Flood water damaged c250d misfire and nox sensor fault HELP!!

[Jon2021]

Hi all i have a problem with my car was wondering if anyone can help. I bought a flood damaged car from copart last month its a w205 c250d bluetec 2015 stated it had a 'slight misfire’ had the diagnostic done on it and it comes up with error code P229F62 and also P026685. The first code is nox sensor is implausible and second one is teach-in of cylinder 2 faulty. I have had both nox sensors replaced however the fault remains and the live data readings on both sensors are 0. Also another big problem i have is the car will misfire when started from cold but not straight away. It takes a few minutes of running the car from cold before it starts misfiring and blowing white smoke stinking of diesel from exhaust. However when the car is fully up to temperature it stops misfiring and smoking and seems like a completely normal car. I have had a compression test done and all is well and i have also had all the injectors checked and they are all fine so what could this be ? I was thinking maybe an ecu fault? Or electrical? But why would the car only misfire when its starting to heat up a little between 40-50 degrees? Im so confused and all these garages cant give me an answer i dont know what to do next.

 

[MJJ]

Hi, sounds like it could be a nice project to rescue and use. I have some thoughts, not solutions, but perhaps the below will help.

Two things to keep in your mind at all times, I hope this is not patronising:
1) Cars are not designed to survive flood water, so you will be encountering things that are rarely seen - thus, unlikely to be able to Google common problems etc. You need to diagnose methodically.
2) ECUs are programmed to give error codes when they encounter an error the ECU programmer expected to see in a failure case. The error codes are not the final answer, more of a pointer.

So, the part of your post that sticks out to me is the NOx sensors - live data readings still zero, even after replacement. I think you need to trace the wiring back from the actual sensors, and see which module (it may well be inside the interior) they plug into - then disconnect all electrical plugs from that module, and remove any water ingress corrosion with a tiny file, and contact cleaner. Make sure all connectors and ECU pins are corrosion free, and dry.

With regard to your coolant temperature mis-fire, a possible culprit is the coolant temperature sensor or as above, it's wiring into whichever module it connects to. I suggest looking at coolant temp live data, and see if that looks plausible as the engine starts from cold.

Lastly, as you know the car has seen a flood, it may well be sensible to do the disconnect/clean/reconnect activity on any module mounted lower down in the car. Boot spare wheel well, and driver/passenger footwells behind carpet and A-pillar trim are the most likely places.

Hope this helps in some way,

Martin.

 

[BULLITTBOSS]

Sounds like a bent con rod to me all the symptoms

 

[E55BOF]

How so? All the OP has described is a diesel misfiring and/or overfuelling while the car is coming up to temperature, and a problem with the NOx sensors. How are those 'all the symptoms of a bent conrod'?

 

[Jon2021]

Hi, sounds like it could be a nice project to rescue and use. I have some thoughts, not solutions, but perhaps the below will help.

Two things to keep in your mind at all times, I hope this is not patronising:
1) Cars are not designed to survive flood water, so you will be encountering things that are rarely seen - thus, unlikely to be able to Google common problems etc. You need to diagnose methodically.
2) ECUs are programmed to give error codes when they encounter an error the ECU programmer expected to see in a failure case. The error codes are not the final answer, more of a pointer.

So, the part of your post that sticks out to me is the NOx sensors - live data readings still zero, even after replacement. I think you need to trace the wiring back from the actual sensors, and see which module (it may well be inside the interior) they plug into - then disconnect all electrical plugs from that module, and remove any water ingress corrosion with a tiny file, and contact cleaner. Make sure all connectors and ECU pins are corrosion free, and dry.

With regard to your coolant temperature mis-fire, a possible culprit is the coolant temperature sensor or as above, it's wiring into whichever module it connects to. I suggest looking at coolant temp live data, and see if that looks plausible as the engine starts from cold.

Lastly, as you know the car has seen a flood, it may well be sensible to do the disconnect/clean/reconnect activity on any module mounted lower down in the car. Boot spare wheel well, and driver/passenger footwells behind carpet and A-pillar trim are the most likely places.

Hope this helps in some way,

Martin.

Hi thanks for the reply, yeah i have checked all the obvious areas for corrosion or water damage but i cant seem to find any. Annoying thing is when the car was put on auction they clearly gave it a little clean up and wash so you can’t really tell how far up the water came. Usually this gives you a good idea as to what extent the damage could be.

In the boot there are some fuses and modules and ive had a look at them and there doesnt seem to be any obvious signs of water intrusion or even corrosion even though it sits right where the spare wheel would be which is quite low down so if the water didnt even reach that area surely it couldnt have digested any water in the engine but i may be wrong.

First thing im planning on doing is doing a physical compression test through the glow plug holes to make sure all 4 cylinders have compression because i have read that those star xentry compression tests are useless and in accurate as they dont give out the actual readings in the cylinders. Its just strange that this problem only occurs as the car is heating up between 40-60 degrees on the coolant temp gauge in the dash. It may well be a coolant temp sensor thats faulty so i will defo look into that too and let you know how i get on.

 

[Jon2021]

Sounds like a bent con rod to me all the symptoms

I have been told this by many other people too it is a likely culprit i had the compression test done on star xentry and it just showed the number 6 on all 4 cylinders and was green assuming this means all good but apparently only way of checking is by doing the physical compression test.

 

[Jon2021]

How so? All the OP has described is a diesel misfiring and/or overfuelling while the car is coming up to temperature, and a problem with the NOx sensors. How are those 'all the symptoms of a bent conrod'?

This is exactly what i was thinking too but so many people have told me this could be a bent rod and apparently as the car is heating up it is expanding within the engine therefore the misfire goes away but i guess there’s only one way of finding out for sure

 

[E55BOF]

I think you've been talking to people who don't know much about internal combustion engines. There are some genuine experts on here, and I dare say one of them will post an opinion sooner or later.

 

[Petrol Pete]

Do a compression test then. It doesn't take long and is simple to do.. Jon I don't really want to state the bleedin' obvious , but if you had of come on here before buying a 'flood damaged' car asking us if it was a good move you would have had more than the 5 replies you have had so far.

You would have had about a 100 telling you not to do it , but it is what it is and you have taken the plunge.(pun intended) The fact that you have told us that the car has not obviously spent ages sitting in deep water is noted but as you are discovering it does not take a lot of water to kill a modern car . I guess you have no way of knowing if it was driven into water or was flooded when parked with the engine off and the electrical circuits dead.

I wish you luck and there are enough experienced bod's on here to help you through.

 

[oldguy57]

Do a compression test then. It doesn't take long and is simple to do.. Jon I don't really want to state the bleedin' obvious , but if you had of come on here before buying a 'flood damaged' car asking us if it was a good move you would have had more than the 5 replies you have had so far.

You would have had about a 100 telling you not to do it , but it is what it is and you have taken the plunge.(pun intended) The fact that you have told us that the car has not obviously spent ages sitting in deep water is noted but as you are discovering it does not take a lot of water to kill a modern car . I guess you have no way of knowing if it was driven into water or was flooded when parked with the engine off and the electrical circuits dead.

I wish you luck and there are enough experienced bod's on here to help you through.

Exactly this. Seems to be very much hit and miss whether you get a bargain on a flood damaged car. The fact that Insurers write most flood affected cars off is a sign of the potential future problems. Having said that a friend bought a water damaged AM cheap and all it needed was a Ford SAM unit - it's been perfect since.

 

[Petrol Pete]

Oldguy. It is a gamble , but whenever I watch those videos where people (usually USA) go to auctions and buy damaged -usually high end cars - to repair they normally avoid anything marked flood/water damaged like the plague. I guess they know what they are doing.

 

[grober]

Hinges on "flood damaged" if the car was driven into deep water then water may have entered the combustion chambers and damaged the internals because you can't compress a liquid like a gas- its called hydrolock

Hydrolock - Wikipedia

en.wikipedia.org

 

[Jon2021]

Yeah to be honest i just saw the exact car I wanted on auction had all the right spec and was marked as flood damaged but I thought i will take on a challenge. I bought the car thinking it will need a whole new replacement engine and with that in mind i bidded on it. I didnt take into consideration all the electrical side of things and that many other components could be damaged. Had i known this i would not have bidded on it. But you live and you learn i guess this has been a learning curve for me i now know what to look out for. I live in london and have been calling around local garages and none of them seem to want to do a compression test on a diesel engine does anyone know why? Im pretty sure its the same as a petrol engine instead of screwing it in the spark plug hole you screw it in the glow plug hole and you may need a different adapter but the process is the same ? Does anyone know anywhere local that i can get this compression test done on a diesel?

much appreciated guys for the responses

 

[MJJ]

Jon,

I have been taking your original comment that the car drives normally when up to temperature at face value. If the engine is performing normally when up to temperature, seemingly producing all its power, and averaging the 50mpg plus you would expect when driven normally- then you don't have any mechanical issues with the car. If that is the case, personally, I would stop trying to test pure mechanical factors like compression. A bent con-rod will mean it misfires all the time, and likely has a knocking bottom end that you would notice instantly.

However, this US document (found by Googling Mercedes P026685) suggests that Mercedes know of an issue with water entering the OM251 engine and hydrolocking it - in this document's case, through condensate from the charge cooler, not flood damage. This triggers the cylinder 2 P026685 error code you are seeing, and triggers a warranty replacement of the engine.

This suggests to me that when water does enter the engine, it is normally cylinder 2 that suffers - all seems quite co-incidental given your circumstances.

If you are not 100% confident on the 'driving normally' statement, perhaps a compression test is wise as a first step. I think garages may be reluctant as removing glow plugs from a few year old diesel can often result in snapped glow plugs, drilling out said snapped glow plug, and a load of extra pain. I think you need a good Mercedes specialist, not a local, back street garage.

For me, it all hinges on how confident you are of the driving normally assertion.

If you are confident, I think a wise investment (£200 ish) of a diagnostics tool, like iCarSoft would pay dividends here. You can read fault codes, but much more importantly, you can view live data - look at the engine ECU value for coolant temperature, does it read c. 10 degrees (ambient temp.) at cold start, and does it climb steadily and consistently when the engine is started?

Then look at injector live data, does injector 2 have wildly differing values than the other 3? If so, and as you say the injectors have been tested, suspect problems with the wiring to injector 2.

The NOx sensor values of zero puzzle me too, but I don't know what engine protection strategies Mercedes would employ to handle implausible NOx data. However, live data for DPF pressures and fill levels will help hugely here - my guess is the car may think the DPF is full, and is always trying to re-gen, thus overfuelling.

Just my thoughts.

Martin.

 

[MrGreedy]

Just interesting to note on the compression test. I was looking to get a compression test done on my previous car (Insignia A20DTH diesel), and I couldn't find anyone locally (within 10-15 miles) who would do a compression test. I have a Bosch diesel diagnostic centre a couple of miles away. They don't do compression tests either.
I figured if I was going to get it done, I would have to buy the kit and do it myself. I find it amazing, but you are not alone in what you have found.

 

[Jon2021]

Jon,

I have been taking your original comment that the car drives normally when up to temperature at face value. If the engine is performing normally when up to temperature, seemingly producing all its power, and averaging the 50mpg plus you would expect when driven normally- then you don't have any mechanical issues with the car. If that is the case, personally, I would stop trying to test pure mechanical factors like compression. A bent con-rod will mean it misfires all the time, and likely has a knocking bottom end that you would notice instantly.

However, this US document (found by Googling Mercedes P026685) suggests that Mercedes know of an issue with water entering the OM251 engine and hydrolocking it - in this document's case, through condensate from the charge cooler, not flood damage. This triggers the cylinder 2 P026685 error code you are seeing, and triggers a warranty replacement of the engine.

This suggests to me that when water does enter the engine, it is normally cylinder 2 that suffers - all seems quite co-incidental given your circumstances.

If you are not 100% confident on the 'driving normally' statement, perhaps a compression test is wise as a first step. I think garages may be reluctant as removing glow plugs from a few year old diesel can often result in snapped glow plugs, drilling out said snapped glow plug, and a load of extra pain. I think you need a good Mercedes specialist, not a local, back street garage.

For me, it all hinges on how confident you are of the driving normally assertion.

If you are confident, I think a wise investment (£200 ish) of a diagnostics tool, like iCarSoft would pay dividends here. You can read fault codes, but much more importantly, you can view live data - look at the engine ECU value for coolant temperature, does it read c. 10 degrees (ambient temp.) at cold start, and does it climb steadily and consistently when the engine is started?

Then look at injector live data, does injector 2 have wildly differing values than the other 3? If so, and as you say the injectors have been tested, suspect problems with the wiring to injector 2.

The NOx sensor values of zero puzzle me too, but I don't know what engine protection strategies Mercedes would employ to handle implausible NOx data. However, live data for DPF pressures and fill levels will help hugely here - my guess is the car may think the DPF is full, and is always trying to re-gen, thus overfuelling.

Just my thoughts.

Martin.

This comment really helped. I just remembered a very important fact. When the car was put on the diagnostics there was indeed a code for the dpf being full of soot but the guy said this is probably due to the nox sensor being faulty and therefore it affects things like the dpf not allowing it to regenerate. He cleared the codes and the only one that was stored was the fault with the nox sensors even the code for cylinder 2 disappears. But once i drive the car a few miles, leave it overnight and then start next day, it starts first time and runs perfectly normal there may be a little more lumpiness and noise but that not unusual for a diesel. However after a few mins of leaving it on it starts misfiring and strongly smelling of diesel. I had a look a the live data whilst engine was misfiring and noticed the readings for cylinder 2 injector was out of range and even number 1 was a little out of range. This would make perfect sense with what u are saying that it is trying to regen and as a result overfuelling which is exactly what this sounds like. I completely forgot about the dpf issue. And with regards to it running fine when warm, it runs great however you do notice that it is not pulling as strong as it should be. I had a c220d previously and that seemed to have more of a pull than this which is not right as this is a c250d. But apart from that issue it seems to run fine when warm no knocking noise or tapping whatsoever. It never struggles to start no matter what the temperature.

What do you think i should do ? Could the dpf being blocked and full of soot be causing this issue? Would the flood water have caused some sort of problem in this region? I really do have my doubts about this being a bent rod problem because it just doesn’t make sense how the car is running fine apart from the problem stated above. It doesnt even give any smoke when up to temperature even if you floor it and the live data readings on all 4 injectors are perfectly in range when its warm. Hence why if u clear the code for the cylinder 2 it goes away and only comes back when it starts misfiring again from cold

 

[Jon2021]

Just interesting to note on the compression test. I was looking to get a compression test done on my previous car (Insignia A20DTH diesel), and I couldn't find anyone locally (within 10-15 miles) who would do a compression test. I have a Bosch diesel diagnostic centre a couple of miles away. They don't do compression tests either.
I figured if I was going to get it done, I would have to buy the kit and do it myself. I find it amazing, but you are not alone in what you have found.

Hi, yes i know it’s ridiculous i have been looking everywhere even today i have spent the best part of the morning calling all local garages and not one of them have so far been able to help. All of them either dont do compression tests at all or if they do its only for petrol engines. Imagine how much money one could make by simply buying one of those kits and offering your services on google i can only imagine the amount of responses you could get from doing so lol. But as for your problem i think the best way is to do it yourself but i have heard that glow plugs can be a real pain to remove so be very careful and if you’re not confident dont attempt it as you could snap it in its hole. Just repeating what others have told me

 

[grober]

The Selective catalytic reduction (SCR) unit in your car has 2 NOx sensors associated with it. THE THING THAT KILLS THEM DEAD is water- thats why they are not switched on till the engine/exhaust is hot- if however you drove into a flood with them switched on---????

 

[GeeJayW]

The Selective catalytic reduction (SCR) unit in your car has 2 NOx sensors associated with it. THE THING THAT KILLS THEM DEAD is water- thats why they are not switched on till the engine/exhaust is hot- if however you drove into a flood with them switched on---????

I think maybe the OP bought the car after it was flood damaged? But, your point is valid; NOx sensors don’t like water and it’s likely that if the car was sitting in say a foot or two of water, the exhaust system will have filled with water and the sensors would have received a good soaking.

 

[grober]

I think maybe the OP bought the car after it was flood damaged? But, your point is valid; NOx sensors don’t like water and it’s likely that if the car was sitting in say a foot or two of water, the exhaust system will have filled with water and the sensors would have received a good soaking.

Meant anyone rather than "you" sorry.