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The mixture (cylinder bank 1) is too rich (0746) and 30 seconds cold start vibration
Hello MBWorld,
I am trying to diagnose an issue as my mechanic couldn't really pinpoint exactly what is at fault, he suggested me to replace the engine intake manifold which is quite expensive.
The car is pre-facelift W212 E300 year 2013 (yes here in Middle East facelift appeared only in 2014). Engine is M272.
The symptoms are two (i wonder if they are related or two separate issues):
1- For about 30 seconds after a cold start the whole car vibrates. The RPM shoots to about 1500 RPM and then decreases gradually over the 30s and once it rests at around 800RPM, the vibration disappears.
2- Check engine light appears intermittently and when checked it gives the fault code: The mixture (cylinder bank 1) is too rich (0746). This happens when accelerating from full stop. However, it doesn't always happen, what I mean is I can drive 500km and not see the check engine light and then one day I drive just 30km and it appears when I accelerate from a traffic light.
Here is the quick test log I managed to get from my mechanic:
If someone can point me in the right direction I will appreciate it very much. If any other data is required I will be able to get it from my mechanic although that guy wants to just replace the whole engine intake manifold. Note that intake manifold was already replaced by previous owner one year back (May 2023) and driven 15000km since then. Currently the odo is at 133,000 km
Aha..sorry I missed the information that the intake manifold been replaced in may 2023. Was it a genuine unit ??
The Xentry showed 32,838KM as last occurrence, how the hell now the car is a 133,000 KM ?
This is a STORED code. Don't tell me over the MANY years it has never been deleted ?
Or.....It could be that the engine ECM is newer than the car.
I know Xentry sometimes has mileage difference between modules, but usually less than 1,000KM
Aha..sorry I missed the information that the intake manifold been replaced in may 2023. Was it a genuine unit ??
The Xentry showed 32,838KM as last occurrence, how the hell now the car is a 133,000 KM ?
This is a STORED code. Don't tell me over the MANY years it has never been deleted ?
Or.....It could be that the engine ECM is newer than the car.
I know Xentry sometimes has mileage difference between modules, but usually less than 1,000KM
It was not a genuine MB unit, it was Pierburg (7.00246.33.0).
I owned the car since 130,000 and I myself got the fault code cleared about 5 times, so not sure about the last occurrence odo reading.
I think you need to data log it on one of those cold starts so we can see what it's doing. You can get a cheap OBD dongle online, and use a phone app like Torque Pro. Net out of pocket cost is like $15 US.
I'd look at Injector Pulse Width when starting it cold, then compare that to after the O2 sensors turn on, which is about that 30 second mark. When it's cold it's adding fuel based on what it thinks it should be doing, but it's flying blind without the O2 sensor feedback. Once the O2's kick on it tells the ECU if it's too rich or too lean and adjusts. So it seems you have one or the other issue during that time.
My car has never given me a code for being to rich, even when it's so rich it'll stall. It will give a lean code, but very rarely and it has to be super lean, like you described with it running rough. So I'd imagine you too could be running so rich or lean that it vibrates, but not give a code. In my experience a rich condition runs much rougher than a lean one.
You can also record fuel trims, all four, and the two primary wideband O2 sensor readings to see what they say when it acts up while driving.
The intermittent thing from a stoplight sounds like it could be an intake manifold issue if those doors and what not aren't opening/closing like they should. Seems odd it would upset the fuel mix that much but I don't know that engine. I do recall several threads on this forum talking about that exact intake not working right, what it does and how to fix it. I didn't pay much attention but I recall it was common so there should lots of info. Maybe it'll help you figure out if that's your issue or not.
So I cleared those old codes and driven for another around 300km. Yesterday, after a cold start, the car vibrated more than usual. When I started driving at slow speeds, the check engine light came on and vibration was worse. I immediately stopped and shut down the engine. After 10 minutes, I started again, the check engine light was gone and the vibration also disappeared. I have since driven around 50km but the symptoms didn't happen again.
Today I checked with Xentry got the following codes
0629 Combustion misfiring has been detected.
0637 Combustion misfiring of cylinder 4 has been detected
Does the combination of intermittent occurrence of 0746 (The mixture (cylinder bank 1) is too rich) and the new codes point to something obvious that I should be looking into?
So the 0746 (The mixture (cylinder bank 1) is too rich) is very likely caused by the
0629 Combustion misfiring has been detected. 0637 Combustion misfiring of cylinder 4 has been detected
Misfire meant the fuel was not burned properly and it also meant RICH combustion.
Usually ECM will shut down the injectors to the Misfiring cylinder, which in this case is Cy 4.
Cy4 is in Bank 1. Bank 1 is where cylinder 1 resides.
Before ECM shut down Cy 4 injectors, the RICH condition already happened, thus that is where you probably got the 0746 (The mixture (cylinder bank 1) is too rich).
Sometimes misfire may not be registered so very soon.
Your COP Ignition Coil is a 4 wire version and has the driver inside the COP.
At the least try swapping the COP and spark plug of Cy 4 to Cy 1 and vice versa, I mean to different bank and is in front of engine, easier access.
If misfire follows the COP & Spark Plug, you have reduced the suspect/s.
U can if you want swap COP only first, up to you.
Attached 2 files, your engine computer wiring diag. M272
Cylinder position/number and ignition firing order for M272
Last edited by S-Prihadi; Jun 10, 2024 at 08:49 AM.
It could also due to the intake manifold gasket not properly aligned when it was replace last time, which caused leak. May need to first take a look at the connecting hoses and the plastic piece to see if anything is wrong. If not you may need to replace a new gasket of the manifold, which will require taking out the manifold. It’s a huge amount of work and the labor is not cheap.
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