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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22919
Experience:  BEng hons Mech engineering, in auto industry 22 years
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1996 r129 mercedes sl500 , been sat for a few months, went

Customer Question

1996 r129 mercedes sl500 , been sat for a few months, went to start it, and it started fine but when it gets warm it stalls and will not restart. There is a strong smell of petrol. Once it has cooled it starts fine again.


edit: its already had a new wiring harness, work done around 2 years ago.

Submitted: 5 years ago.
Category: Mercedes
Expert:  Matt replied 5 years ago.

If you've got a strong petrol smell I'd 1st suggest checking under the bonnet for any signs of a leak - especially around the fuel injectors and their manifold rail and the associated hoses as fuel leaks are pretty dangerous and can also result in the engine not running correctly

If this is all Ok and there's no sign of a fuel leak then I'd suspect that there's incorrect fuelling due to a faulty water temp sensor. If this is the problem then the ECU will be leaving the car constantly ' on choke' and not reducing the fuelling as the engine warms - so unplug the sensor and clean the contacts on both sides with some switch / electrical cleaner ( not WD40) and inspect the wiring for any signs of damage to the insulation especially where it runs over other components and through the bulkhead- if this doesn't help consider replacing the sensor as they're not expensive
Customer: replied 5 years ago.

Hi, I did check for leaks but the smell is only there when the engine is running, probably becuase it is very rich like you said. I will go ahead and check the water temp sensor, do you know roughly where it is located on this engine?. I guess there is a water temp sender that goes to the temp gauge and a seperate sensor for ecu...





Expert:  Matt replied 5 years ago.

the water temperature sensor shoudl be located near to the coolant top hose exit

so if you follow the top hose back from the radiator and look around the point where this joins the engine you shodul be able to locate the sensor

to my knowledge there's only the 1 sensor which should be a 2 wire type
Customer: replied 5 years ago.



I have located 3 sensors next to each other where you said. I cleaned all 3 but it made no difference. 1 of them I know is the sender because when i unplug it the gauge stays on zero.


I am a bit confused to which one of the other 2 sensors controls the fuel mixture/choke...

1 has a round connector with 4 prongs, the other has a smaller connector with 2 prongs, when i unscrew either of them water drips out, so there both going into coolant.


here are pics of the 2 sensors....




both describe them as coolant temp sensors so its confusing...


I guess 1 of them might control fan switch but that's just a guess.


Do you know....?



Expert:  Matt replied 5 years ago.
Hello Luke

its the 2 pin one with the blue plug that is the ECU sensor

If you disconnect it and run the engine without it, the ECU should default to a standard warm engine setting so its worth trying that before replacing it
Customer: replied 5 years ago.

Hi, ok I unplugged it but it made no difference. Still stalls when warm and wont restart, greyish/white smoke that smells strong of fuel out of exhuast like before.

Expert:  Matt replied 5 years ago.
Hello Luke

in that case check over the condition of the connector plug for any corrosion and the wiring for any damage as if the wire is shorted out anywhere this will effect the ECU
Customer: replied 5 years ago.

Hi, there is no corrosion, the wires and connectors look clean and almost new. Just a thought would a crankshaft position sensor cause this problem?

Expert:  Matt replied 5 years ago.
Hello Luke

yes you're right a faulty crank sensor and its probably time to go back to basics a bit1st check for spark by removing one of the spark plugs and plug it back into its lead and rest it on the metal of the engine ( DO NOT hold it on place by hand) then have your assistant turn the engine over whilst you watch the plug. It should give a fat blue spark at regular intervals.

If not then check that there's power to the coil pack (at the other end of the plug lead) using a meter. If there’s 12V at the pack but no spark then check the ECU power supply (fuse and relay) and also the connections to the crankshaft speed sensor.

If there is a spark and its still not starting then its likely that the ECU isn't opening the fuel injectors, if the injectors are working you can hear them click - its difficult to hear them with background noise so use a long screwdriver like a stethoscope, placing the handle end in your ear and the metal tip against the body of the injector whilst your assistant turns the engine over.
If you can't hear anything check the ECU power is present by measuring voltage at the fusebox and on its relay socket if OK then check the connections to the crank speed sensor and consider replacing this sensor if the wiring / connections look OK

If the car will start using a squirt of quick/ easi start into the intake then this confirms the problem is lack of fuel

Also its worth checking that the electric pump that feeds the injectors is working correctly, it should run for a few seconds then switch off when you first turn the ignition on. If you remove the filler cap to the fuel tank and listen inside while your assistant flicks the ignition you should hear it clearly
If the connections to this are corroded or damaged then the pump could stop at any time. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse, ideally measure the voltage at the pump and check its getting 12V.
You should be getting around 3 bar measured at the injector rail check to see if the plugs are getting wet indicating fuel is flowing – if not and the injectors are opening then there’s a problem with the pump or the filter or even the pressure regulator

Customer: replied 5 years ago.

after spending a lot of time doing what you said, was not getting anywhere, In the end i search online and found the answer it was moisture build up in the dizzy caps, making the spark arc when warm. Cleaned out moisture and now running fine.

Expert:  Matt replied 5 years ago.
Hello Luke

well done for finding that one and thanks for letting me know