2007 CLK550 with 60k miles. Runs great. Never an issue.

Drove car for about 1 hour. It is hot here (90+). Parked it and it sat for about 3 hours in the early evening. Came out around 10:00 and no start. It cranked but no start. Went inside and did some reading online and it seemed like it could be the CPS. Engine was still warm. Took a hose and ran cool water down the drivers side firewall area (location of CPS) for a few minutes. Got back in and on 3rd or 4th try it came to life. Drove it home 1 hour and all was well.

Next morning I took wife's car to MB dealer and got a genuine MB crankshaft sensor (needed my car) and went home. Installed sensor and it fired right up.

Later that day I took car out for another drive of about 1 hour. All ran well. Parked it for about 2 hours and it re-fired right up. Drove home about 1 hour and parked it for about 3 hours. Came out to take a nice drive out for dinner and back to a no start (cranks but does not start).

Had dinner (about an hour) and came home and it fired right up.

Tons of reading tonight and everything suggests the sensor. There are no codes. When it starts it runs like a dream.

Could sensor #2 also be bad? Something else?

Read a ton last night trying to educate myself.

One video suggested you could test the sensor by using heat and checking for continuity. So I took the old sensor, used my tester between two of the three prongs and when cold it read1 (on the 20k scale....meaning no continuity). Used my heat gun and warmed it up and it started dropping like a stone after about 1 minutes of warming it up. Eventually it got down to about 98 (from 2000) and using my infrared sensor it was about 220 at its hottest part.

Pulled the "new" sensor out and tested it and it did the same thing.

So...both sensors good and I am chasing my tail? Or the new sensor was bad as well? The symptoms were the same with either sensor.

So this morning I went out and got another OEM Mercedes sensor thinking maybe I got a bad one the first time. Installed it and the car drove great. Even changed the coolant on it.

Drove it around for about 45 minutes and all was well. Parked it, came out after about an hour and it started right up. Went and mowed the lawn. Came back about an hour and a half later and no start/ This time I had my OBD2 scanner on it and it was counting rpms while cranking so I guess its not the sensor.

Thought about swapping relays for the fuel pump. Swapped one from the front SAM and guess what...it fired right up. Thought maybe I was onto something.

Drove to the grocery store (about 15 minutes), shopped for about 20 minutes, came out and guess what.....no start. So this time I went inside the grocery, bought a ice cold water, came out and tired starting it (it wouldn't start) so I poured a nice cold bottle of water on the ecm. Got back in the car and it started right up!


Drove home, came inside, checked emails, posted some updates, and went back outside with a cooler and a cold bottle of water. Got in and it started right up. Drove anout 20 minutes. Went inside to shop for about 20 minutes. Armed with a cold bottle of water, came outside and it.....fired right up.

So now its sitting outside and I will let it sit for about an hour and see what happens.

Would love some guesses.

My experience is that there only two things not connected to a computer that would give fault codes: the crankshaft sensor and the fuel pump.

That said, can you check the fuel pressure? You could try it the dangerous way and loosen a fuel line and see if gas leaks out.

Several MB models powered by the M273 V8 of that vintage had issues with the engine control units failing under the conditions you're describing. I've diagnosed them by using a heat gun to get the control unit hot even when the engine is cold. The problem is with the signal to activate the fuel pump...........