Hey guys! I just picked up a beautiful 2005 SL55 last month and have run into a big problem that nobody else has been able to track down for me.

So, I have a surge in the RPMs. It happens at idle and feels like a misfire. I also see the gauge fluctuate upon acceleration and also when driving with cruise control on.

It's about 25RPM's. Only feels like a misfire/shaking at idle. When driving normal speeds, can't really feel it. Here's an example. At idle, the car will go from 600RPM to 625, then back to 600 within 2 seconds constantly. Same upon acceleration. 2000RPM to 2025, and back down to 2000 in about 2 seconds constantly. Every once in a while I catch a break at idle and I will see the gauge stabilize, but not for long, and the car still feels rough even when stable.

The car is in great shape, but I think they put sub-par fuel in the car before. When I changed the fuel filter about 1,500 miles ago it was totally nasty and grey.

I have no codes whatsoever, and I do have a star diagnostic computer. Still no codes. Live-data of the engine doesn't give me any insight either. Fuel pressure bounces from 75-78PSI at idle and stays constant even when accelerating.

Along with the fuel filter I have replaced the air filters, fuel pump relay, intake Y-pipe, the PCV hoses, valve cover gaskets & breathers, pressure damper on the fuel rail, air filters, intake manifold gaskets, cleaned throttle body, & multiple runs of Techron fuel cleaner. All genuine Mercedes parts from the dealer. No impact has been made.

I purchased the fuel pump and the fuel pressure regulator located above the fuel filter but do not want to replace them yet as I would have to remove both axles to do so.

What do you guys think it might be? Where should I start poking around? Could the injectors be poorly performing from the sub-par fuel used before my ownership, or would it most likely be the fuel pump?

Thanks in advance!!!

 

What’s the mileage? Have you checked the Plugs/wires? Health of fuel injectors?

 

105,000 miles. I forgot to mention that yes the coil packs, wires, and spark plugs are new as well. Genuine Mercedes parts as well.

As for the condition of the injectors I have not pulled one off yet. I figured something like fouled injectors would have shown up as a problem on STARS.

I also checked for vacuum leaks, and replaced the problematic metal EGR vacuum pipe behind the driver-side intake.

 

It took me forever to track a vacuum leak under the SC. No codes but started with RPM surges and then I could hear a very loud noise followed by the codes

 

I really hope that’s not it. I’ve essentially attacked the engine with carb-cleaner while I was hunting down the vacuum leak from the metal EGR tube. No other vacuum leaks I could find.

The surge got sharper up and down when I replaced the fuel damper/regulator on the rail. It was more “flowing” of a surge before that.