Hi, I'm new to the forum but a long time reader. In 2017 when I was stationed in Germany I bought a used (like new) 2013 GL550 X166 with about 65k miles on it. I believe it was originally sold to someone in Canada and they imported it to Europe. We needed the space because my mother-in-law lived with us and we liked to travel Europe and uhh..... Twin-Turbo V8+Autobahn.....!! We've had little trouble with it aside from the Soft-Close doors going out within the first year. Fast forward to today and I no longer live in the city so my car isn't as neat or clean as I would like it but it is what it is. It has about 130k miles on it now and is beginning to show its age. I typically only drive it in winter except to get the fluids flowing in it once in a while. I've had a few problems creep up and I have ideas on how to fix them. I'll list the problems and then my plan as it is now. If you have any input, advise, wisdom, experience to share please do so because that's what I am after.

About 4 years ago I began getting an intermittent CEL for a cylinder 5 misfire. I'd sometimes have to clear it but many times it would go away. As it has increased I noticed that it does this when idle more than any other time. Once, the car started right up but didn't have all 8 firing and I thought it was a goner. I have since been running fuel system/injector cleaner through the fuel system and that seems to help prevent the issue for a while (a month?) but then it comes back. Today, I was pulling my climate control head unit to replace it and I decided to pull the engine codes. I didn't have any active codes but I did have stored codes which were P0300 Random/Multiple misfire, P0305 Cylinder 5 misfire, P0301 Cylinder 1 misfire, P21A0 Unknown (emissions??), and P00A8 Low Pressure System Pressure Too-Low.

Last year we put a new fuel pump system in the fuel tank and had a crimped hose fixed because the vehicle couldn't tell when it was going to run out of fuel and it hesitated occasionally. Toward the end of last winter, I noticed a clicking in the front end, especially during cold start up. When I did my last oil change, I ran oil system cleaner through the engine and I noticed no difference in the sound leading me to believe this was not related to lifters. After doing much reading, I see that timing chain tensioners can be a cause of this clicking in the front end. So, given these two problems, here is what I think the best course of action is-

1. Replace both high pressure fuel pumps since I have a cylinder on each side misfiring (typically throws code at idle).
2. Replace the fuel lines that lead around the rear side of the engine using a pre-packaged fuel line kit.
3. Inspect/replace both timing chain tensioners.

I have little experience working on Mercedes vehicles so I feel like this will be a bit of a challenge but one that I can complete. There are quite a few online tutorials and videos on how to do these things. I haven't found the BEST one yet though so if you have one to share I would appreciate it. Please, tell me what I am not thinking about, what I should try first, what trouble I might run into, what risks I'm not seeing are. Considering that I'm on a budget, is there anything else I should replace while I'm in there? Any input you have is greatly welcomed. Thanks.

 

Your engine most likely toast. Cylinder 5,1 - most likely burned exhaust valves, plus do cylinder scope for scoring

 

Oh no. Say it ain't so. IF this is the case, aside from babying the engine, is there anything to do to get the most longevity? Thanks for your response and thoughts.

 

Don't spend any money on useless shiet. Do Xentry dynamic compression test to rule out issues. If you have compression loss - well, either invest and get it fixed, or keep driving till wheels fall apart.

 

Thanks for the great advice.

 

Question- is it a MUST to use Xentry dynamic compression test or will a standard compression test give some desired results?

 

I’ll be doing the scope this Saturday.

 

Dynamic faster and don't need to pull spark plugs which is a task

 

Understood. Thanks!