steps backwards - Is my vacuum understanding correct - PeachParts Mercedes-Benz Forum

Junkman · #1 07-26-2017, 06:15 PM

84SD 617. Vacuum pump replaced with a new pump from whoever the OE manufacturer is. There are approximately 8,000 miles on this vacuum pump.

Recently and intermittently, brakes get hard and car won't shut off. There are 2 small vacuum line branches in the main engine compartment and another larger one behind the false wall that goes to the door locks.

The large branch is plugged awaiting my tracking down leaks and getting the door vacuum working. I measured only 10" of vacuum at one of the smaller ports then disconnected and plugged the other small port. No change. Next up was to disconnect and plug with my thumb the line at the brake booster in case the booster was leaking.

Next was to connect another vacuum gauge to the 2nd port to confirm the 1st guage. No change, still 10" of vacuum.

The car will shutoff by connecting the mity vac to the shut off line.

Are there any other leak sources or one way valves that could be leaking? If not, I'll look into another vacuum pump or rebuilding this one.

Ports that are plugged:
2 small lines in the engine compartment
larger port going to door locks
end that goes to the brake booster.

Is there anything else? Shouldn't vac be 18" or so?

Diseasel300 · #2 07-26-2017, 07:06 PM

Vac should be >20" if the pump is healthy and the vacuum lines are in good shape without leaks.

Is it possible the check valve at the pump isn't sealing?

Diesel911 · #3 07-26-2017, 08:32 PM

As previously stated a low vacuum can often be the Check Valve the screws into the Vacuum Pump. However, a new Vacuum Pump I believe comes with a new check valve so one would think that would make a check valve issue unlikely.

They way to know for sure would be to remove the Tubing where it goes into the Vacuum Pump and use one of those tapered rubber adapters and push that into the Vacuum Pump Check Valve so it seals and measure the exact vacuum that the vacuum pump puts out.
If you are getting good vacuum there thest the Brake Booster to see if it holds vacuum. After that all that is left is the main vacuum line.

Junkman · #4 07-26-2017, 08:40 PM

I'll work on it tomorrow. There may be some useful fittings in an old compression tester kit.

tyl604 · #5 07-26-2017, 10:06 PM

The door locks work from the vac in the trunk, not the one under the hood. Sounds like your shut off valve (inside the back of the injection pump) is OK. Probably just a leak; check all the rubber Y connections. The plastic lines are probably OK. Mine is a 1981 300SD and the plastic lines are just like new.

Clemson88 · #6 07-26-2017, 10:19 PM

Quote:

Originally Posted by Diesel911

As previously stated a low vacuum can often be the Check Valve the screws into the Vacuum Pump. However, a new Vacuum Pump I believe comes with a new check valve so one would think that would make a check valve issue unlikely.

They way to know for sure would be to remove the Tubing where it goes into the Vacuum Pump and use one of those tapered rubber adapters and push that into the Vacuum Pump Check Valve so it seals and measure the exact vacuum that the vacuum pump puts out.
If you are getting good vacuum there thest the Brake Booster to see if it holds vacuum. After that all that is left is the main vacuum line.

I agree, starting at the source and working your way outward is the most efficient way to examine a vacuum system. Pump, check valve, main vac line, brake booster then each branch off the main trunk line.

On an SD I would remove and isolate the the cabin and that horrible mess of pods and actuators under the dash from the system first.

Junkman · #7 07-27-2017, 02:48 PM

Any way I measure, I get 8" at idle. Vacuum goes up to 20" if the engine is revved and stays there as long as the engine is revving. Vacuum slowly settles to 8" when the engine returns to idle. The one way valve tests good.

I'm going to check warranty but will likely end up replacing or installing a kit. Any suggestions on what kit or what brand of pump?

I won't explore further to fine reasons for failure until parts are in hand because I want to keep the car mobile. Strange that this crapped out with out many miles.

Edit: I think the pump was a Pierburg

Diesel911 · #8 07-28-2017, 11:25 AM

Quote:

Originally Posted by Junkman

Any way I measure, I get 8" at idle. Vacuum goes up to 20" if the engine is revved and stays there as long as the engine is revving. Vacuum slowly settles to 8" when the engine returns to idle. The one way valve tests good.

I'm going to check warranty but will likely end up replacing or installing a kit. Any suggestions on what kit or what brand of pump?

I won't explore further to fine reasons for failure until parts are in hand because I want to keep the car mobile. Strange that this crapped out with out many miles.

Edit: I think the pump was a Pierburg

If you are measuering directly at the vacuum pump you have some issue in the Check Valve or Vacuum Pump itself.

When I had a low vacuum and I mean way back in like 2008-2009 I removed the Check Valve and found it full of Gunk. I holsed the innards of the check valve out with WD-40 and re-installed it and had no more issues from the Check Valve. That was a long time ago.

Later I took off the the vacuum pump and took it apart and cleaned it and re assembled it.

I have also pulled the pump of for inspection of the Arm at least 2 times. Never found any issues.

Not sure about the avaliablity of Kits as they were getting scarce and never were cheap for the piston types.

There was 2 kits. One for the Arm and Bearing end and the other for the Piston end with the valves.

Junkman · #9 07-28-2017, 01:36 PM

@ Diesel:
I measured directly at the pump even with the closest metal line removed. Next, I pulled the check valve (22mm socket) and it did have some gunk but easily cleared. I reinstalled the check valve & re tested. Vac was still 8-10" unless the engine revved.

I've read threads where pumps failed for various reasons and am not looking forward to tracking down internal engine parts to repair. I'm contemplating getting a compression gauge as that also needs to be checked as a step in figuring out what is causing an engine shake.

Vac pump repair kits are apparently NLA. I've seen a few on Ebay at ~$170. I'd buy a new Pierburg pump at that price and keep this one for parts. I may have kept the old one somewhere.

Current thoughts are to: 1. run compression test while vac pump is installed so I have compression information on engine health. Right now I only know that it starts easily with 1 glow even at temps in the teens.

2. remove and disassemble the current pump before deciding the next step.

I have a parts car available but want to keep that engine complete without scavenging parts.

SD Blue · #10 07-28-2017, 02:57 PM

Some of that gunk you found may have made it inside the pump to the two small internal check valves. That is what happened on mine. If you are getting at least a reduced amount of vacuum, you may have similar.

Junkman · #11 07-28-2017, 04:00 PM

It would be great to be able to clean and install. That would leave the open question about where the gunk came from.

Junkman · #12 07-31-2017, 01:49 PM

Brakes are hard at startup and engine doesn't always shut off immediately with the key. Fuel cut off valve tests good and shuts engine down with a mitivac.

Vac at idle measured at the pump with 2 vac gauges measures 8". Vac quickly goes to 20" when engine is revved. Both gauges match each other and also go to 25" when connected to the Dodge Cummins engine.

Pump itself is Piersburg bought new with approx 8,000 mi on it.
Main check valve works - ie lets air into the pump but not the other way.

The attached pics are how the pump came off the car without any cleaning.

Is the next step to disassemble the pump? No kits are available so I'd either have to reuse gaskets or make some with by using a ballpeen hammer.

Edit: I didn't disassemble the ramp but there is no obvious side to side play. The bearing in the back of the pump is nice and smooth.

SD Blue · #13 07-31-2017, 04:22 PM

Doesn't yours have the cover with several small screws? If so, all that is needed is to remove it and the two small check valves are inside.

Junkman · #14 07-31-2017, 04:30 PM

Quote:

Originally Posted by SD Blue

Doesn't yours have the cover with several small screws? If so, all that is needed is to remove it and the two small check valves are inside.

OK, I'll check on it tonight. I was just uncertain about whether the seal could be reused and whether there was a trick to reassembling. The spring that resists the roller is nice and strong.

this PN. 000-230-13-65

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Diseasel300 · #15 07-31-2017, 05:16 PM

Have you blocked off and disconnected everything from the pump (including the booster) to test the vacuum directly at the supply fitting on the pump? Sorry if you've answered this already it isn't clear. If you're leaving the booster attached and you have a leak in the diaphragm, low vacuum in the system is exactly the symptom you'd be getting.

The brakes should be holding vacuum even if the rest of the system doesn't. There's a check valve in the brake line to have it hold vacuum. It should have enough vacuum stored to last a pump or two of the pedal before going hard.