Originally Posted by SSB3435

so what is missing for cyl #8 to work at cold start?

Originally Posted by John CC

I only mentioned that because when preforming a leakdown test it is no more work to identify the source of the leakage, and that would give you one more data point to confirm the diagnosis.

Originally Posted by John CC

About 5 years ago we had a brand new vehicle. Within about 500 miles it started throwing a misfire code for one of the cylinders. The dealer tried a compression test, new coil, plug, injector, etc, all to no avail. Finally they put a new cylinder had on it and it's been fine ever since.

Did your mechanic scope any of the other cylinders to see if they looked the same?

Originally Posted by SSB3435

Dear Forum Members,

I used to read MB World forums for years but this is the first time that I feel hopeless and need help...

I have a 2018 GLS550 and around 20k miles, a very weird misfiring problem started. This is what happens:

- Sometimes (30-40% of cold starts), engine starts with immediate misfiring at cyl 8 after cranking, check engine light blinks and stays on, turning off/on does not help, after driving for 2-3 mins and engine gets a bit warm then I turn off/on the engine, everything turns back to normal, car runs perfectly smooth with all cylinders on, no performance issues. Even sometimes, check engine light goes off too!
- The main problem is the misfiring when driving. A very hard shaking starts if I drive at high gears (6-7-8-9) with high load (more than 50% of throttle) and engine rpm between 1,800-2,200 rpm. Under these conditions, an immediate shaking starts (not everytime but 60-70% occurrence), check engine light starts blinking and stays on, engine control unit shuts off the misfiring cylinder and everything comes back to perfectly normal if I turn the key off/on and restart the engine. When I check the fault codes, I see 100% cylinder 8 misfire and 75-80% of the times cyl 7 and 6, and very rarely cyl 1 and 4.

To overcome this very annoying problem, I tried these:

- All spark plugs have been replaced, ignition coils replaced, nothing changed.
- All injectors sent to Bosch center, cleaned and tested, reinstalled, nothing changed. If I change the position of the injectors and recode them for new locations, still the same old cylinders misfire. Nothing moves with injectors.
- I opened up the engine wiring harness top section, checked and rewired all injector/ignition coil wires, common +12V and ground points. Nothing changed.
- Cold compression test done. All cylinders are around 215-220psi and above, very very good. No problems seen.
- All 8 cylinder tops and combustion chamber checked with flex camera, cyl 8 has more carbon deposits on the piston than the remaining 7 but nothing seems severe. I removed the complete intake manifold, cyl 8 intake port seem more CLEANER than the other 7 but all of them shows heavy carbon deposit, more or less. SO I decided to clean the intake ports and valves. With brake cleaner and thinner, I removed all carbon deposits from the ports and valve bodies. Problem is still there.
- I replaced all 4 knock sensors, nothing changed. If I drive the car with knock sensors unplugged, the problem is 90% goes away but performance goes significantly low, naturally.
- I test drove the car with Xentry connected, low/high fuel pressures, intake and boost pressures, engine temp, 4 camshaft angles etc all values are in the range. When I increase the load and start the shake/misfire, I see cyl 8 and cyl 7 misfire counters goes up like crazy and eventually engine computer shuts off cyl 8 and sometimes 7 too.
- Finally I tried STP intake valve cleaner fluid to clean the possible remaining carbon deposits (an experienced MB mechanic told me M278 engines are very sensitive to intake valve carbon deposit), no help.
I have ZERO performance issues, car literally flies if you push hard on high rpm's, no smoke, no hesitation. Throttle response is very very good. If you go over 2,200 rpm and floor it, car is like a rocket.

Under this circumstance, I think the engine is good mechanically. I am considering to change the engine wiring harness thinking about my past experience on my ex Ferrari 599GTB, showing 'transmission failure - hydraulic pump pressure low" error and putting car to neutral which makes car non-driveable for 5-10 mins and comes back to normal until next occurrence in maybe 1-2 months. I saw many similar 'mystic' problems caused by wiring harnesses.

Last note: I drove a 2013 GL550 some years ago, that one was showing similar cold start misfire thing but I discovered a service bulletin about it, reprogrammed the engine computer and problem solved forever. This time, there is no such solution

Any other experiences/comments on this mystery?

Originally Posted by thvmcs

By any chance do you know which wire supplies 12 volts to the ignition coils?

Originally Posted by SSB3435

There is a thick red and brown cable coming from the engine control unit for all ignition coils common 12V power supply. Trigger signal comes on a separate cable individually for each coil, for sure. Same for the injectors. Why you need to know?

Originally Posted by thvmcs

im trying to install a switch/button where it cuts ignition whenever i press the switch/button.