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I have a 1991 Mercedes 190E (gasoline, not diesel), and while it's still a reliable car, it has a strange problem with starting the engine that left most people I asked puzzled (including mechanics), so I'm posting the situation here hoping that someone might have experienced something similar.

If I try to start the engine after not using the car for at least a day, or if I start it up within a few minutes of stopping it (e.g. dropping a friend off or waiting for a train to pass), it's all fine.

However, if I try to start it within a a few hours (but less than a day) of stopping it (typical scenario: leaving it in a parking lot when going shopping, to the cinema etc.), I hear the starter "buzzing" and all, but the engine fails to start up. It usually takes many-many tries, to actually get it started. I've found that it helps if I step on the accelerator pedal while turning the ignition key, but even then it takes a few minutes (and a few key-turns) to get it running. While it never actually left me in the parking lot, I'm very afraid of this scenario so I'm using it less and less for shopping. I've tried taking it to a mechanic of course, but they haven't found anything.

Has anyone experienced or heard of anything similar? If so, what was the solution?

It might be worth noting that while the battery has been replaced quite recently, so it should be reliable, we routinely use a nonstandard "master switch" to cut the car's circuit from it, to make sure it doesn't drain unexpectedly. I don't know if this is relevant to the issue.

(Sorry for my limited mechanical vocabulary, I rarely talk / write or even read about cars honestly.)

  • Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Oct 2 '18 at 18:48
  • I'm not clear on exactly what happens when you try to start and it fails. Is the engine turning over, but not starting? Or is there no engine movement at all, and just a clicking or buzzing noise? The former would suggest a fuel problem, the latter more likely electrical. If the engine is turning, you should hear a rhythmic sound and feel some vibration. – barbecue Oct 2 '18 at 21:44
  • I think there's engine movement, but I'm not sure. Probably "buzzing" wasn't the best word to describe the sound. I tried to find a similar sound online, and the one you hear at around 2:00 in this video is what I usually hear: youtu.be/6ulUl0lHors?t=120 Do you think the engine is turning over in this case? – Dániel Kis-Nagy Oct 2 '18 at 22:23
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I suspect a bad check valve is allowing the pressure and "head" of the fuel tank to bleed off, requiring a lot of substantial fuel pump work to get the air out, eliminate cavitation at the pump, and reestablish sufficient fuel pressure.

Try cycling the key on and off a dozen times, without actually turning to the "start" position. Just ignition on, wait ten seconds, off, ignition on, wait ten seconds, repeat. Then attempt a normal start.

If the car then starts immediately, investigate the check valve.

Also, before starting, if you hold the accelerator to the floor, this probably initiates a "flood clear" mode. This prevents injection, but allows time for the fuel pressure to build. After a few cranking tries, no more than 10 seconds long each, attempt to start normally.

Again, if the vehicle starts immediately, suspect a bad fuel return check valve.

While I have not specifically investigated this for 1991 190E, the symptoms you describe suggest a fuel pressure/flow problem after a long rest period.

The fuel check valve is screwed into the fuel pump body on the underside of the vehicle, I believe. It's an inexpensive part ($20 ?) if you don't get the OEM one from a dealer - which is probably a few thousand Deutschmarks...

Another possibility is a bad fuel pressure regulator, which has a ruptured diaphragm. This allows fuel to leak into the vacuum control line over time, creating a flood condition. If the first test I suggested is unsuccessful, try the second test. If that works, you are clearing a flood condition and the fuel pressure regulator is more likely the culprit. You could also simply disconnect the vacuum line at the fuel pressure regulator and see if there is any liquid gasoline. There should be NONE. It might smell like gasoline, but there should be no visible liquid.

These are simple DIY tests you can perform in your driveway. The professional route would be to connect a fuel pressure gauge and observe the readings while starting, as well as after turning the engine off. But not many of us have access to a good [expensive] fuel pressure test set with all the adapters, especially Euro/German...

Best of luck. Let us know what you find.

  • Thanks very much for the comprehensive answer, I'll try working through this checklist next time I experience the issue! I'll let you know which case it was. – Dániel Kis-Nagy Oct 3 '18 at 8:13
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Some update on my situation, hoping it will help Carlos Thomas Murphy and others.

During the past few weeks, I conducted the checks that SteveRacer suggested above and I decided to replace the fuel check valve based on that. Sadly, I didn't actually manage to do so yet (basically my car spent the last 2 months in service for unrelated reasons – had rusty body parts replaced etc. – and they forgot this one request of mine, so I'll need to visit them again), but until that happens, this "workaround" worked for me so far every time:

"[...] if you hold the accelerator to the floor, this probably initiates a "flood clear" mode. This prevents injection, but allows time for the fuel pressure to build. After a few cranking tries [ignition on, wait ten seconds, off, ignition on, wait ten seconds, repeat], no more than 10 seconds long each, attempt to start normally."

Sometimes I don't actually hold the accelerator but "pump" it with my foot. After a few tries, the buzzing turns into audible (but weak) cranking, and after even more tries, the engine starts normally. At that point, be sure to keep the accelerator pressed (if you let it go it will die again), then let go of the clutch and get going. If you drive for a few minutes, it usually gets into a stable state.

Wonky, but it gets me out of the parking lot.

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