2001 R129/M112 SL 320 misfire developed after battery renewal

[PAT722]

Hi, the recent cold spell did the battery in on my SL320. Failed to crank, charged for the day, cranked fine, failed to start when left overnight in garage with outside temperature around minus 5. Bought a 100aH Exide 5 year guarantee battery and all was good. I did not drive the car on the open road until last weekend, just regular starts and moving in and out of garage since battery replaced. I have a misfire after about 2000 revs, starts and idles perfectly and was not noticed when just moving car in and out of the garage. Last weekend, when the sun was shining and the roads were dry, I took it out and struggled to get above 3000rpm. Could the change of battery be to blame, e.g. could the alternator be struggling to keep the new battery charged as well as maintain a constantly high level of power to the ECU / ignition system? All plugs are good and only done around 600 miles since MB main dealer service although this was 18 months ago. Any ideas what to check and in which order would be appreciated. Thanks in advance.

 

[Wearsthefoxhat]

Hi, I would book the car into a good MB independent for a diagnostic report.. In the long term it will probably be cheaper than guessing and replacing parts. Not sure that a battery swap would cause this fault..

 

[PAT722]

Hi, I would book the car into a good MB independent for a diagnostic report.. In the long term it will probably be cheaper than guessing and replacing parts. Not sure that a battery swap would cause this fault..

Thanks Wearsthefoxhat - not confident it would get to the nearest one around 15 miles away. There are no codes thrown on the OBD2 scanner so cannot even pinpoint which cylinder(s) are problematic.

 

[Smiley]

Battery swap is not the cause. You say that you have started the car up several times since and not driven it - how many times and how long did you have it running for each time? If you have not allowed the engine to warm up properly then you may have unburned fuel in the catalytic converter or gummed up sensors. I would run the car at 1500 - 2000 rpm for at least 20-30 minutes; ideally take it for a drive to get it nice and hot and see if that fixes it. If it does not fix then it is time to get it on a diagnostics.

 

[PAT722]

Thanks Smiley. After replacing plugs & leads and driving 50 miles after 30 minutes warm-up the SL is back to it's 'normal' grand tourer self, i.e. a bit lethargic when compared to the EQC400 but that's a different story . Not sure if replacing the parts actually did anything but grateful for the return to the norm!