Wednesday, 28 May 2014

e46 n42 Bottom Hose Confusion

So I was about a week away from visiting my brother in the South of France when I got in my touring to find the coolant light was on. I lift the bonnet and unscrew the expansion cap to discover an empty header tank.

It had been a slight loss of coolant over time since I got the car which I hadn't been able to trace so I just put it down to that, filled it up and forgot about. A couple of short journeys later she was empty again.

There was a lot of coolant on the outside of the expansion tank so I assumed I had fallen victim to the well known cracked expansion tank issue. I ordered up a replacement expansion tank and fitted it.

However, when I filled it back up with coolant my original top and bottom hoses leaked, it seems the o-rings that seal them don't like to be disturbed. Unfortunately BMW don't sell them separately and you have to fork out for the full pipe. I am sure I could've found o-rings to fit, but I was in a rush as I was about to drive to France so I wanted a permanent reliable solution.

However, getting the pipes from BMW was harder than expected, the BMW diagrams are not 100% clear and the car differs quite a lot from the six cylinders. I needed to replace the top hose and the pipe which attaches to the middle of the expansion tank.

So, originally the technician ordered completely the wrong pipe. I had to take pictures to get him to believe me that it was the wrong part. So I will try and describe the problem as best I can just in case it helps someone else:

The problem is this parts diagram which I've taken from (the dealer's was identical) ... According to this diagram the large pipe (no 4) that runs underneath the radiator from the thermostat/water pump attaches to the middle of the expansion tank:

I thought the pipe was incredibly expensive when I ordered it (even for BMW) because the pipe I actually wanted is this one (no 7):

No 7 runs from the middle of the expansion tank to the heater matrix and is quite short, and has one of BMW's plastic attachments at one end and a jubilee clip at the other...

However, the dealer looked at the first picture first and it took quite alot of too-ing and fro-ing to get the correct pipe.

The large pipe actually attached to the bottom of plastic bracket that holds the expansion tank to the radiator (no10):

This is a different set up to the six cylinders which don't have anything going into the bottom of that bracket. You can see in this picture that there is a connector for a pipe sticking out of the bracket, this is where that long bottom hose attaches...

Hopefully that makes sense and saves somebody some time with the dealer! Unfortunately, after all that, the car still had a serious cooling problem... check my next post to find out why.


  1. The ideas are very fine and attractive about the E46 expansion tank. Here this types off tanks are being used in the BMW motors.

  2. hi do you know what is the part number for hose no. 7in the second diagram?

  3. You mean pipe number 4 not number 7 in the second diagram

  4. Lol...for my 2001 330i they wanted $85 for that hose. I spliced it with three inch hose and two connectors. Since I didn't have the $100 for the hose and antifreeze, it turned out to be a $15 fix that has lasted over four years. Since these BMW "special fitting" hoses are so pricey it's a great idea to inspect them once a year for rubbing, cracking, or other damage. It will save a tow truck ride and an unexpected headache. PS ...if this hose gets kinked it will cause you to have no heat and be unable to bleed your system!

  5. A great thing to know about a cooling system with a problem about to occur. Before your pump goes or if there's any blockage in the system my 2001 330i always let's me know by #5-#4 valves will run hot causing a tapping. If you hear this infamous tappet noise it's a good bet that your cooling system has poor flow,there's an air bubble in the system, your oil is low, or your coolant is very low. To check for a water pump failure put the end of a large screwdriver on the vanos right above the pump and listen to the handle end. If you hear a whirring noise or bearing crunching you need to replace your water pump asap. When they let go when driving the water pump will throw the clutch fan into the radiator shredding the blades throwing them throughout the cooling system and even through the hood of the car!! I had it happen twice due to a bad diagnosis. Just because it's not leaking or move doesn't mean it's not ready to explode. The running hot and screwdriver trick could save you $2500 or more!! When M52-M54-S54 water pump let go it's catostophic....wish I knew this six years ago!!