I have been taking care of a friend's SLK320 while she is abroad. Basically all I do is drive it around for 30 mins once a week.

It has been about 7 months and now when I start to acelerate slowly, the car picks up in speed and then seems to stop acelerating, then starts to speed up again, then doesn't... It is not as though the car is rocking about but it is subtle and yet feels like I might not be able to build up much speed. And now the engine light has come on.

Any idea what might be the cause?

Thanks.

 

if the gas tank lid when open says "do not top off" and if you had it resently filled, maybe it overflowed into a vapor canister, that will cause you not to be able to go over 45 mph for about a 100 miles or so. I had that happen on my 230. I don't know if that was an issue on the newer 320 R170s. check the lid. if you haven't put new gas into it for awhile, like seven months, it could be bad gas. I had a fuel pump that went out on my sl that acted like that too. you'd need a fuel pressure test to check that.

 

Thanks for your ideas.

The car gas tank was about half full 7 months ago. A month or two ago I filled it back up to half a tank. So I guesss your first theory doesn't apply.

Your second idea - stale gas - is interesting. I asked two Mercedes mechanics if I should drain the gas and add new. Both said it was unlikely stale gas is the problem; they have only seen that as a problem with cars that have been sitting unused for a couple of years.

But I will add more gas a bring it up to 3/4 of a tank to see if that helps.

 

I just noticed the dashboard shows a diagnostic code when the car is started: 143. If that is P0143, then this explanation makes sense:

P0143 O2 Sensor Circuit Low

02 Sensor Circuit Low Voltage (Bank I Sensor 3)

The Catalytic Converter is used to help control emissions. The PCM (Powertrain Control Module) uses the signal from the oxygen sensor to the rear of the catalyst to monitor the catalyst's efficiency.
This is the bank 1,3 sensor. It's a four wire sensor being supplied a 0.5 volt reference voltage as well as a power and ground circuit for the o2 sensor heater element. There is also a signal wire from the oxygen sensor to the PCM that can vary between about 0.1 volts to about 0.9 volts according to oxygen content of the exhaust. A properly working post-cat sensor will exhibit small changes at a slower rate than front o2 sensors.
To over simplify: a post-cat sensor's main job is to monitor catalyst efficiency, not control fuel management (although they can have a measure of influence over the front o2 sensor's operation). If the PCM determines that the post-cat o2 sensor signal voltage is below a certain threshold for too long, P0143 will set. PCM detected the HO2S signal was less than 156 mv for 28 seconds.


Symptoms
Usually rear o2 sensor problems won't cause drivability issues since they are an input that measures catalyst efficiency (unlike front o2 sensors). However if you have other codes present, like lean codes or pre-o2 sensor codes, then they may cause drivability problems as well as P0143. The following symptoms may be present:

• MIL (Malfunction Indicator Lamp) illumination
• Engine performance issues
• Engine running rough
• Engine running rich
• Poor idle

Causes
An engine that is running lean could set this code, however there will likely be other codes present.

• Air leaks in exhaust in front of o2 sensor giving false reading
• O2 sensor connector damaged
• O2 sensor signal circuit is shorted to ground
• O2 sensor ground circuit is open
• O2 sensor has failed (it may be contaminated with fuel or coolant)
• PCM has failed

 

Where are you seeing this 143 code displayed.? never heard of that.