Intermittent vaccuum pump - PeachParts Mercedes-Benz Forum

cirrusman · #1 01-14-2010, 08:47 PM

Ok, so I'm driving my '83 300SD and everything vacuum fails; brakes are long and hard (that's what she said...

) tranny starts shifting rough and the engine won't stop unless it's stopped with the engine lever. I'm thinking "total pump failure, goodbye $400, it was nice to have you with me". So the car sits for 2 days and then i have to drive it and I find that the breaks are slowly responding and then after some 20 miles the car is fine again... The pump is back in business and now I'm wondering what the... Any ideas on what can be going on? what can I check to find out what's going on so this doesn't happen again? Because Murphy says it WILL happen again at the worst possible moment, and I'd kind of like to avoid that

4x4_Welder · #2 01-14-2010, 08:51 PM

Sounds like a stuck valve in the pump itself- If you undo the nut on the hard line where it runs past the fuel filter, is there an oily residue in the line?

kerry · #3 01-14-2010, 09:08 PM

Malfunctioning check valve in the pump.

Diesel911 · #4 01-15-2010, 01:27 AM

As kerry said and there is other valves inside of the pump that can get gummed up.

If cleaning the Check Valve does not work remove the Vacuum Pump (I am assuming it is a Piston Type Pump) and remove the end plate to gain acces to the litte valves (not wihch direction they point; take pics).

I did this with mine. The only problem I encountered was I sheard off 2 of the little Screws that hold the Vacuum Pump Plat on.
The rest I heated with a Propane Toarch and used an Old School type Manual Impact Wrench on and they came out.
When I put it back together I put a light coat of Silicone Sealant in areas that needed to be sealed as I did not replace any gaskets or O-rings.
I did nothing to the sheard off screws; counted on the Silicone to seal that area. That was well over 1 year ago; and no problems.

snookwhaler · #5 01-15-2010, 08:58 AM

I went through this exact problem recently... Check your vacuum at the pump and see what you got at idle. Hook the hard line back up to the pump. Then unhook the line to the brake booster and plug it. Observe the readings again off the main line. While the brake booster line is unhooked, hook up your mighty vac and see if the booster is holding vacuum. Doing these couple things will tell you a lot (Thanks again to everyone that helped me with this problem!). Takes about 10 minutes to check all of the above.

In the end I found out that my brake booster had a small leak. However, rebuilding the pump with a kit from Fastlane fixed the shut down and hard pedal problems because now I have stronger vacuum. The kit is relatively easy to install but is a little time consuming. I just rebuilt it with the pump on the car. Now I'm looking for a brake booster that does not leak.

I also found that the top check valve in my pump was an oily gooky mess. But, no oil made its way into any of the lines (thank goodness).

tangofox007 · #6 01-15-2010, 09:22 AM

The intermittent nature of the problem is highly suggestive of a failure of the check valve in the fitting where the hard line connects to the pump. The parts of the valve rattle around in the pump inlet and cause intermittent blockages. You can remove that part with the pump in place on the engine.

Note the small disk in the right-side photo:
http://catalog.peachparts.com/ShopByVehicle.epc?q=1982-Mercedes--Benz-300sd-Diesel--Injection&yearid=1982%40%401982&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6206%3AMBC%7C1510%3AED%7C10000134%40%40300SD&catid=240706%40%40Diesel+Injection&subcatid=240735@@Vacuum+Pump+Check+Valve&mode=PA
If you remove the nipple and that disk isn't there, you will need to fish the disk, valve and spring out of the pump inlet, then replace the valve with a new one.

fdanielson · #7 01-15-2010, 09:39 AM

I had a similar problem and rebuilt the vacuum pump only to find out that wasn't the source of the problem. Turns out my brake booster was leaking intermittently and I found it by disconnecting the vacuum line from the booster and plugging it with a rubber cork from the hardware store. A few careful test drives with very stiff brakes showed that everything else vacuum related worked fine so I installed a replacement booster from the wrecking yard. If you do decide to replace the booster you can get a rebuilt one from A1 Cardone through several of the parts chains for under a hundred dollars.

kerry · #8 01-15-2010, 09:51 AM

Quote:

Originally Posted by fdanielson

If you do decide to replace the booster you can get a rebuilt one from A1 Cardone through several of the parts chains for under a hundred dollars.

Any more details on this? When I thought I needed a booster replacement a few years ago on my 77 300d, the best price I could find was at least a couple of hundred dollars.

fdanielson · #9 01-15-2010, 11:11 AM

I bought mine from Autozone, $82.99 plus the core. The website (www.autozone.com) shows the same thing for your 77 model. ******** (www.********.com) lists the same booster for about $100 plus the core. I've been using Cardone's rebuilt brake products for about 20 years and have always had good success with them.

4x4_Welder · #10 01-15-2010, 11:16 AM

Cardone is the low-priced alternative at Checker/Schucks/O'Reilly and Advance Auto stores. I have found their products to be acceptable.

cirrusman · #11 01-15-2010, 12:29 PM

I really appreciate all the suggestions folks... The booster doesn't seem to be the problem here as I disconnected the main line from the booster and it did not read any vacuum at all, so I'm thinking it's the check valve which I'll proceed to check first... I'll try and post pics too.
Dang I love this forum!

GRIESL · #12 01-19-2010, 09:20 PM

I'm watching this thread closely--my 85 300d started having consistently inconsistent vacuum related issues. Sometimes slow to shut down, but then other times functioned perfectly. Now, my door locks stopped working and very long (10 sec or so) shut down. Hoping it's an easy fix like the check valve.

tangofox007 · #13 01-20-2010, 04:02 AM

Quote:

Originally Posted by GRIESL

I'm watching this thread closely--my 85 300d started having consistently inconsistent vacuum related issues. Sometimes slow to shut down, but then other times functioned perfectly. Now, my door locks stopped working and very long (10 sec or so) shut down. Hoping it's an easy fix like the check valve.

You might try plugging your door lock circuit at the yellow check valve near the firewall in the engine compartment. If your brake boost is not affected, your problem is probably not the check valve at the vacuum pump. More likely leaky door lock actuators.

Diesel911 · #14 01-20-2010, 12:45 PM

Quote:

Originally Posted by GRIESL

I'm watching this thread closely--my 85 300d started having consistently inconsistent vacuum related issues. Sometimes slow to shut down, but then other times functioned perfectly. Now, my door locks stopped working and very long (10 sec or so) shut down. Hoping it's an easy fix like the check valve.

The Vacuum Pump Check Valve is actually the part that your Main Vacuum Line Screws in to at the Vacuum Pump.
Remove the Line and use a Socket Deep enough an unscrew the Check Valve. After that it is easy clean.

Alternativly; in my case I just squirted some WD-40 inside after I had removed the main Vacuum line.

Not Cheap but you can buy the Check Valve by itself. But, there are other valves inside of the pump that can also get gummed up.

GRIESL · #15 01-23-2010, 10:20 PM

I'm pretty sure mine is in the ACC, but only when the front vents are activated. I was fooled by the intermittentness, if that's a word, because I wasn't paying attention to the ACC switching from top to bottom vents. With the ACC temp knob turned all the way up or down, the system doesn't adjust vents, I don't think, so that's my fix for now. Thanks for the input, and sorry, didn't mean to hijack the thread. /Brian