It sometimes makes lots of racket before it is about to fail, sometimes it just pukes its guts into the chain without warning. Low vacuum is usually a vacuum device leaking or a bad check valve.

 

So, what you're saying is that something like that needs to be monitored... also, that sometimes even monitoring the thing is not enough, and it could harm the chain, and thus the engine...

So, are there any mileage or time guidelines for when a vacuum pump should be rebuilt in order to avoid the complications of it dying quickly and without warning, and possibly taking your engine with it???

 

Not really. Some will make it past 400K without any kind of issue, some will fail before 100K. Its a matter of chance, but, I'd bet oil quality plays a significant role in its lifespan just as it does with the chain.

A preventive inspection/rebuild wouldn't hurt anything except your wallet and time.

If you think about it, the vacuum pump lives through literally a billion+ strokes throughout its life.

 

Hey let me ask you mechanical guys an undoubtedly stupid question: I'm the guy who had suspension totally rebuilt on a 300d turbo diesel and one week later had signs and symptoms of vacuum pump issues and the next day had the timing chain break. I'm not looking to blame my mechanic in any event at all but is it possible that anything he did in a complete (including springs) suspension rebuild he could have done anything to affect the vacuum pump or lines, etc.? It just seems like a weird temporal connection.

 

Very unlikely unless he was tinkering where he was not paid to go.

 

I`ve owned many old mercedes diesels (and some gas), some with very high km`s(mileages) on them. And so have many of my friends. But i havent heard about vakuum pump faliure that harms the engine.. Usually the two small valves in the pump stop working first and then the pump is changed/rebuildt, not a cheap job. But maybe this is the reason i can`t get/buy those small (probably cheap) valves as spares. As to oil quality i`ve allwas used ordinary 15/40 turbo diesel motor oil, but the fuel quality here is very good. There are different types of vakuumpump, witch kind is prone to faliure?

 

There's an o-ring that seals the vacuum booster against the brake master cylinder. I've had one of these go bad before, resulting in a hard brake pedal (booster not operating as it should).

It's a cheap, quick (15mins), and easy job to replace the o-ring. Might want to give that a shot before replacing any expensive parts.

AFAIK, there is no set interval for rebuilding the vacuum pumps. If you're getting good vacuum at the main vacuum line going to the booster, and you're not getting any oil in the vacuum lines, I'd say that your vacuum pump is in good shape.

 

Thanks, IQ678... the problem I had with the brakes was one that occurred with a car we previously owned, but I'll keep your fix in mind if anything happens in the 'new' wagon...

 

I have to weigh in on this one. The engine from my 1983 300TDT is in my basement undergoing a total rebuild because the timing chain failed. I am thoroughly convinced it was caused by vacuum pump failure.

When I disassembled the engine the vacuum pump housing was filled with mangled bits of the pump internals. The two bolt heads on the top of the pump timing mechanism were battered and the ramps were galled. It took a while to get the injection timer out because the chain broke between the timing sprocket and the main sprocket on the crank and some chain parts were wedged between the timing sprocket and the block. I removed the injection pump and when I went to remove the intermediate shaft it wouldn't come out because it was bent.

My conclusion is that something inside the pump came loose and got tossed around in the space between the pump and the timing sprocket until it finally jammed the sprocket. Once the sprocket got jammed the tension in the chain bent the shaft and broke the chain. This is consistent with the noised I heard during the failure. I was just slowing down to pull off of the highway when it started banging, grinding and carrying on. I just got off the highway when suddenly everything went totally quiet. A couple of seconds later I realized the engine had died when the steering got really difficult. The old girl didn't leave me totally stranded - I had just enough momentum to pull into a gas station mini mart and park it perfectly in line with the other cars. Felt just like I was driving an electric vehicle.

The moral of this story is - well, I'm not sure what it is. I never expected this component to fail and I'm not sure what I would have looked for even if I had suspected it was marginal.

I love the car and have about $10k tied up in it between the purchase price and the restoration work I have done so far without the engine rebuild that is currently underway. I can't tell you all how thrilled my wife was to get the news. I am slowly rebuilding the engine with the hope of selling the car so that I can work on another old diesel Benz I own. But that will be another post some day.

 

I checked my vacuum pump last summer, and it was producing upwards of 20 mm Hg of vacuum. I know that is within spec, so I thought nothing more of it...

No it isn't within spec, 21-21 inches of Hg is.

20 mm Hg of vacuum would be like no vacuum at all.

 

Upwards of 20... I think it was more than 25, but at least 22... Anyway, I'll check it before I get at the rest of those pesky door actuators and behind the dash... which section of the manual did you pull those numbers from? I have the hardest time finding little tidbits of info from that manual when I need them... and what should it be, 21-??

 

My SD is pulling around 10 inHg vac, measured on the big line with the other outlets plugged. I checked because it won't shut off now. When I pull that much vac on the shut off valve, it shuts off. No other symptoms.
I've been looking through the FSM, and all I can find is .5 bar. Does that translate to around 10 inHg? My compression tester has two gauges, but it's on the low end hard to tell, and that is bar and psi vs inHg.
Any help is appreciated.

 

.5 BAR is about 14.764 in/Hg

 

Thank you!

 

MBZ300 - Dude... dduuuuude... I sooooo feel your pain. Not quite 10k in her, but hell, 2500 the week before she committed suicide. I think the sound she made was more like a garbage disposall being fed pepsi cans by a burly Bulgarian guy with a personality disorder. And then the silence. The loss of power steering (hydraulics on my brakes went the night before - kind of supporting your vacuum pump failure contribution). The rolling to the curbside. Mine was in front of a vacant house in a shitty section of town. This is killing me. Still looking for a suitable donor heart.

 

I had to rebuild the vacuum pump on my 83 300dt...I got in it one morning to find I had no brakes and the engine would not shut off...I found a link in this forum with step by step instruction and pictures on how to rebuild and tips like the new o-ring wouldn't fit and you have to reuse the old one...great instructions and worked just fine...it also helped with the transmission shifting...this is one great site that I'm glad I found...now that I have owned and drove a MB, there will be no others for me.