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Pete, Mercedes Mechanic
Category: Mercedes
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Experience:  Over 15 years experience as a Mercedes dealer technician
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Mercedes-Benz S500: 2003 S500 car starts fine when cold, wont

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2003 S500 car starts fine when cold, wont restart when engine is warmed up.
check engine light comes on, had a mechanic yesterday tell me it was the gas cap not sealing properly. cranks over, seems like its not getting any gas, the cap is on good and tight
advice please need to drive it home through the mountains today, 175 miles no cell phone coverage most of the way.
thanks Paula
Hello,I will be glad to try and assist you with this,Welcome to Just Answer.I may need more info in some cases to help with answering your question.

Do you know what code(s) have set with this check engine light and if so can you post the code number/info here?

Let me know,thanks Pete.
Customer: replied 4 years ago.

dude said the one code was saying there was some kind of fuel vacuum problem, he said it was associated with the gas cap getting old. resealing the cap seemed to fix it, check engine light went out, car started. but by then the engine was cool. seems to not re-start when engine is warmed up

O.k.,how long does the vehicle have to sit before it will restart again?

Do you hear a hum or buzz noise coming from the fuel tank area when the key is first turned on when it wont start?

Let me know,thanks Pete.
Customer: replied 4 years ago.

no sure on how long, first time it didnt start was last night, went out ran in to a shop came out 3 mins later, wouldnt start. decided to stay for dinner, in restaurant for 2 hrs, came out started up with check engine light on.

this morning started up, sounds a bit rough no check engine light on now?

my concern is getting home today,XXXXXthrough the mnts, sketchy cell coverage.

Do you recall if you heard a hum or buzz noise from the fuel tank at all the first few seconds the key is switched on when it wouldn't start?
Customer: replied 4 years ago.

can be sure about hearing a hum or a buzz coming from fuel tank.

what would that mean, i can try to get it in to the dude who saw it yesterday, not too sure if he is really qualified, so that is why i am seeking pro help

O.k.,there are two common components that can cause this type of fault here on your vehicle.This doesn't mean that either of these two are the cause but they are the most common.

First and most common is a faulty crank position sensor.This sensor is located on the drivers side of the engine below the drivers side cylinder head mounted to the block where the top of the transmission bolts up.

This sensor is common to heat up with engine temperature and when it becomes weak it will cause a signal loss.Then when you go to restart the vehicle it may not restart as this sensor is not sending out a signal until the vehicle has sat a period of time to cool down.Once cooled down it will usually restart but repeat the problem soon after or after it has been shut off.This sensor is around $100 from a dealer and a local parts store and about 1 hours labor to have replaced.When this sensor fails when cranking over the engine if a scan tool is plugged in and the engine rpm data observed on the scan tool there would be a 0 or very little rpm signal,this helps in identifying if this sensor is the cause.

The other component is the fuel pump in the fuel tank itself.If no hum or buzz noise is heard most likely this pump is failing needing replaced.This would need to be checked when the vehicle is acting up with a fuel pressure test gage threaded onto the test port on the engine to see if there is any fuel pressure present.

I would say with what is described here I would lean more towards a failing crank sensor as this is most common and fits what you have posted here.This problem should be corrected before driving the vehicle to far as it may fail all together and not restart.If it were mine I would replace this sensor.

Thanks Pete

is my profile if you need anything in the future on your vehicle here,you can save it to your favorites and come back if you have any future questions on your vehicle (besides this question here).
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I am following up with you to see if you still needed help with your question you had asked.If my help didn't satisfy your question please let me know so we can continue.

Thanks Pete