also found the Mercedes-Benz Continuous Injection System (CIS) Diagnosis & Adjustment – (1976-1979) Manual very useful, as well as the M110 engine workshop manual.

To perform the K-Jet fuel pressure test the first step is to connect the gauge set.   The K-jet system doesn’t have the test port like KE-Jet.    From what I could see, the gauge set is plugged in between the fuel distributor and the warm up regulator.

The outlet looked like it went into the top of the fuel distributor.    The other side went into the warm up regulator.   The instructions called for it to go into the larger fitting.  However, I had one metal fitting and another one looked like a flexible hose.    I kind of figured it was probably the metal fitting.   At this point I also had to disconnect the electrical connection.

Once I bypassed the fuel pump relay, I was able to test the system with the help of a friend.   However, while the gauge started registering pressure, I was getting a steady stream of fuel from the disconnected metal line at the warm up regulator.    Tracing this back to where it came from, it looks like it is coming out of the return line from the warm up regulator.

I figured that I probably have the lines mixed up, but I find it odd that petrol would be coming out of the return line.    I wonder if this if this is normal or points to some fault with the fuel distributor? Before I go any further it is worth investigating why fuel would come out of this line.   I also need to check if I have the lines back to front.

Since I am not sure if the lines are connected properly, the results may be meaningless, but I registered 5.2 bar on the test gauges.

Update 22/2/22: Looks like I had the wrong port on the WUR.   I will need to change this and re-test.  

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