The story of the 240D and the Diaphragm

All engines have "blow-by" gasses (from leaks past the piston rings) in their crankcases and these are piped into the engine's intake manifold and so eliminated. Some lube oil is carried along with this mist so there will always be some oil in the motor's intake manifold. It means nothing.

Now on to your question. How much oil do you have to add to the motor between changes. If 1 qt/ltr or less in 5K miles stop, its "normal."

I can't understand the, if any, connection between the injection pump and any oil in the intake manifold. Perhaps I'm missing something.

Does the vacuum pump (mounted on the front of the engine) have an air line connected to the engine's intake manifold?

If so, I suggest you think VACUUM PUMP and finding another mechanic. Franz has been pulling your chain, IMHO, and should KNOW (very well) that vacuum pumps' diaphragms fail after many years use and should be replaced.

Longer trips heat the oil to a thinner consistancey allowing it to go places where it shouldn't.

You should also know that vacuum pumps can be rebuilt from kits so there's no reason to replace the vacuum pump $$ vs. rebuilding it.

Thanks for your response, T.G. -

If I take the car on the Freeway, it'll use a quart of oil after two hours or so at 70 mph, give or take. Around town at average speeds of 45 mph, the engine doesn't seem to use any oil at all. According to the gauge in the vehicle, the engine temp rarely rises above 175 degrees (except on very warm - 80 degree summer days).

I will ask Franz again on Monday to re-explain his Injection Pump Diaphragm theory, but I believe it went something like this: Because the Diaphragm in the Pump is weak, it is letting engine oil somehow into the mix at high speeds. I know that there is a line going from the Injection Pump to the Intake Manifold assembly (going from memory - the car isn't here at the moment). He also thought that this would explain the pre-ignition noises that I hear emanating from the engine after a Freeway run, stating that excess engine oil is being burnt along with the Diesel Fuel and therefore is causing the pre-ignition knocking in the engine. The knocking lasts for probably 30-45 minutes after a Freeway run, and is usually gone by the next day. All the while however, the engine does run quite well. It produces some smoke, but it's not generally black unless I back off the throttle entirely while at Freeway speeds (such as if someone cuts in front of me). Then I notice black smoke on deceleration in the rear view mirror, but only for a few seconds.

I replaced the Vacuum Pump entirely last year; the old one had begun to disintegrate it's bearings and thankfully was caught in time. Yes, their does exist a line from the Vacuum Pump to the Intake Manifold assembly. I've always had the valves adjusted at least once a year, and have replaced the Timing Chain as well (as soon as I bought the car).

I'm currently using 20/50 weight oil in the engine, and it doesn't seem to mind it. After a Freeway run, the Oil Pressure Gauge will fall to around "30" from it's top mark at "45" while the engine is idling. The Oil Pressure always comes back up to top as soon as any throttle is applied.

Thank you again for your advice,

- Bill Schmidt

How many miles has this old 240D done?

I once saw a picture of a diesel that had a throttle on its intake - like a gas engine - as part of, I guess, an emissions control system. Does this 240D have anything like that?

If > 250K I believe the basic problem is a lot of blow-by gases which carry oil as a mist getting sucked into the intake manifold. Yes the oil is being burned like fuel and yes it will preignite because it's in the air, not a fuel whose ignition timing is controlled via the injector. During slower driving there's much less blow-by supplying oil so the intake manifold cleanses itself of oil and the preignition ceases. There's a well known diesel phenomon in which an engine runs away - it runs on its own lube oil, faster and faster and, lacking controls, destroys itself. This is exactly how that occurs.

This motor's crankcase is vented from its valve cover to the air cleaner or intake manifold. In the top inside of the valve cover is a baffle through which the blow-by gases should pass to separate the air and suspended oil which is dripped onto the cam. The baffle may be clogged, or missing or mispositioned so that the oil being flung off the chain is thrown directly into the vent. I'd check that vent baffle before messing with anything else.

IMHO neither the injection pump, nor the fuel injectors have anything to do with this problem. The injection pump is lubricated with engine oil, there's even a small oil pipe to it, but there's no mixing of the fuel and oil. Injection pumps are complicated, not for us home DIY guys, and rarely need to be rebuilt. I really doubt there's anything here but some $$ for Franz.

The engine's oil pressure is completely normal, in fact it's very good,

15 psi at hot idle would be acceptable.

Hope this helps you sort it out.

Thanks very much for your responses, T.G. -

My 32-year old 240D has only 92k on it - and yes, it's original.

As I recall (the vehicle is currently being stored 20 miles from me) along with the air butterfly valve, there is also what seemed to me to be another contraption in that (the air intake) assembly; sort of a governor maybe. No cable or other apparatus going to or coming from this mechanism. This aside no - there is no emissions equipment on this engine.

I know the old Diesel "run-away" phenomenon all too well - it happened to me, personally in an old VW Rabbit, years ago. I certainly do NOT want a repeat performance of that.

I'll investigate the baffle on the underside of the valve cover for damage or cleanliness and hopefully repair or replace as necessary.

Thanks once again for your advice - it's appreciated!

- Bill

Restoration place I think it's near Atlanta, Georgia, USA:

Here is some photos and info on a 240D being restored If the link doesn't work go to the first one above and click on "Links" on that page then select the second one on the list :

" MBDiesel.net is a site created by one of our clients detailing their Mercedes experiences

"

Helen

Bill,

Since the motor has done only 92K miles its blow-by should be "normal" and NOT the creator of this problem.

That butterfly valve on the air intake is the reason for the puff of black smoke when you let off the throttle - the air is cut off but the fuel rack and "other oil" isn't cut off as quickly - so the motor is briefly over fueled - smoke.

I suggest you check the operation and adjustment of this butterfly valve

- if it isn't sufficiently open at speed the engine will pull a vacuum on the intake manifold and suck the crankcase vapors (any anything else) into the intake. Diesels shouldn't have any native vacuum, like gas engines do. I believe that's what's been happening to suck lube oil into the motor's intake.

I'd investigate the removal of the butterfly valve from its intake airflow, or disconnect its linkage and wire the butterfly in its "open" position, if only for a test. Remember, a diesel is controlled by its fuel input, not its air input.

Hope this helps solve the problem.

Tom

1980 300SD W/ 109K Miles, owned since new.

That butterfly valve may provide vacuum for the power brake, so go carefully.

Thanks again for your suggestion(s), Tom - I'll check into a butterfly valve test as you describe below.

Thanks again,

- Bill