vacuum issues: hard brakes, no shutoff - PeachParts Mercedes-Benz Forum

WannaWagon · #1 11-22-2005, 12:42 PM

All of a sudden this morning, while pulling up at work, my brakes got hard and when I went to turn the engine off, it would not shut off but slowly idled itself down until finally it stopped. This was the first time I drove the car after a nearly 800-mile round-trip weekend journey in which there were no indications of problems like this.

I assume this is some sort of vacuum leak. But it's strange that it happened so suddenly. Do these two problems together point to a particular problem or place to look for a leak?

#2 11-22-2005, 12:53 PM

It has to be a fairly large leak to affect the brakes. Either that or you have a problem with the vacuum pump. I would start at the pump and look for loose vacuum lines, then put a vacuum gauge on the main line with the branch connections blocked.

boneheaddoctor · #3 11-22-2005, 01:23 PM

Quote:

Originally Posted by Craig

It has to be a fairly large leak to affect the brakes. Either that or you have a problem with the vacuum pump. I would start at the pump and look for loose vacuum lines, then put a vacuum gauge on the main line with the branch connections blocked.

yes.....look closely for cracked line etc...cracked check valve...plastic gets brittle with age...

Do you have a vacuum guage of some sort? if not buy or borrow one.

WannaWagon · #4 11-22-2005, 04:26 PM

Thanks. Yes, I have a vac gauge. So I guess now I know how I'm spending my weekend.

WannaWagon · #5 11-22-2005, 04:47 PM

BTW, how much vacuum should I see on that gauge when I test? What is the "normal" vacuum in the system at idle?

Pete Burton · #6 11-22-2005, 04:56 PM

I got a steady 23" last time I checked.

WannaWagon · #7 11-26-2005, 01:13 PM

Well it looks like I have vacuum issues. I just tested the vacuum by connecting a gauge to one of the small lines between the vac pump and brake booster. I got only about 10" of vacuum at idle. More importantly, the vacuum did not rise with engine speed, as my Haynes manual says it should.

If this is a problem with the vac pump, I'm puzzled that it could fail so suddenly. Is this possible? Again, this follows a long weekend road trip in which there were no signs of vacuum trouble. The next time I drove the car, on a short trip to work, everything was fine until arrived, and suddenly the brakes were hard.

Also, I don't understand how this problem would cause the engine not to shut off properly. Again, at the same time the brakes got hard, the engine didn't shut off sharply but slowly idled down to 0 rpm....?

Biodiesel300TD · #8 11-26-2005, 01:35 PM

Also try plugging in your vac gauge on the main line form the pump. Then unbolt the line to your brake booster, then plug this line. Turn the car on and see if your gauge reads high enough. If it reads good then the booster is leaking. If the reading does not change than the booster is good. Have fun.

WannaWagon · #9 11-26-2005, 02:16 PM

Thanks for the quick reply.

OK, I tested like you said and this time got around 20" of vacuum when the gauge was connected just to the main line from the vacuum pump. But the reading was jumping around sharply between 17-23" of vac, and the reading did not increase with engine speed. If anything, it dropped.

Whaddya think?

Biodiesel300TD · #10 11-26-2005, 02:23 PM

If it's jumping around you may not have had a good seal on the brake booster line. Try licking your thumb or one of your fingers and plug the booster line that way. Then you can take your finger on and off as well to test is a few times. What happens then?

Brian Carlton · #11 11-26-2005, 02:57 PM

Quote:

Originally Posted by WannaWagon

Thanks for the quick reply.

OK, I tested like you said and this time got around 20" of vacuum when the gauge was connected just to the main line from the vacuum pump. But the reading was jumping around sharply between 17-23" of vac, and the reading did not increase with engine speed. If anything, it dropped.

Whaddya think?

If you have 20" vacuum with the line disconnected from the booster and the gauge attached to the end of the line, this most likely eliminates the vacuum pump as the culprit.

Now, disconnect and plug all other hoses leaving from the main line. Attach a T to the main line and read the vacuum level again. Be absolutely sure that there are no other devices or hoses connected to the main line.

If the test is done properly and the reading is 10", as you mentioned above, the problem is a leak in the booster. The leak is a decent sized leak and the vacuum pump can't keep up with it. The engine is trying to shutdown with 10" vacuum and I'm very surprised that it will shutdown at all with this low vacuum level. Usually they just keep running.

To condemn the booster requires that everything else coming from the main line be disconnected and/or plugged.

unkl300d · #12 12-03-2005, 01:56 AM

Wierd, My 300D just started with the periodic hard brake syndrome. I had my Vacuum pump rebuilt several months ago (new diaphram) and now it started with the original problem again. BUT there is no problem with shut off (so far).

Anyway, thanks to this thread, I can begin checking the lines etc.

WannaWagon · #13 12-03-2005, 03:33 AM

Followup: Turns out my vacuum pump was shot. I didn't have time to deal with it myself, so I ended up enriching my indy mechanic by about $550 to do the job. Brakes are good now.

#14 12-03-2005, 04:58 AM

Quote:

Originally Posted by WannaWagon

Followup: Turns out my vacuum pump was shot. I didn't have time to deal with it myself, so I ended up enriching my indy mechanic by about $550 to do the job. Brakes are good now.

BTW your engine uses vacuum to shut itself off and this is why your engine slowly idled down and finally shut off...you had weak vac.

Many people forget these are not gas powered cars where you can just shut off the electricity

rmmagow · #15 12-03-2005, 01:24 PM

How difficult is it to rebuild the vacuum pump? This would be on a 617 300D Turbo. Pump housing and all is part of the block no? Are the kits worth the trouble? Thanks.