mcheung

I have a 2003 C320. The car cranks but doesn't start. It all started in winter 2021/2022, shortly after my alternator died and was replaced. In the morning, after the car sits overnight, it cranked but at the moment I tried to ignite the engine, it couldn't. Then I realized if I depressed the gas half way, the engine would start. After the car stared, it drove normal and smooth. And there wasn't any error code. When spring came and weather was warmer (above 10C), the car would start normally without me needing to depress the gas. When weather turned colder, the problem came back . This winter, I had to depress the gas all the way down when cranking. At times, it wouldn't even start. And now the weather is warmer again, at 10C, I needed to press the gas half way to start. It seems like whatever the problem is, it's getting worse.

My mechanics checked all the fuel pressure and everything he could get a reading on. Everything is normal. At the time the alternator died, the lights on the dash flashed like crazy and there was misfire. But the alternator shouldn't cause this to happen. After the alternator was replaced, everything was normal except for the starting process. And the car only has hard start when temperature is below 10C or so.

I have the crank position sensor and coolant temperature sensor replaced, thinking the dying alternator might have fried the sensor and caused it to give wrong readings the the ECU. But they didn't help.

Any thoughts of what could be wrong? Without error code, I am shooting in the dark.

Thanks,
Michael

slammer111

What mileage is your car?

I'm wondering if your fuel pump has an intermittent fault. Did your mechanic check the fuel pressure when the car was having trouble starting? A failed fuel pump doesn't trigger an error code.

The W203 has an electronic throttle. Stepping on the gas may change the throttle body position, but I'm not even sure if that happens when the car is cranking.

The other thing that comes to mind is the voltage regulator, which is attached to the alternator. Not sure about the V6, but for the I4 there are 2 versions of the alternator (Valeo, Bosch) and my understanding is they are not interchangeable. Did you get an OEM alternator and VR?

I assume your plugs and ignition coils are working properly. Failed coils will trigger a CEL. I assume failed plugs will too.

mcheung

The mileage is about 240000km.

My mechanic said he checked every reading he can possibly got. And fuel pressure is within the spec. He said there's nothing leading him to think the fuel pump is the culprit. He wouldn't mind changing the fuel pump for me, but even he couldn't justify

Regarding the alternator, if Valeo and Bosch alternators aren't interchangeable, I think a lot of things in the car will behave erroneously. And he always uses the VIN of the car to make sure he gets the right part for it. Although I can't be 100% certain, I am pretty sure he got the right alternator.

Spark plugs are relatively new. Ignition coils are still the original from the factory.

One thing I didn't mention. Over the past winter, while I needed to floor the gas while cranking to get the car started, in colder weather, the car might need a few crank before I could be started. After that, white smoke got out of the exhaust with gasoline smell. And I got P0151 error code. The code would go away after a few days.

paulzale

I've been having this exact issue for a few years. The car is injecting too much fuel when cold, I can smell it. If I depress throttle it will start, but once too cold that doesn't even work. At that point, I crank it then quickly remove fuel pump fuse in trunk, car will start then I quickly reinsert fuel pump fuse. It will be fine the rest of the day.

I've replaced the following:
1. MAF
2. Fuel Filter
3. EGR Valve
4. Fuel Injectors
5. Cleaned throttle body with Seafoam
6. Coolant Temp Sensor
7. Crank Position Sensor

Here is other thread where I've discussed that went nowhere: https://mbworld.org/forums/c-class-w...ml#post8669813

mcheung

Thanks so much for the reply, paulzale. I've replaced (within the last 4 years)
- Fuel filter
- Spark plugs
- Spark plug wires
- O2 sensors
- Throttle body cleaned
- MAF
- Coolant temperature sensor
- Crank position sensor

I was suspecting aging fuel injector o-rings. But after reading your other thread, I start to doubt it has to do with it. Since it's more like guessing, I changed the coolant temperature sensor and crank position sensor because they are relatively cheap to do. Changing fuel injectors are more expensive.

If you have to remove the fuel pump fuse to get the car started, would it be suggesting a faulty fuel pump? Perhaps it is pumping too much fuel at the very first "pump" of the day.

paulzale

I agree too much fuel, don't know how the fuel pump could over pump. I'm guessing throttle position sensor or some other electrical component, no idea what.

mcheung

It's a guessing game

paulzale

I'm still guessing, but haven't done any more since it's warm out. Wondering how the c320 knows if there e85 vs regular fuel. I've read a few threads about being stuck in e85 mode causing computer to dump more fuel for cold start?? Grasping for what this could be, also read o2 sensors, but that shouldn't impact cold start and you've changed yours without success???