07/03/2026
The other day, this beautiful Euro market 500SL all the way from Southern California came into our shop.
It had a very peculiar problem that a few mechanics have tried to diagnose and fix, without any results.
As someone who grew up in the era of this car, and intimately familiar with the K and KE Jetronic fuel systems I was certainly up for the challenge.
The issue at hand was hard to diagnose because it was ALWAYS different. Normally the car would start up fine, idle beautifully, and would rev up no problem.
Sometimes it would barely start, and would struggle to rev up.
When it ran, it ran great, could be driven halfway around the block, but then it literally struggled to make it back to the shop.
The spark plugs were EITHER soaked, or dry. Did not make any sense
We checked fuel pressure, it was spot on, replaced the Warm Up Regulator (which was out of spec) that had a torn diaphragm. Calibrated the new one to spec with the pressure gauge.
Everything seemed great, until the first test drive. Then seemingly the same thing happened.
Now the replacement WUR was out of spec again!
Why haven’t we checked the fuel filter at this point you might ask. Great question. Firstly, we were told the previous shop had just replaced it, and it DID look new. Also, a clogged filter would not explain why the head pressure was so high randomly.
However as a final last straw, we did decide to inspect the fuel filter. To our shock it was draining rust when the lines were cracked and worst of all it was completely broken up inside. (See the last slide). This totally explained all the weird random failures.
When unblocked, it managed to send full pressure to the WUR. But upon driving , and bouncing around eventually it blocked the outlet. Also once the WUR was replaced it worked fine, until the rust blocked the fine screen, making it impossible to bleed off the excess pressure, sending full pressure to the fuel distributor.
After the fuel filter replacement, and re-cleaning and re-calibration of the WUR, the car ran amazing, a real joy to drive.
Now it’s just waiting for the right time for a MANUAL SWAP. 😁
