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Misfires and poor gas mileage 2003 c230 Mercedes

I have a 2003 Mercedes c230 with low mileage 70000. This was its first season not garaged. About 2wks ago when very cold started abruptly to misfire badly at idle with poor gas mileage and modest loss of power. I got codes on OBD II for multiple cylinder misfires p0301-4 (all 4 cylinders). No other codes. Also the MAF and O2 sensors seems to be giving normal readings, and I cleaned the MAF sensor (didn’t take off. cleaned through screen connected to air filter housing). 1 or 2 of the spark plugs seemed fouled as had black oily fuel on them and the O2 sensor voltage readings were consistently close to 0.9v (high meaning reading rich air/fuel right?). Changed all 4 plugs and swapped around ignition coils. Seems a little better but still misfire. O2 sensor voltage reads now consistently around 0.7v and the new plugs are dry but already blackened (indicating rich air/fuel I think). Any thoughts?

Yes it sounds like you are running rich and since it is causing issues on all 4 cylinders then you’d look at things that affect entire fuel delivery. I don’t know this engine but it probably has a fuel pressure regulator and that should be checked - a general check of fuel pressure is actually a good idea. You’d also want check the coolant temp sensor and actual coolant temp once the car should be warmed up. Cleaning the MAF is fine, but I’d also want to look at what it is actually reading on a scantool. If you’re reading O2 sensor voltages it seems likely that you can probably use whatever scanner you’re using to read the coolant temp and MAF readings. Are you able to access fuel trims?


You have the 4 cylinder, correct?

271 engine, I believe

I worked at a Benz dealer for many years, and I’m quite familiar with that engine.

I’m going to give you my opinion right now. There’s a fair chance you’ll be needing a cylinder head, or at least you need to find an excellent machine shop. Those engines had a tendency to get worn valves and seats, which would lead to rough running and misfiring.

There was a software update, but the real fix was usually the head.

I seem to remember Mercedes-Benz said the valve seats were not replaceable. However, an excellent machine shop might be able to do something

Another bit of advice . . . if you do eventually go for a new head, I strongly urge you to have the repair performed at a dealer, or at the very least, at an independent shop which is very familiar with this engine. Many special tools are required. Putting the cylinder head on is kind of tricky.
There is also a point where you need to apply rtv, and if you apply it wrong, there can be consequences. It has happened.

FWIW . . . many of those engines had the cylinder head problem before new car warranty expired

I gave you the worst case scenario.

For your sake, I hope I’m wrong

It wouldn’t hurt to call a Mercedes-Benz dealer and ask them if there are any outstanding campaigns for your specific vehicle. They’ll need the VIN number

@db4690, presumably if the valve seats are an issue this would show up by compression and leakdown testing? Given that these issues exist with that engine it seems a way to either confirm or rule it out at the outset would be nice.


Yes, you’re correct

The factory way to diagnose the issue was to do a leakdown test. However, low compression also accompanied the problem

By low, I mean below specs. Sometimes it was surprising how well one of those engines seemed to be running, given the compression. Sometimes the compression on problematic engines was 100psi.

Thanks for the advice. I checked with Mercedes and no outstanding campaigns on my vehicle. I think that I am going to need to take it to my local mechanic (not Dealer!) and have them run a compression test. I have noticed over the last week on OBD II readings that the O2 sensor is fluctuating more and sometimes down to <0.1 volts. I’m not sure if this is normal (I’m hoping that it is not!). this car has only 70K and if is the valves and seals I’m going to be pissed. Needless to say that this will be last Merc I ever buy. I’ll stick with Honda in future.

An O2 sensor is supposed to bounce (switch) between about .1V and .9V according to how much O2 is in the exhaust - that’s how they work. If yours isn’t switching and used to be somewhat “stuck” up around .7-.9V and now seems “stuck” down in the lower voltages then you might just be looking at a fouled up and sluggish O2 sensor. This can easily be the case without any O2-specific codes being set. OR whatever you’re using to read it isn’t capable of sampling at intervals small enough to truly see what the sensor is doing. Can you get the fuel trims?

I would check out the compression either way. Even if it isn’t related, given what db4690 said above I would want to have a baseline knowledge of compression right now - to compare to any future readings if there are later problems.

Another good way to see if the valves are burned is to put a tiny camera in the spark hole and examine the valves.


Here’s some thoughts about that oxygen sensor staying low

It’s possible that it’s staying low because it’s picking up the excess, unburned oxygen, due that misfire

It’s something I’ve seen before

Fix the root cause, and the oxygen sensor behaves correctly


" . . . put a tiny camera in the spark hole . . . "

As a matter of fact, Mercedes-Benz also suggested that. But if the compression was low, and the leakdown test revealed bad valves and seats, the head had to come off either way

@db4690, I just brought up the O2 sensor because in the original post he said it was hanging high up in the .7-.9V range. Now the report is that it’s hanging low. What I really suspect is that whatever scanner is being used doesn’t give a good running record of the voltages. However, if it is saying something about it then an O2 sensor issue shouldn’t be ruled out. I would have checked the compression already, but I also have a gauge in my garage.


Come to think of it, that engine might be using a wideband sensor, in other words, an air/fuel sensor, rather than an oxygen sensor

Anyways, I still am leaning towards a valvetrain problem, purely based on my experience with this engine