Costco Premium Gas + Engine problems
 

For the last 3-4 months, I've had problems with my 2015 S550 with only 11,000 miles on it. On Park while in IDLE the engine sounds slightly rough & louder than normal and the car shudders slightly. When I accelerate on Gear 2 (rough throughout Gear 2 with my feet on the pedal) & the beginning on Gear 3 the engine sounds & FEELS strained and rough (almost truck like). I've taken the car to two different dealerships. The service advisors initially admit they can hear the the slightly rough engine sounds when I first show them the problem and take them on a ride in my car.

But inevitably the techs (including the shop foreman) claim that that there is no problem and it sounds normal to them after they run their diagnostics. My service Advisor now claims that its the exhaust/muffler that is making the noise due to the gas I use (Costco Premium gas) and that I should put some engine additives and switch gas brands. Does this sound right to anyone? Anyone else use Costco brand Premium gas?

I use Costco hi test all the time. No problems to report.

We've been using CostCo premium fuel in both the E400 and the GLK since new (almost 3 years now). Absolutely no problems. CostCo pledges to sell only top-tier gas that meets or exceeds all manufacturers' specs. Of course, it is always possible that your particular CostCo has water seeping into its underground tanks or other such problems. Can't hurt to try another name-brand, first without any additive, then if the problem goes away you know it is the gas. If the issue remains after a couple of tanks of name-brand gas, then I would try the additive (something like Techron - ask the Dealer which one they use in the service department). If name-brand gas with additive works, you can either stay with the name-brand but stop the additive, or go back to CostCo. You may have deposits or contamination in your fuel system, and the additive may help clean it out. If so, then you can drive for quite a while before it builds up again. At that time, just add another bottle of the magic additive and keep going.

I remember that with my W201, at each service the dealer service department always added a bottle of Techron to the fuel. I never had an issue, and that car ran perfectly for the 137K miles I drove it.

I can also vouch for CostCo gas. I have been using it on all my vehicles since new, and never ever had any problems. Even with the infamous BMW F10 with N63 engine. I do use a bottle of Techron for the second from the last tank before any planned oil change though. And that one I can also vouch for.

I forgot to mention that i am getting about 13.7 mpg city/highway mixed. That seems low to me as well. what kind of MPG are you guys getting on your cars?

If my math is correct, you are putting less then 3,000 miles per year on the car. (MY 2015 + 11,000 miles) Could you be making only short trips whereby the car is not getting up to full operating temp? If so, this could be allowing deposits in the engine (intake/exhaust valve train?) that would normally be eliminated by extended full temp running. Try some kind of fuel additive for cleaning valves and give the car some good runs.

I was about to say it sounds like he just needs to get it out and open it up really hard. Drive it like a race car and drive it on a long trip. It might be gummed up.

The issue is not the Costco gas. I have been using their fuel for over 10 years in all on cars, some with superchargers and turbos and no issues reported...

My SA gave me that same BS when I had trouble with my E350 restarting after a fill-up. The root cause was a vapor vent valve problem. I've been running Costco for a long time without any issues other than that day. Most MB SA are simply salesmen who haven't a clue how these cars work.

My 2014 had a couple of fuel injectors go at about 5000 mi. Some manufacturing issues I don't know for how long, maybe a TSB on your car. There is a TSB thread.
Usually/maybe a check light results

COSTCO fuel is Spot Market fuel and is probably the same stuff you get at any name brand fueling station.