N55 Expansion Tank Replacement

IA EURO illustration of one failed N55 coolant expansion tank from our bench: level sensor plug at the bottom, crack marked across the plastic weld at the top — Figure 1One failed tank. The crack ran across the plastic weld at the top left of the back. From our bench.

Figure 1One failed tank. The crack ran across the plastic weld at the top left of the back. From our bench.

On an N55 the tank is a small part. What matters is the tank number, which changes by car, and the bleed (getting air out of the cooling system) afterward.

How long: an X3 owner reports about 30 minutes for the tank plus 10 to 15 minutes to bleed it, and a parts site says about an hour at home. No report says if that counts draining or underbody work.

Is it the tank?

The tank is the plastic coolant bottle under the screw cap in the engine bay.

Low-coolant message. BMW's sensor trips it only well below minimum, though dirty or wrong-mix coolant can trip it sooner. Corroded sensor contacts can fake it and clear when warm (F30 owner).
Leak signs. Owners report topping up often, dried white, blue or crusty residue on the tank, cap and engine cover, a sweet smell after driving, and a wet passenger-side fender well.

IA EURO illustration of the failed tank's screw cap neck — Figure 2The screw cap neck. Dried coolant collects around the threads. From our bench.

Figure 2The screw cap neck. Dried coolant collects around the threads. From our bench.

Crust doesn't convict anything. BMW says the cap's valve can vent on hot days and leave coolant traces that don't show the cap is bad. Shop rule of thumb, not BMW's: crust shows coolant got out, not which part. Pressure test it.

Shop-blog advice: if it overheated or the coolant looks milky or oily, find that cause first. If the warning returns after a top-up, stop driving.

Prove it with a pressure test

Owners say looking misses leaks that only show hot and under pressure. Owners and parts sites report them at the seam where the two halves join, the plastic vent screw, the hose ports and the bottom by the level sensor.

Can't find it? Check the back. Shop tip from our own N55, not BMW's: the crack ran right across the plastic weld at the top left of the tank's rear face, next to the power steering fluid reservoir, where you can't see it with the tank in the car. Experienced techs missed it, and so did a shop visit. The coolant trickled down the driver's side of the front subframe and dripped near the driver's front wheel, nowhere near the tank. A leak on that side that nothing up front explains: look at the back of the tank, then the back of the engine. We saw the same kind of hidden leak at the back of an N52 E70 that other techs had missed.

IA EURO illustration of the failed tank from the side with the level sensor plug circled — Figure 3The level sensor plug at the bottom of the tank. From our bench.

Figure 3The level sensor plug at the bottom of the tank. From our bench.

1.5 barBMW's test pressure, on a cold system.

Engine fully cold, cap off, tester on the tank neck. Never open a warm system. Goggles and gloves on, as BMW says. BMW uses a hand pump with a tank adapter: special tools, listed under What you'll need.
Pump to 1.5 bar and watch the gauge. A tight system drops 0.1 bar or less in about 2 minutes. More means a leak.
Test the cap on its own. Screw it onto BMW's cap adapter and note where its valve opens. BMW prints no N55 number and residue can make the valve stick, so retest up to three times. Replace the cap only after three failed tests.

Which tank is yours

N55 cars only. Not for ActiveHybrid 3, 5 or 7L: high-voltage cars with a second tank. Find your chassis code (E90, F30 and so on) below. A 335i is in two rows, so the code decides. Not sure of yours? Send us your VIN. Four of these tanks also fit other engines, so order by car and engine together.

Group 1: E82, E88, E90, E92, E93, E84135i, 335i, X1 xDrive35iTank 17 13 7 640 515. 135i and 335i built from 03/2010 (earlier ones are N54).

Group 2: F22, F23, F30, F32, F33, F34, F36, F87M235i, 335i, 435i, M2Tank 17 13 7 642 160.

Group 3: F01, F02, F06, F07, F10, F12, F13535i, 640i, 740iTank 17 13 7 647 283. F01 and F02 built 07/2012 to 05/2015.

Group 4: E70, E71, F15, F16X5 and X6 35iTank 17 13 8 621 092.

Group 5: F25, F26X3 and X4 xDrive35iTank 17 13 8 616 418. Some X4s built 12/2015 to 03/2018 are listed without the xDrive35i name: check by VIN.

A wrong number that looks right. 17 13 7 640 514 is one digit off the E-series tank and fits N51, N52, N54 and N20 cars, not N55.

BMW's catalog lists a separate level switch (the sensor on the tank's bottom), 17 13 7 553 919, for every group except Group 2. On some cars BMW says the sensor comes only with the tank: check the listing before you buy a switch.

No N55 tank in stock right now: order the number above anywhere.

Cap and coolant

Cap, most N55 carsBMW 17 11 7 639 020Screw cap on nearly every N55 car above.Genuine cap $43.99 →

Cap, E70, E71 and early F07, F10, F12, F13BMW 17 11 7 639 021The only cap listed for the E70 and E71. Early F07 (to 12/2011) lists it, and early F10, F12 and F13 (to 03/2012) list both: send us your VIN.URO cap $24.99 →

BMW blue coolantBMW 82 14 1 467 704Nitrite-free, 1 gallon concentrate. Mix 50/50 with clean water.Blue coolant $37.99 →

BMW green coolantBMW 83 19 2 468 442HT-12, 1 gallon concentrate.Green coolant $43.99 →

Match the color already in your tank and never mix blue with green: BMW says mixing coolants can gel and damage the engine. Late F16 (from 07/2018) and F06 (from 08/2018) may hold green, and BMW put it into some older cars through campaigns from 03/2018.

Never reuse drained coolant: replace what you drain with new. A full fill is 6.7 to 10.9 L by car (send us your VIN for yours). BMW plans on 2.0 L of top-up when the hoses are disconnected.

Small parts to check while it's apart

Vent screw 11 53 7 793 373. The plastic screw on the tank that vents it. Listed only for Groups 1 and 2. O-ring 11 53 7 793 248 on Group 2.
Rubber grommet 64 12 6 910 076. The tank's locating pin (a small peg) sits in it. Replace it if cut, hard or out of place. Listed for Groups 1 and 5 and Group 2 except the F87. None for Group 3. Group 4 (X5, X6): send us your VIN.
Hoses. Replace if hard or porous (parts-site advice). X3 and X4 vent hose 17 12 8 651 298, X5 and X6 vent pipe 17 12 7 536 235, F15 and F16 return line 17 12 7 623 272.

Figure 4One failed tank. The crack ran across the plastic weld at the top left of the back. From our bench.

What you'll need

BMW's special tools: hose-disconnect tool set and pressure tester (kit 17 0 100, pump 17 0 101, adapter 17 0 113 for the tank neck, 17 0 114 for the cap).
Also needed: battery charger, hydrometer (antifreeze tester), jack and stands if underbody parts come off.
From owners: 10 mm socket and ratchet (two bolts on the E-series and X1 tank), catch pan, towels, funnel.

Safety

Never open a hot system. BMW's scald warning: cooled engine only. Goggles and gloves on.

Coolant on the floor is a skid risk. Drip pan under it, dispose of it under local rules. Shop rule of thumb, not BMW's: wipe spills at once, keep used coolant sealed.
Using a vacuum filler? Cover the alternator. BMW says coolant on it can cause alternator failure.
Car on stands, level ground, never on a jack alone, before any wheel, liner or underbody work.

Your car's version of the job

BMW has several versions of this job and doesn't always say which car each is for. Unsure? Send us your VIN.

How much to drain. BMW's versions: partial drain to the tank's lower edge, full drain, or clamp the hoses and top up. Shop rule of thumb, not BMW's: drain at least to the lower edge, all of it if you pull hoses off the radiator.
What comes off first. Most of BMW's drain texts start with the front underbody protection. Some cars also need the fan cowl, the charge air cooler, or the intake port and a cross brace out. F23, F33, F87: BMW adds the right front end strut. Dynamic Drive cars: BMW has you remove the left trailing link and intake port and set the Dynamic Drive reservoir aside.

Out in this order

Let it cool
Car on standsIf underbody parts come off
Parts off first
Cap off, drainDrained coolant: never reuse
Hoses off the tank
Mounting screws
Lift the tank
Sensor plug
Lower hose
Tank outPin leaves its grommet

Out

Engine cold, car on stands if your version needs it. Take off the parts it lists.
Cap off, pan under the drain point, and drain as your version calls for. Old coolant goes in a sealed container for disposal, not back in the car.
Photograph the hoses and sensor plug, then take the hoses off with the disconnect tool. Unlock each, pull straight off.
Mounting screw or screws out, if you see any (some cars have none).
Lift the tank far enough to reach the sensor plug and lower hose at its bottom. Unlock and unplug the plug, then unlock and pull the lower hose, pan underneath.
Tank out, without damaging or moving the rubber grommet its locating pin sits in. Check the plug's contacts.

In

Hold the new tank up to the old one. Same ports, plug and pin. If the listing says the level switch is separate, fit it now.
Fresh parts: a new cap if the old one failed the test, a vent screw and O-ring where your car lists them, a grommet if the old one is cut or hard.
Tank in, locating pin into the grommet.
Lower hose, plug, then upper hose (BMW's order on the X3 and X4; elsewhere, reverse your removal order). Each lock should click. Check the lower hose sits in its bracket (BMW warns of this on the four-cylinder F30; your six-cylinder isn't confirmed).
Mounting screw or screws in, to the torque for your car (Torque, below), not by feel.
Fill slowly with new 50/50 coolant to the lower edge of the filler neck, then close the cap until its arrow marks match. With a vent screw, BMW's older pour-in text opens it while you fill and closes it before the cap.
Bleed it as below. Don't skip it.

Torque

Tank mounting screw or screwsBMW's documents give different numbers and don't all say which car. Send us your VIN and we'll send yours.

Plastic vent screw, where fittedNo BMW N55 number. Go gently: a parts site reports plastic ones strip.

Bleed it BMW's way

BMW wants a bleed after any cooling-system part change or refill. Its five versions share this core. These are the pour-in steps, with the closed cap and low beams that four of them call for.

Battery charger on.
Cap closed, arrow marks matched. Don't open it during the routine.
Ignition on. Don't start the engine.
Low beams on. Without them the routine switches itself off.
Heat at its hottest setting, blower on its lowest speed.
Not in ECO PRO (driving experience switch).
Floor the accelerator pedal for 10 seconds. This starts the pump.
Let it run. The electric coolant pump shuts off by itself after about 12 minutes.
Top the tank up to MAX with new 50/50 coolant (one version: about 6 mm, 100 ml, above MAX). Done when the level holds and nothing drips. BMW says to check the mix with a hydrometer. Shop rule of thumb, not BMW's: recheck the level after the first drive, once cold.
Running it again? Take the key out for about 3 minutes so the engine computer sleeps, then restart from the ignition step.

Where people mess it up

Skipping the bleed routine. Topping up and driving off leaves air in the system, which can leave the engine running hot. Watch the temperature gauge on the first drive and pull over if it climbs.
No charger. Owners report the routine won't start on a weak battery.
Reusing coolant. BMW says never. New 50/50 coolant, every time.
Prying hoses. Parts sites say prying cracks the brittle ports. Use the disconnect tool and make every lock click.

Steps, test values and part rules come from BMW's repair instructions for the expansion tank, the cooling system leak test, and cooling system filling and bleeding. The F30 hose-bracket warning comes from BMW North America service bulletins. Owner, forum, parts-site and shop-blog tips are not BMW's. Part numbers come from BMW's parts catalog up to 2020. Check yours by VIN.