Thats no my vacuum setup. Close, but not identical. There is no vacuum line to the distributor on my car, it goes to an advance module/coil. And some other lines do not match. This is the diagram I used.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Well so much for the notches in the harmonic balancer. Useless they would seem. I can feel the bolt I have to remove, but its closer to crank then the notches allow to get at. If the notches were maybe a half inch deeper it would work. So not sure which half-brained engineer designed this thing, but getting the water pump out is back to being a total pain.

So back together it goes for now with deep loathing for german engineers. I did do some more searching online too, indeed the balancer must come out on the 560.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Perhaps when faced with a major disassembly one might as well replace all the things in that area which might go soon rather than repeat. Not sure what that might be in this case.

Maybe the SLS hose. I noticed its easier to get at with the water pump out.

Well I thought this would be more of a pain than it turned out, sorta. Online indicated air tools to get the 27mm bolt at the crank out.

Well for one thing, that write-up is dead wrong. I locked the engine and got it out with a breaker bar easily. But where its wrong is that it does not need to come out. All that need to come out are the six 13mm bolts. Once those are out, the pulley assembly and then the harmonic balancer will come off. Mark them before removing.

Also the distributor is of course adjustable. Remove the adjustment screw and the pillar that it goes into does pop out. I did not know this and removed the entire distributor. No one in any write up mentions this fact. So once the screw is out, just tap that piece and it will come out. Then you can easily get to that water pump bolt. Now I have to re-time the engine a little.

Do not over-tighten the pump. I noticed a small rip/ripple in the gasket. Did not think I did, but must have.

When reinstalling the pulley assembly, be VERY careful when tightening the six bolts. The first time I had it skewed just a little and when I started to tighten it was not aligned at all. Just go slow and tighten all a little at a time to make sure it seats right. One bolt did not go in and never will. I had a heck of a time getting the last one in. Might be why I had the above issue since I backed off the other to try and get it. After several removal and reinstalls, I gave up and will have to settle for five bolts...

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Update: Well she is still running hot. The more you guys mention other items, the more I had doubts it would help. But like I said a few posts back, even if it did not fix the issue I will have a new pump and can rule it out for sure. The old one is in pretty good shape. No play in it and no wear of the impeller.

Granted, I have not burped the system like crazy like I did before. But I am not giving hope to that being the issue this time. I will of course do so tomorrow anyway.

So next, another t-stat to be sure its not the issue. Takes me 10 minutes to change now after all. I am going to run an O2 test for setting mixture again, been a while. Then I will remove the radiator for closer inspection. If the radiator was not so pricey, I would toss one in for peace of mind too.

One thing is for sure, my timing may have been a touch off. All I did was set the engine to TDC and line up the marks on the distributor. That may not be the correct method, but the car had loads more power. And to think I thought it had power before. I can actually chirp the tires now on take off.

And if all else fails, the intake is coming off. That was the plan anyway. When I did it the first time, the boot did not look so well. I passed doing the donuts too because I was going back in. So in a coupe weeks it will be out for a full overhaul anyway. Maybe I will find something then if I still have the issue.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

You are aware that the pully/blancer/hub will only go back 1 way? All the holes are not the same distance apart. One is off set so it will always go back together the same way. Sounds like you may be off 1 hole and that is why you can't get the 6th bolt back in.
On the running hot problem, I usually don't repeat myself but I have to mention again about the engine grounds. If the engine is not grounded the way Mercedes intended then it can cause gauge problems. More grounds is OK, but it still needs the original grounds. I know you are convinced that the gauge must be correct because the aux fan comes on at the correct engin temperture and they use different sensors. This is correct and generally would be a good assumption but remember they also use the same grounds. From everything I have read so far, I don't think the engine is running hot, you just believe it is.
While going down the road at 50 mph or more, you could take off the engine fan and aux fan and the car will be happy. They are only there for slow or stopped. Thermostats either work or they don't and I believe you have tried 2 or 3 with the same results. Burping these engines is easy. Just fill it up to the top (over full) and drive it. If it is too low when it cools, refill to the full mark. I just did the upper timming chain guides and flushed the cooling system while I was there and had no problems getting the system full or getting the air out.

Paul

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84 500 SEL (307,xxx miles)

LYL:

In checking the condition of various vacuum lines you noted that removing and reconnecting the line to the ignition unit (EZL) caused a change in engine speed; that's a good sign; at least that portion of the EZL is working.

A couple of additional checks are in order. Do you have a timing light, and an ohmmeter?

With the timing light, check the timing at idle speed, and up to about 3500 RPM. With vacuum, idle: 10-14 BTDC, 3500: 40-44 BTDC. Without vacuum, idle: 3-7, 3500: 24-28.

With the ohmmeter, check for continuity from the temp. sensor to the temp. terminal in the plug on the EZL. Which plug on the EXL do you want? If you were viewing the EZL such that the two plug connections were "down", and the vacuum connection were "up", the plug on the lower left contains the temp connection. It also contains the throttle valve switch connection, and the reference resistor connection, and an unused connection. On the dual temp. sensor the two circuits are connected diagonally, one supplying the temp gauge, and the other the EZL and the ECU. If you pull the insulation jacket back a bit (at the sensor) the green with black tracer is the wire you want.
What's all this about? If there is an open circuit between the sensor and the EZL, the advance range of the EZL will be partially inhibited, resulting in less advance than is required for proper warm-engine, cruise operation. This inhibition of advance with a cold engine (high resistance) is a design feature to speed the heating of a cold engine!!! Seems rather consistent with what you are observing. If the EZL never gets a warm signal (low resistance), the advance remains inhibited, and the engine runs too hot.

Just a thought, but do you have a temp gun? The kind with a laser dot that gives you the surface temp of what you are pointing it at. Also you can unplug the coolant temp sensors and check the ohm readings. This will get you past the potential ground issue.