Heater Plate Removal and Coolant Hose Re-Routing
It’s dark and dank underneath the intake manifold assembly, so if you come out of this repair only missing an arm and a small wax-covered paper boat, you’ll come out ahead.
Take your M20b-whatever, with it’s easily accessible coolant and vacuum lines and shove ’em! We’ll bare all of our compensatory machismo through our teeth and tell you that you’re not a real automotive enthusiast unless you have to disassemble upper and lower intake manifolds, detach the throttle cable, move the intake boot and MAF aside, remove the throttle body, and tell your wife’s boyfriend that the suit he’s wearing for his date night with her fits him well, just to be slapped in the face by the swarm of emissions-related vacuum and heater plate coolant hoses on the vehicle that must be removed to facilitate the bypassing and liberation of said heater plate! Apart from all of that, it’s not terribly difficult, in theory.
The hardest decision to make, is whether you want to effect this modification or not. If you live in a warmer climate, it’s a shame your car came with it at all as it almost appears to be a cog in a conspiratorial machine to get you into a service bay. There’s a tangled combination of wet and dry hoses tucked snugly underneath and behind the intake manifold assembly and a wiring cache that make it impossible to do any of this work without removing both halves as if it were an aluminum chastity belt with a black plastic heart for a belt buckle. Its purpose is genuinely served in sub-zero temperatures as a preventative to a stuck throttle plate, but for everyone else, it’s a must-go. By now in 2019, even if the car hasn’t been driven much, these lines are dry-rotted and if they’re not leaking coolant already, you should play the lottery.
Upon first glance, this is a rather confusing diagram of how to re-route this system, but from personal experience we can confirm that it works and is accurate. Fast forward to having the intake manifolds, throttle body, and everything adjacent removed, you’re looking at pile of hoses that remind you more of overcooked spaghetti noodles and watery sauce than a precise German automobile. We won’t complain about over-engineered emissions components because honestly, we’re as tree-hugging as the next co-op volunteer, but we won’t admit that it didn’t make us grit our teeth. The basic route of coolant flows from the head to the heater plate and then into the block. The heater core circuit flows from the head to the heater core, to the block, through the radiator, and eventually into the head again through the magic of the coolant passageways.
Get all of your tooling together. Mostly hoses. Get one or two foot lengths of 5/16” hose, 19/32” hose (or close to it), 5/8” hose and enough hose clamps to cover each end of each hose. This would also be a good opportunity to replace the two temperature sensors located near the front of the head. A great enthusiast would take notes or pictures to ease the load on the memory or identify which sensor goes in which hole, but a good-enough enthusiast would notice that the holes are two different threads. As an FYI, one of the sensors reports to the dashboard cluster, and the other to the ECU. Get your coolant ready because you’ll have to replace a little bit of it afterwards, especially if you forgot to tighten the rearmost coolant hose on the head after you already reinstalled the intake manifolds. Fortunately we had a cheap USB-micro borescope handy to help us identify exactly where it was leaking from so we could use a small 1/4″ drive ratchet to tighten it. Hopefully you have a BMW compatible coolant left over from replacing your thermostat housing because our theory is that the corrosion we experienced on the aluminum housing was likely from an off-the-big-box-parts-store-shelf green coolant. Take some time to study the above diagram, because you don’t really need a step-by-step if you can make sense of it, but we’ll give you a link to a great tutorial on it anyway (linked in the description of the re-routing diagram). You’ll be happy with the results because it relieves the stress of ever having to chase down hard-to-reach vacuum leaks again and if you like to play around with injectors, it makes removing the manifolds twice as easy, and nearly downright pleasurable to do. Use this is a guide or follow the dozen good instructional forum posts out there and enjoy the fruits of your labor because just like Secret Santa they’re completely invisible to the on-looker unless you tell them.