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Mercedes-Benz SL65 AMG - Report 003

Updated: Jun 24

SL 65 AMG running costs

After a three and a half month period of winter hibernation, I pulled the dust sheets off the SL65 AMG in mid-march and put it back into service as my daily driver. You may remember from my last update that my trusted mechanic @jhd_ltd was working on the car back then, chasing a couple of issues that the vehicle had developed - mainly an intermittent power loss, that felt a bit like heat-soak, and an intermittent stalling issue.

The car spent a couple of weeks in Jamie’s workshop, where we thoroughly overhauled the Chargecooling system, including installing an uprated header tank and a new BOSCH pump. The engine was smoke-tested and Jamie found a split map sensor vacuum line. When replacing this, he thought it prudent to completely replace all vac lines throughout. We also de-clogged and cleaned out the charcoal canister as, after hours and hours of searching the worldwide web, I found reports of another car in the US that had this intermittent stalling issue and a clogged charcoal canister turned out to be the culprit!

Jamie phoned me to say the car felt much better, and he thought it was back to full fitness. I collected the car later that week and it did immediately feel vastly improved. The engine felt so much smoother throughout the rev range and fuel economy has improved drastically - to the tune of about 50%! Also, the stalling issue had disappeared, which was great. However, I still felt like it wasn’t producing full power, and a late-night test with my draggy box confirmed it was still a good bit slower than it should be.

SL 65 AMG running costs

Contemporary road tests indicate the car should be doing 0-100mph in somewhere around 8.5 seconds, but my rolling 30-100mph tests were showing that, with a clean launch, this car would be achieving something in the 10’s. I was starting to feel frustrated at this point, and another chat with Jamie didn’t really help! He explained that he had done everything he could, he simply doesn’t have the diagnostic equipment necessary to dig any further. These engines and their electronic management systems are so complex it really needed to go to a specialist. So I did nothing else about it for a couple of weeks, as the car was still absolutely lovely to waft around in and, let’s be honest, wasn’t exactly slow.

I then realised that it was due a service and promptly booked the car in with my local main dealer. I normally wouldn’t trust a main dealer with any repair work outside of servicing, but I thought what the hell - it won’t do any harm. And so, the day before the service, I dropped them an email explaining this power loss issue and a detailed list of all of the work Jamie had done with the car since I’d had it - asking them if they wouldn’t mind plugging it into their diagnostic system and seeing if there was anything they could find. I must admit I didn’t have a lot of faith that they would find anything.

However, when I collected the car a couple of days later, to my amazement, the guy at the service desk explained that they had done a thorough diagnostic test, and found an intermittently faulty boost sensor….hallelujah!! The dealers had replaced this sensor with a new one (which cost me £72) and it has transformed this vehicle. The mighty SL is now running as it should, and performance is eye-watering for such a huge car - 2.1-tonne vehicles have no right to feel so absurdly fast!

SL 65 AMG running costs

This outrageous straight-line punch is at its most vivid from a 70/80mph roll-on, when - in the dry at least - traction is not an issue. The way this thing pulls when you open the throttle fully from that kind of speed is hilarious and completely unnecessary! This increased power (my butt-dyno reckons it gained around 150-200bhp from replacing that sensor…) is exposing the car’s chassis flaws somewhat, however.

When full boost and 1000NM of torque arrives in second or third gear, the car feels like its chassis twists, the rear end lunges and sleuths around, as the car’s structure tries desperately to contain the fury that its power plant is delivering to its back tyres. The rudimentary traction control system aggressively cuts in to try and control the chaos, but does so in a very blunt manner - meaning it’s better to drive with the DSC switched off, or even better with the car in ‘dyno-mode’ which is accessed by a specific sequence of dash buttons and disables the system altogether.

Following this triumphant return to full fitness, I thought I would also try and allow the car’s monster V12 to have a bit more of a voice, and booked it in with @chis_tech for a backbox mod. I am delighted with the results, which have seen a subtle increase in volume along with much more pronounced spooling sounds from the turbos. There are some videos for you all to see for yourself on my Instagram page @pissed_on_petrol


Date acquired - Sept 2021

Total mileage - 51,437

Mileage this ¼ - n/a

MPG - 15.1

Expenditure - £1950 (service, exhaust mod, repair work)


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