[MD335n55]

Hey guys, sorry if this is the wrong place for this post, but I wasn't sure exactly where to put it. This weekend I was driving and my n55 335i and I got a yellow light for my engine overheating and then a red one right after. I immediately pulled over, turned off the car, and scanned for codes which showed 377a (Water Pump Blocked). I know this likely means the pump/ thermostat is bad but when i looked under the hood i saw coolant bubbling out of the expansion tank. Could the tank leaking cause this? I also did the water pump test once the car was cooled down and it was pumping water. Has anyone had any experiences like this? Thank you

[robthewrench]

Because the waterpump is electric and not mechanical, it can fail intermittently. This means it can work for 10 minutes, stop working for 3, start working again etc...

The first thing you should do is get the waterpump replaced. After this, you can check the expansion tank. And no a leaking expansion tank will not cause the waterpump to fail, unless the car loses enough coolant to run the pump dry, but this would also mean the engine has overheated to failure and is ruined.

Replace the waterpump.

[MD335n55]

how would i know for sure the water pump is bad? Id just like to know for sure before i just start replacing things. Thanks

[pehamm]

iirc I haven't had that code, but I've had those exact symptoms & have had to get my water pump replaced 2x. I also had a cracked radiator leaking coolant but if you see it bubbling out of the expansion tank, you're getting extreme temps likely caused by lack of cooling circulation

[MD335n55]

I did the water pump test a few more times and each time the pump works. I also started the car last night and let it run to see if i saw any problems. while it was running the heat was blowing hot and suddenly started blowing cold. Is it possible the heater core is blocked? Thanks

[robthewrench]

While I appreciate your curiosity about what else could fail besides the waterpump, if you do some research on this forum, you will find that a failed waterpump is extremely common and other have reported problems similar to yours. You are welcome to continue to investigate other possible causes, but my advice to you is that your waterpump is failing and needs to be replaced. As note, since the pump can fail any time, you could get stuck at a very inconvenient time. You will also find others will warn you about trying to drive the car when the pump has failed as you can damage the engine.

You get heat and then suddenly no heat from the heater core because the waterpump stops working for several minutes and then resumes working. When it stops, you have no hot coolant flowing from the engine to the heater core. The water pump is having a motor electric control failure and not a mechanical blockage.

I'm sure no one else on this forum is happy with the fact that the 335 waterpump tends to last only 40-80K miles and costs ~$800 to be replaced, but that is a known fact about this car / engine.

If the heater core became blocked, it would most likely stay blocked until you flushed it or mechanically removed the blockage. Good luck.

[TheMidnightNarwhal]

Originally Posted by robthewrench

While I appreciate your curiosity about what else could fail besides the waterpump, if you do some research on this forum, you will find that a failed waterpump is extremely common and other have reported problems similar to yours. You are welcome to continue to investigate other possible causes, but my advice to you is that your waterpump is failing and needs to be replaced. As note, since the pump can fail any time, you could get stuck at a very inconvenient time. You will also find others will warn you about trying to drive the car when the pump has failed as you can damage the engine.

You get heat and then suddenly no heat from the heater core because the waterpump stops working for several minutes and then resumes working. When it stops, you have no hot coolant flowing from the engine to the heater core. The water pump is having a motor electric control failure and not a mechanical blockage.

I'm sure no one else on this forum is happy with the fact that the 335 waterpump tends to last only 40-80K miles and costs ~$800 to be replaced, but that is a known fact about this car / engine.

If the heater core became blocked, it would most likely stay blocked until you flushed it or mechanically removed the blockage. Good luck.

I agree good explanation on the heat core theory would make sense. I wasn't 100% sure it was water pump but now I agree OP should probably replace it.

OP how many miles on your car and do you know when is the last time it was changed?

[MD335n55]

Ok thank you Rob I will hopefully get that done this weekend and will post my results. TheMidnightNarwhal the car has 62000 miles so its right around the time the water pump fails. Its the original pump

[TheMidnightNarwhal]

Originally Posted by MD335n55

Ok thank you Rob I will hopefully get that done this weekend and will post my results. TheMidnightNarwhal the car has 62000 miles so its right around the time the water pump fails. Its the original pump

Ahhh yep most likely your pump.

Man I'm exactly at 62 700 miles I hope might doesn't go out now. I don't know if it was replaced before I got the car at 55k.