Cold start stall Mercedes V12

Richard Diagnostician Alberta Posted   Latest   Edited  
Resolved
Driveability
2005 Mercedes-Benz SL65 AMG 6.0L (275.981) 5-spd — WDBSK79F65F098103
P0300 — Random/Multiple Cylinder Misfire Detected
P0309 — Cylinder 9 Misfire Detected
P0310 — Cylinder 10 Misfire Detected
Start Stall
Misfire

I'm getting killed by this Mercedes and am out of ideas. 

This car and I have a lot of history together. It has a bunch of Renntech aftermarket performance stuff on it, including a tune and air filter boxes. The original complaint was a lack of power and a cold start stall. I drove the car and it was missing a few hundred hp. I found the ignition mains module was only supplying 10v to the left coil pack on the main power supply and the ionic misfire detection, while the right side was getting 180V and 23v. I put a new mains module into it. The engine started and ran well, it is now back to being terrifyingly fast. I don't think I have ever hit WOT because I don't want to die. 

A few days later we get a call from the customer saying it runs well but if the vehicle sits for a few days, it will start and stall about three times then run perfectly. If it sits in the shop that is about 20c it will start and run every time, put it outside when it is 0 to -30c it will start and stall. When it stalls it does not sputter, the engine shuts off like someone turned the key off. Sometimes but not always when it starts cold cylinders 9 and 10 will flag misfire codes and a very faint misfire can be heard out of the exhaust. Once it runs for about 20 seconds it does not miss a beat. After it has sat for a few days and before starting all of the temperature sensors all read within 5 degrees of each other and match the ambient temperature. All of the map sensors just start to read vacuum before the engine stalls. I was thinking it could be fuel drain back into the tank, this is a returnless system. There is a threaded connection on the fuel line under the hood before the rail. If you undo this connection and flip the key on to prime, about a half-liter instantly comes out of the line. 

I scoped the cam and crank sensors during engine stall, the crank sensor is an ac sensor, and the waveform winds down like a normal engine shut off. The cam sensors stop reading when the engine speed gets too low. Using an amp clamp on the 180V lines to the coils, I found a few weak coils in both coil packs. These coil packs are a marvel of engineering and are about a meter long, replacing them is about $8000. At this point, I have found something, but I'm not 100% sure it's going to fix the problem. The vehicle goes to our sister shop and they think we should replace the coils. The customer is informed that the coils may not fix the problem and decides to replace them. Coils did not fix the problem. 

Currently, my running theory is cylinders 9 and 10 injectors could be dripping and draining the fuel out of the left rail. If the rail has no or little fuel in it, it could cause the stall and the misfires. I removed the throttle body and inspected 9 and 10 injector tips with a borescope, cylinder 10 injector had a small drop of fuel collected at the tip but nothing around the valves or in the port. I removed the two injectors and applied 30psi of air pressure to them, they held the pressure. I put them in the freezer for a few hours and they still held pressure. I swapped 9 and 10 with 7 and 8 to see if the misfire follows the injectors. I started the engine this morning, it stalled twice but with no misfires this time. I'm going to let it sit over the weekend and see what it does on Monday. 

The customer is in Asia somewhere for a few months and have the vehicle for the winter, he just wants it fixed. The vehicle has 20 000km on it and is in very good condition. Am I missing something? does anyone have any ideas?

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Marty Technician
California
Marty
 

Fuel supply/sample? can you hookup a pressure transducer/scope into the fuel rail to see if there are ant anomalies/variation which may give you a clue?

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John Technician
Utah
John
 

I'd look at the circuit 87 relay while the stall is happening. The temperature aspect of the issue is making me think it's caused by a relay

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Dave Owner/Technician
California
Dave
 

I'm pretty familiar with these cars and I don't see where you've missed anything. You could put a pressure gauge on the rail and make sure system holds residual pressure, but even if not that would cause long crank, not stall. This is a long shot: There is a situation when these cars undergo a hot soak (stop for a short time when fully warmed up, underhood temps rise higher than when driving) in…

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Chris Diagnostician
Maryland
Chris
 

Do you have a file or screen grab of the cam and crank when it stalls? Or can you get a screenshot of the crank sensor waveform? This sounds like a issue I had on a Mercedes a few years back and would like to see the crank signal to see if it is a similar issue. Thanks

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Glenn Owner/Technician
Texas
Glenn
 

I recommend check the fuel Ethanol content. Along with that it needs to run on a high-grade fuel. These are a twin turbo performance monster. The #10 injector is the only questionable item in your description and that may be the issue. Possibly some BG 44K may resolve this issue. At 0 to -30c nothing works as designed. Have you checked the temp PIDS for the engine and intake inlet air when the…

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Helpful
Obie Technician
Washington
Obie
 

You stated the car has a performance tune. I would watch the 02 sensors and loop status on cold start for mixture control and out of spec voltages. In my experience hot rod tunes do not react like the OE programming. They aren’t too concerned with smog. Also, some of those programs actually prevent some codes from appearing. As it seems there are no other problems it may be a way to go.

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Fermin Owner/Technician
California
Fermin
 

I agree with both Obie and Glenn. It could also be related to the fuel status. Open loop operation may be what's affecting how the vehicle runs after a cold soak. You could try unplugging the upstream oxygen sensors to force it into a default state. Alternatively, you could monitor fuel status to see if the start/stall condition correlates to open/closed loop. The heaters in the oxygen sensors…

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Randall Technician
Pennsylvania
Randall
 

Richard, Don’t get bagged on poor quality fuel. I agree with the others who suggest replacing the gas. I have a homemade external fuel source for confirming bad gas. Especially when temperature is a factor. Randy

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Richard Diagnostician
Alberta
Richard Update
 

The saga continues: I have figured out that if I leave the vehicle sitting in one of my bays it will stall once every morning. I have taken advantage of this. I wanted to find out if the engine module was shutting the engine off or if it was stalling from some type of mechanical means. I tagged my scope into the number 10 fuel injector (yellow trace) and the ignition timing signal for cylinder…

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Marty Technician
California
Marty
 

Having trouble determining what is the injector on time (?) Not used to the Snap on format. If ambient is -15C ( +5 f) I would expect a pretty high value…above 15 or 20ms… maybe more, during cranking…(just a guess)

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Richard Diagnostician
Alberta
Richard
 

I have that scope capture super zoomed out so you can see the whole event. It was taken in the shop at around 20c. With the engine running before the stall injector on time is about 5ms, during the stall it grows and the last injection event has an on-time of 11ms. I would assume that the engine module is dumping in more fuel trying to keep the engine alive.

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Marty Technician
California
Marty
 

Just for clarification, engine stall occurs regardless of temperature..? (range of -15C to +20C) May want to see if throttle is clean and what throttle value is doing during failed state. I came across some info referencing fuel systems that I attached as it may be helpful. ..

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Richard Diagnostician
Alberta
Richard
 

Thank you for all of that information, that helps a bunch! Back a few months ago, it would only stall after it sat for a few days regardless of temperature. Once it got colder outside it started stalling every morning if left outside. Now it will stall every morning regardless of temperature. The colder it is the more times the engine will stall, if it is 20c it will stall once, if it is -20c…

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Richard Diagnostician
Alberta
Richard Resolution
 

I think I have this one fixed. I disconnected both batteries and performed a capacitor drain to force a software restart. It has not stalled since. This problem started after the ignition mains unit failed. The mains unit turns 12v into 190v, so there is a high potential for a voltage spike that may have scrambled some software. I would like to thank everyone for their input. I have invested…

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