PeachParts Mercedes-Benz Forum - View Single Post

JimSmith · #3 12-14-2003, 04:34 AM

sohj,

I want a place in Heaven too, maybe then I will actually get to finally meet leathermang since he has already answered your questions pretty thoroughly. I am a little late but typically much wordier than Greg, so as long as my place is within walking distance of his I will be ok.

1. The crank bearings are the stationary supports at the center of rotation for the crankshaft, and MB typically has a set of them (they are normally split rings, with the split at the horizontal centerline of the of the bearing "journal" of the crank support bearings that are captured on the upper half by the structural ribs of the crankcase, and by bolt-on caps that look a lot like pipe hanger clamps but beefier, on the lower half) between each cylinder. Since this comprises the rotating elements of the lower part of the engine (it is rare to find a crankshaft at the top of the engine but it has been done so, like in Nature, there are no single solutions), it is commonly referred to as the "lower end."

On the parts of the crankshaft that are offset from the center of rotation there are bearing journals for the bearings that support the connecting rods, which are connected to the pistons on the other end. Similar kind of bearing. They are fed by pressurized oil from the oil pump and have specifically sized radial gaps for the oil to enter and leave while maintaining a continuous film thickness needed to prevent metal from contacting metal while the parts move relative to one another. The oil is fed to these bearings through passages that are drilled in the crankshaft support bearings and then through the crankshaft out to the connecting rod support bearings. The oil leaves the bearings on the axial ends of each bearing and gets flung around inside the engine crankcase/block area and drains back down to the pan where the pump suction line is located.

2. The rack dampner bolt is the subject of lots of discussion. Do a search to find out all about it. I don't have one on my 240D, so I have not encountered this item.

3. Throttle linkage travel is as leathermang described, how far the throttle moves when you push the throttle pedal from its normal "idle" positon to the floor. There are a couple of places in the linkage where the motion is converted from linear motion to rotational motion. At these joints there is also often a rubber part involved and over time the rubber shrinks and hardens, and the linkages get misaligned. Have someone sit in the car and exercise the throttle pedal while you watch what happens and you will see what I am talking about. The first place where you loose travel is the bushing/bearing bolted to the firewall where the linear motion of the pedal gets converted to rotational motion that goes to a cluster of levers and bars supported off the valve cover, where it is converted back to linear motion. The firewall bushing can just loose a chunk of the rubber and then instead of pure rotation you get some vertical translation before the rotation starts, and loose that much "travel" of the throttle linkage on the injection pump. The cluster of linkage bars supported off the valve cover has a similar propensity to get loose and waste some available travel. Finally, these things can get misaligned with all this unwanted motion and actually bind. This is a common problem on a twenty or more year old car.

4. The thermostat is a regulator like the one on the wall in your house, but it does its regulation by controlling water flow volume and direction by changing the shape of the flow passages in it in response to the temperature of the water that it is in contact with. It is not powered by electricity - it is powered by heat and the differential expansion rates of a set of springs inside it. They wear out after time, and fail to properly regulate the water temperature. When this happens another item in the flow passage of the engine coolant that has a part in it that changes its electrical resistance in response to temperature changes, and is connected on one terminal to the battery and on the other to the temperature gage in the instrument pod, which then sends a specific voltage to the gage where that voltage causes the needle on the gage to take a specific position from the pegged cold spot to the pegged too hot spot. Where the needle sits is dependent on the water temperature, and the condition of the "sensor" or the thing responding to the water temperature. The sensor can fail, as can the mechanism that moves the needle, but the sensor is in a generally harsher environment and is typically more likely to fail.

5. Do another search on engine mounts. When you are in drive and forcing the car to stop with the brakes, the engine speed is dragged down a little, which means the frequencies of the sources of normal vibration excitation for the "system" are reduced. If the car starts jumping around this is a sign the rubber in the engine mounts has aged and hardened, increasing the natural frequency of the mounts. This means the mounts, instead of absorbing the vibrational energy are being excited into a resonant condition, and the vibration level increases even if the level of the input energy is lower. Time for new mounts and there are lots of posts on changing mounts. Not a really difficult thing to do, but it can be really dirty if your car has oil leaks and the like and is oily and dirty. I have learned to wash the engine and the area I will be working on at the do it yourself high pressure spray bays.

6. Yeah, don't sweat those lines between injectors unless they leak.

Now, most of this stuff, except the rack dampner bolt, is pretty straight forward and you should be able to find a "How Things Work" type book at the local library on engines with great illustrations and pictures (crankshafts, bearings, journals, bearing caps, oil passages, the head and its working parts, pistons, and so on). If you do the search you will likely find a post or dozen with photos of the actual old, new and installed rack dampner bolts.

I hope this helps, and good luck with your Diesel, Jim