BMW M4 Restoration

Unfulfilled with losing money through traditional recreational means, I decided to up the stakes and buy a Convertible BMW M4 from salvage auction. This particular car was classed as flood damaged, but runs and drives.

The roof, boot and rear windows were all inoperable, on top of this the rear interior of the car is stripped. The pace of this write-up is quite quick as there's a lot.

Upon arrival of the car - it was dead, a jump pack turned the speedo on but the infotainment and AC controls weren't responding, neither were any of the electronics in the rear of the car.

However - the car on the whole seemed to be in really good condition - the body was fine, interior also fine (although in various pieces). There was a little hint of mould on the rear strut mounts but only in this location, fishy.

After trying and failing to start the car using a jump pack, (it turned over though, thank christ) I started inspecting the rear a little more.

Knowing BMWs' a little my instant thought was to REM, the Rear Electronics Module - this module controls all electronics in the rear of the vehicle and also was around the location of the mould found previously.

Someone had clearly investigated this before as the REM was off it's mounting plate. I unplugged the REM and checked the CAN bus on the loom side and confirmed - there was a short.

I moved on though for now - the engine CAN bus is on a higher speed network and this shouldn't prevent the car from starting. I fired up ISTA and attempted to connect to the car, where what could only be described was a wall of DTC's were displayed, most of these were low voltage codes however - from the dead battery.

I cleared these and hooked the car up to a fresh battery and attempted to start the car.

The engine in this thing is the most valuable part by far. If this didn't fire because of mechanical issues - that, was it.

I clicked start and to my absolute surprise the car roared into life. However, the rear (roof, windows, lights etc) were still dead.

Upon further inspection of the rear, (rear left, NSR in the UK) I noticed an aluminium module that was absolutely covered in corrosion. I'm unsure if I'm right in this assumption but voltage, water + aluminium was what I believed to be the cause of this.

I checked out what this module is actually responsible for - it was the ASD module or "Active Sound Design", not critical. it’s basically the amplifier for the car that also injects intake noise into the cabin.

Enthusiasts are rather torn on this module I later learned, long gone are the days of the V10 S85 in the E60 M5 and the twin turbo chargers attached to the inline 6, really isn't the same sound.

Regardless, I temporarily unplugged this module, checked for shorts on the pins of the module and yes - this might be the culprit.

Surely not though - this is an M4 for over half off the retail value, with the rear interior stripped out and clear evidence this module has sat in water. Surely, I'm not the first to see this.

On inspecting the ASD, the state of the interior of the module was too far gone - I've done board repair electronics and still have all the gear, but this is ridiculous. Plus a used replacement is only £80.

Nethertheless I fired up E-SYS and coded the replacement module and gave the car an entire software update now the CAN bus is functioning correctly.

Again, I tried the ignition, and f**cking jackpot, the infotainment and A/C controls are back!!

My celebrations were short lived however as the rear windows and roof were still inoperable, back to ISTA.

I had a code for window not normalized, which upon research means the car doesn't know exactly what position the window is in. I've had convertibles before and due to them having a roof that might, or might not be present and no B-pillar, I believe the location of the rear windows is fairly crucial to the operation of the roof. Please God don't be something more sinister.

Upon removal of the window regulators, which was a total pain in the arse due to the windows/roof not dropping and me being unable to find a manual release for the roof. Which honestly wasn't my favourite option anyway - I don't have a garage, and opening a convertible roof on the chance that you may be unable to re-install it, in British Weather no less, is just asking for it.

The first regulator was a pain, I used a towel so as not to scratch the trim, but getting that sodding thing out of the door while fully extended was a nightmare. It confirmed my suspicions though that the regulator was toast.

Check out the water line too, it got pretty high.

Likely the same on the other side too - I found a neat trick for the second window though. Once removing the door card you can just snip the cables in the regulator and actuate the window manually.

After getting a shopping list (2 rear regulators, 1 motor drive) and rushing to my local BMW Garage for parts, I did still have a question of how this even happened? It's a coupe so no rear doors.

with evidence of water in both wheel arches and the boot too, I found this:

https://static.nhtsa.gov/odi/tsbs/2016/MC-10147023-9999.pdf

...which gave me my answer.

Water normally creeps past the seals in the window and if the drain is blocked (leaves, debris etc), this water accumulates in the rear "doors"? and slowly makes it way to the rear of the car under acceleration.

There must have been some noises or sloshing in the car, or a clue that this was happening. Although I did see a Trance Megamix left in the CD slot, so perhaps that excuses the previous owner’s lack of hearing.

It also seems that the recommended remedy has already been applied to this car - both drain plugs are removed and a small 10mm hole in either sill.

After fitting all the parts, I nervously raised and lowered the windows a few times, in the hope it might re-learn positioning. I then tried the roof and holy hell it worked.

I just had a few things to sort out - valvetronic motor needed replacing + new headlight washer jets (needed for MOT, non-LED headlights).

After all this though I had a totally functioning car, time to get the rear interior back in and inform my car insurance company of my stellar financial choices.

My plan was to sell the car, I have two kids. A 2 door convertible just isn't practical. I figured though it'd be a waste to just get rid without enjoying the car first. So I planned a trip to the Nurburgring, The green hell.

I always wanted an e92 M3 or e60 M5, however the fact the engines didn't share any common parts with any other BMW models and the fact internal engine components (rod bearings/main bearings on the e92) are a serviceable item. I could never justify it.

I'll leave this here but predictably I fell in love with this car and it's still with me today.

I doubt I'll ever sell it.