oily trigger points-continued - PeachParts Mercedes-Benz Forum

berfinroy · #1 09-08-2009, 10:32 AM

I have read many posts on this forum concerning the chronic problem of D Jetronic oily trigger points as used on the first generation 3.5/4.5 engines. I now strongly suspect that an inherent deficiency in connection with Mercedes' closed loop crankcase pressure relief system is to blame. Even when the cam cover breathers and PCV valve are functioning properly, the system does not adequately relieve the buildup of crankcase pressure, which in turn forces oil up the inner distributor shaft into the trigger point chamber. Consider the pumping pressure of those eight cylinders at highway speeds and the relatively puny demensions of the pressure relief system.

Obviously, I still have the problem after following all advised steps to ensure that my system is operating properly. I am now looking for ways to improve crankcase ventilation by other means. Any ideas or comments?

meltedpanda · #2 09-09-2009, 01:02 PM

have you cleaned out all the little screens that are in the air passages, ( look like christmas trees)

berfinroy · #3 09-09-2009, 05:09 PM

Assuming that there are only 2 of these, one at eachcam cover opening, yes. Are there any others?

Chas H · #4 09-09-2009, 07:01 PM

I have read that worn bushings in the distributor can allow oil to get to the points. I think excessive crankcase pressure might cause leakage elsewhere such as main seals.
You could make up a pressure guage to monitor crankcase pressure while you drive.

meltedpanda · #5 09-09-2009, 09:31 PM

no others,but a common miss for some

Graham · #6 09-09-2009, 10:01 PM

I have a '72 D-jet - I think it has a crankcase vent line that runs across the front of the engine and then down the drivers side back to the fuel tank vent near the rear wheel well. Sort of black plastic just below PS pump. It is apparently used to vent overpressure from the fuel tank and relies on there being a negative pressure in the crankcase

There is an orifice where the driver's side valve cover vent pipe connects to the idle air valve assembly. If that was blocked you may get pressure build up.

The '73 may be different, but perhaps has something similar. If this was blocked, you may get excessive crankcase pressure.

berfinroy · #7 08-28-2013, 09:49 AM

I'm following up on this earlier post with a "solution" that seems to be working for me.
Make a disc 1/16..1/8 thick out of relatively stiff absorbent material, the outer diameter being equal to the inner diameter of the distributor housing, and the center hole being equal to the diameter of the distributor shaft below the trigger points. Make a cut through the disc from the center hole so you can insert the disc around the distributor shaft below the trigger points without disassembling the distributor. The disc will absorb oil creeping up the shaft before it can foul the trigger points.
How long? Can't say for sure...I haven't had a problem for several thousand miles , but it surely depends on how much oil is creeping up the shaft, which in turn depends on degree of distributor wear, crankcase breathability, etc. But its one "solution" to a common, vexing problem with old D jet systems.

meltedpanda · #8 08-28-2013, 11:48 AM

nice work, chance you could post some pictures?

Benz Dr. · #9 09-01-2013, 04:15 PM

There is a seal inside the distributor that's supposed to prevent this from happening. The problem is, it's a bit of a job to get at it as are most jobs on this unit, which was probably built with no service in mind.

The seal is made of felt IIRC.

Graham · #10 09-14-2013, 11:22 AM

It might be a chronic problem, but I have never had it on my 72 D-Jet. Oily trigger points are often blamed on the forums for poor performance, but not so sure that is always the real problem.

Later versions of trigger points had covers over the point contacts, but I don't even have those. Maybe I am just lucky or my distributor seal and bearings have held up better than most.

Perhaps problem as cars age, could be increasing crankcase pressure caused by worn rings etc.? Perhaps check the vent orifice as well as the screens. Some time ago, I saved this diagram. Helped me understand vent system a little better:

mercmad6.3 · #11 09-22-2013, 04:23 AM

It has been my experience that oily points are caused by excessive blowby in worn engines. Once the blowby reaches a point where the crankcase is pressurised rather than vacuumed ,oil will leak out out anywhere and the distributor shaft is one place where it leaks easily .