Hi there, first of all thanks! I have a ML320 1999 with 105k miles. Has big problems starting when temperature drops below 6 Celtius. I was told it is due to the Oxigen Sensors (asked price 550£ or 1100USD to replace). What are these sensors and where can I find them on the engine? I'm a flight Engineer on helicopters, would like to give 'em a look, maybe clean them before such an expensive intervention. Are they easy to locate/replace? is the price correct? Thanks a lot for any information on this topic.


[IMG]local://upfiles/31739/77C1CA753B7A4483B2D75479E6EEB822.jpg[/IMG]

 

A starting problem doesn't sound like O2 sensors to me.
The O2 sensors check the O2 level in the exhaust and feed back to mixture control ECU to keep within specific range.

There is however an external temp sensor which determines ambient since the colder it gets the more fuel is required to help the engine START. The ambient temp also influences the fuel map that is used.

Next step. Is your servicing up to date. Filters, plugs etc.
Ha=d any other problems?
Get the diagnostic codes read. Basically any good garage although some places will apparently lend you the scanner to DIY. Plugs into a socket under the dash and takes about 1 min to do.

The codes define what systems are not operating correctly. This helps us in deciding what might be causing the problem. It does not necessarily say "Change Plug No. 1" or similar. More likely Misfire.

Changing parts without good reason (i.e. data) is expensive. Don't use cheaper generic parts. MB's are very fussy about having genuine or at least spares made by the original equipment manufacturer. i.e. Bosch.
FYI the O2 sensors are fitted in the exhaust downpipe upstream if the small catalyst cans which are also positioned in this location. dependent upon territory you may also have a further catalyst downstream of this same cat. So if V6 which I believe yours is you will have either one per side or two per side.

OK?

Stuart

 

Thanks Stuart!

the next service is due in 4100 miles and I have always taken great care of the car. The Mercedes authorizedmechanic that did the test, said it was the O2 sensors the only error that came up... but will pass by again to read the codes and let you know.

I Had exactly your same impression that there must be a temperature sensor that has gone crazy and does not give theright mapping for the cold start of the engine.

Every other start is smooth and nice. I have a consumption of 210 miles per tankwhich must be around 14.5 miles/gallon. The ride is always smooth. Only once, I speed tested the car and pushed it on a safe track at almost 120m/h which really impressed me... few puffs of black smoke came out during acceleration, but nothing else to worry.

Can you please help me identify the temperature sensor? where is it (somewhere on the air intake conduct?)... should I try changing this first if less expensive?

Thanks