BMW Z4 M40i Upper Radiator Hose Replacement (G29 B58)

Fits US G29 Z4 M40i (B58)

US G29 Z4 M40i (B58). The Toyota GR Supra shares the same long-hood layout, so these steps fit that car too. Send us your VIN and we'll send yours if your hose part number differs.

Replace the upper coolant hose when it leaks or the quick connectors will not hold pressure. The hose runs from the radiator to the heat-management module. On the Z4 M40i B58 you remove the DME, integrated supply module, and control unit holder to reach it, not the cover-only path BMW lists for B48 and B46 four-cylinder cars. Plan for underbody panels plus a deep engine-bay teardown.

Holder screws: 8 Nm
Cross brace: 19 Nm
Stiffening plate: 56 Nm + 45°

Before you start

Hot coolant can scald: wear PPE. Catch coolant and dispose per local rules. Disconnect the battery negative before you unplug the DME or supply module, and do not connect a charger at the jump-start point. Touch control-unit housings only with ESD care. Do not drive without the front stiffening plate reinstalled.

Out in this order

Front underbodyplus lower side covers
Rear underbodythen stiffening plate
Front trimbonnet seal fixtures, cross connection
Acoustic covertop of engine bay
DME and PDMwith control unit holder
Upper coolant hoseradiator, secondary port, HT module

What you replace

Upper radiator hose

17 12 8 654 803

Coolant line holder

17 12 8 654 893

Stiffening plate bolts

M10 at the front axle support. New bolts every time.

Steps, torque values and part rules come from BMW's repair instructions for Z4 M40i upper coolant hose replacement. Part numbers come from BMW's parts catalog up to 2020. Check yours by VIN.

Drawing of the upper coolant hose quick connectors at the radiator with numbered dots: 1 Upper coolant hose at radiator; 2 Secondary coolant port; 3 Quick connector lock; 4 Coolant line holder — Upper coolant hose at radiator

Secondary coolant port

Quick connector lock

Coolant line holder

Generic layout: yours may differ.

Tools

Socket set and trim tools

Underbody, trim, and cover hardware.

Drain pan and shop towels

Battery wrench and ESD-safe workspace

Coolant fill and bleed gear

Steps

Remove underbody panels
1. Front underbody protection
2. Underbody TS5 screws
3. Lower side cover screws
Generic layout: yours may differ.
Remove the front underbody protection and lower front side covers at the front of the car so you can reach the stiffening plate bolts.
Drop stiffening plate

Remove the rear underbody protection under the car, then the front stiffening plate at the front axle.
Front cross connection

Remove the left and right bonnet seal fixtures, then the front cross connection.
Remove acoustic cover

Remove the engine acoustic cover without jerky force; release each snap lock on its own.
Disconnect battery and ECUs
1. Supply module connectors
2. Supply module lock
3. Integrated supply module
4. Control unit wiring
Generic layout: yours may differ.
Disconnect the battery negative, then remove the DME, integrated supply module, and control unit holder.

Caution: read fault memory after disconnect and clear entries if needed before reassembly.
Remove upper coolant hose

Unlock the upper hose at the radiator, secondary port, and heat-management module, catch coolant at each port, unclip from the holder, and remove the hose.

Caution: hot coolant. Each quick connector lock must click audibly when you reinstall the hose.
Install new coolant hose

Install the new upper hose, connect every port until the locks click, and clip it into the holder.
Remount control unit holder

Reinstall the control unit holder and torque the M6x16 screws to 8 Nm, routing harnesses so nothing pinches.
Reinstall PDM and DME

Reinstall the supply module and DME and reconnect every plug.
Refit front trim

Reinstall the acoustic cover, front cross connection at 19 Nm, and bonnet seal fixtures.

Caution: install the acoustic cover only above 20 degrees C. Use distilled water as an install aid, not lubricants.
Refit stiffening plate

Reinstall the stiffening plate with new M10 bolts to 56 Nm plus 45 degrees, then refit underbody panels at 2.6 Nm on the lower side cover and 3 Nm on wheel arch and underbody TS5 fasteners.

Caution: stiffening plate bolts are new every time.
Refill and bleed coolant

Fill and bleed the high-temperature coolant circuit after the hose job. Send us your VIN and we'll send yours for the bleed routine.