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Michael Warfield
Michael Warfield, Factory Mercedes-Benz Technician
Category: Mercedes
Satisfied Customers: 2924
Experience:  Mercedes-Benz Master Certified technician
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1998 ML320 with P0300 P0303 and P0306. I found that I have

Customer Question

1998 ML320 with P0300 P0303 and P0306. I found that I have spark but the injectors are not turned on by the ECM on Cyl 3 and 6. What turns them off? Key off and restarting does not clear that out and turn the injectors back on. ECM problem? I know the ECM can selectively shut an injector down. Where do I go from here.
Submitted: 3 years ago.
Category: Mercedes
Expert:  Michael Warfield replied 3 years ago.
The ECM will shut the injector off to a cylinder if it detects a misfire. Turning the key off and on again should reset the ECM but it sounds like your misfire is severe enough that it is detected right away so the ECM shuts it down right away. I would try clearing the fault codes with a scanner and see if the injector will turn on again. If it doesn't make a difference I would try swapping the ignition coils and wires to see if the misfire moves cylinders. If it does then I would suggest replacing the coil and wires. Even though you have spark at those cylinders, it could still cause a misfire.
Customer: replied 3 years ago.
So why would 2 coils go bad at the same time? I did clear the codes out and they come right back. This engine has 2 SP/Cyl. Why, as you state, that if you still have spark at those cylinders, it could still cause a misfire? How is that possible? Engine ran fine and while driving, this problem started. It actually died and then only restarted and idled. When steping on the throttle it would just break away and die. Now it will rev up on 4 cylinders.
Expert:  Michael Warfield replied 3 years ago.
Sometimes under compression, the spark is not strong enough to completely ignite the fuel air mixture as designed causing a misfire. This engine is sensitive to misfires and can turn off those injectors very easily. If the problem only happens when you rev then engine it is a good possibility the mass airflow sensor is bad. If you have access to one I would test fit a known good one or new Bosch one. These are very common problems and are known for causing misfires and engine stumbling off of idle.
Customer: replied 3 years ago.

Sorry but I cannot eccept the answer since non of your suggestions have fixed the problem. I spent hours and pulled my hair out ... Here is what fixed this vehicle.

 

When I was flushing the fuel rail and found water in the fuel, that was an eyeopener.

Since I found water in it I had to drain the tank to make sure there is no more water going to come to this fuel rail from the tank, to call this a fix. Here is the explanation as far as why cyl 3 and 6 injectors were shut off. The important thing to note is that the fuel rail on this V6 engine is about 1.5" lower towards the firewall then the front of it, on both sides/banks. So where does the water go to? The low point. So Cyl 3 and 6 were getting the water and missfiring and the injector got shut off with the selective shut off so there is NO damage caused to the cat. In this case I do not think that the water would damage anything, it would be steam by the time it hits the hot exhaust manifold and cat. After draining the tank and flushing the fuel rail I had 1 qt of water out of the system. Now it runs once again the way it should and no more codes.

 

So do I now get paid to educate you?

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