Story by
the backyard hack mechanic: Sean750
Document history:
12 Feb 2005 : added full intake gasket resealing on both
manifolds and rewrote some parts
22 Jan 2003 : corrected a wrong intake manifold torque value with
thanks to Patrick C 88 750i
17 Jan 2003 : initial version
Problem : the last couple of months my V12 developed a rough idle. These engines can run like sewing machines, but not in my case. More like a 2-stroke chainsaw...first I looked for the obvious problems. Vacuum hoses, bad ignition wires, bad spark plugs....you name it, but I couldn't find the real source of the problem. Changing the ignition wires seemed to help a bit, but I still didn't have that smooth idle. Glad I didn't spend 600 Euro/USdollar for a complete new spark plug wire set....
At a Saturday afternoon, I
decided once more to search for the problem, but with a different
approach. I removed the plastic engine cover that covers the fuel
injectors and I disconnected the 6 injectors of the left bank:
After this, I started the engine. The engine should run on 6 cylinders almost even smooth when running on 12 cylinders (unbelievable, but true). I repeated the procedure at the other bank and noticed a big difference between the left bank and the right bank. The left bank was running smoothly and no vibration whatsoever, and the right bank was stumbling and hesitating. Now I knew which bank caused the problem.
Engine running only with the hesitating bank, trying to investigate where the problem lies, I leaned forward with my elbows at the top of the left intake manifold (which is connected to the right bank, still getting it?). At a sudden the engine was running even worse. I wiggled the manifold slightly up and down, and the engine almost stalled. Right. This one is gonna cost some serious bucks. I was already afraid of this one: the infamous intake manifold gaskets where leaking....aaarrrggghhhh.
These intake gaskets are expensive. Very expensive. I mean really expensive. You'll gasp for air when you are laying on the floor before the counter of the local BMW dealer. We are talking about a stunning 100 Euro/USdollar for each gasket. And you need 4 of them.
At that particular Saturday afternoon, that wasn't simply the solution for the problem. The local dealer was already closed (and most likely he had to order the parts because they where not in stock), my wallet was empty due to the formerly replaced front wheel bearing and spark plug wire set, and the car must run at Monday because I had to go to my work (to earn the money needed for the parts to keep my car running.....what a life).
Decision: to quickly remove the left intake and put some sealant between the intake manifold and intake gasket, reinstall everything and hoping the leak isn't between the gasket and the cylinder head.
This was added at Feb 2005: this turned out to be a bad decision after all. Later on, I did both manifolds and also resealed the surfaces between the gaskets and the cylinder heads. I added this procedure to this story. This was a better solution.
Models : 750i(L) with M70 (V12) engine
Background: the large intake manifolds gaskets aren't the usual, paper thin gaskets. These are thick 'rubber' gaskets, with a nasty tendency to shrink over the years, causing vacuum leaks and poor idle. Someone else reported that the gaskets are thick because it prevents transferring heat from the cylinder head to the intake manifolds. Nevertheless, its probably a bad design, virtually every produced V12 has had this problem or will suffer from it in the nearby future.
Lets start unbolting things : REMARK! The photos are showing that the black plastic cover that protects the complete injection harness is removed, exposing the complete wiring of the injectors, engine sensors and DK's. You don't have to remove this cover. I removed it due to some changes/experiments with the injection system. You can lift the entire harness, stuck between the fuel rails, up and set aside.
Also, you DON'T have to remove the DK motor (throttle body), instead of what the Bentley Book and the factory manual are saying. Keep it attached to the manifold.
After removing
the plastic engine cover and removing the coolant expansion tank,
remove the brackets above the injector harness:
Disconnect every
injector connector, use a flat-blade screw driver to push the
retainer clip up:
Disconnect both
DK motors, sometimes a screwdriver is necessary (a piece from the
connector disappeared):
Remove the
cylinder identification sensors and crankshaft positioning
sensors at the front of the engine:
Remove the
clamps that hold the connectors:
Now lift the
entire harness up (don't get scared of this picture and read the
remark at the begin of this section, you can leave the black
plastic cover onto the wiring harness. I made some modifications
and didn't reinstall it, ending up with this myriad of wires) and
set aside:
Remove the air
mass meter and crankcase vent hoses:
Detach the vacuum hoses of both fuel pressure regulators:
Loosen fuel hose
clamps (2 supply hoses, 2 return ending up in 1):
Remove the Allen
bolts of the fuel rails (one rail is already removed to make
things more visible for the photo):
And remove
entire rail with injectors (pull it gently straight up):
When both rails
are removed, it should give a nice picture like this:
for the next page click the arrow ....
