K-Jet fuel pressure test part 3

This is part three of a series where I am trying to diagnose poor cold starts on my 1979 280SE.  Please click here for part 1 and here for part 2.   In the last part, the fuel pressure tests had pointed to high control pressure.  This would most likely be cause by the Warm Up Regulator (WUR).   However, after a while the pump became very noisy, and since rebuilt WURs are expensive, I wanted to rule out a bad pump.

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.