Vacuum Pump Rebuild (?) - PeachParts Mercedes-Benz Forum

KDHBenz · #1 11-26-2005, 02:07 AM

I have just under 15psi of vacuum at the supply to the power brake booster vacuum line when I'm looking for 15-20psi. Barring any leaks in the lines, of which there aren't too many (vacuum lines that is) beyond the rigid lines out of the pump, is this symptomatic of the vacuum pump needing a rebuild kit?

Is so, there are two options as I know it for rebuilding the thing with one being significantly more expensive then the other. Both kits are shown below and I'm assuming the first one is all I need, if anything, but am unsure...

Option #1 (valve, approx $80):

Option #2 (rocker arm, approx $260):

As always, any advice is appreciated...

Thanks!!!

Biodiesel300TD · #2 11-26-2005, 10:46 AM

If you can pull the vac pump and look at it. If you the rocker arm in your car look fine I wouldn't buy the rocker arm kit. You will probably only need the valve kit, since the pump is still creating vac. The rebuild is pretty easy. Getting the pump off is the hardest part.

P.E.Haiges · #3 11-26-2005, 11:49 AM

KDH,

15 PSI (actually 14.7 at sea level) is the most vacuum you can get with any vacuum pump. You can't get more vacuum than atmospheric pressure.

I'd say your vacuum pump is working fine if you get ~15 PSI vacuum.

Or are you talking inches of Mercury instead of PSI?

P E H

KDHBenz · #4 11-26-2005, 12:24 PM

Okay, so here's the story...

The procedure I was using for checking the vacuum circuits(s) came from here and said to look for 15-20" of vacuum at the supply to the brake booster, so I thought I might've been on the low end. If 15" is good, then so be it. My current problem, related to the car shutting off rather slowly, has only recently remanifested. It did so before, in fact wouldn't shut off at all, due to a faulty check valve at the pump. The diaphram and plastic insert were broken off rendering the check valve as nothing more than a coupler. We found the pieces laying inside the inlet passage and temp-fixed it to reconstrict the line and get the vacuum pressure back up by inserting a small rivet inline (

), pending receipt of the new check valve. I also needed a new vacuum switch that goes on top of the valve cover (2 stacked together) because one of them (the top one) had a broken connection and ordered it at the same time as the check valve. When we received those two items we installed them without incident. Then when we decided to check the vacuum system prior to making needed transmission adjustments (throttle pressure and vacuum modulator), we noted what we thought to be a low end reading at 15" of vacuum. We called it good for the purpose of doing the rest of the checkout and in the process discovered that the other vacuum switch on top of the valve cover (the bottom one) also had a broken connection. In the process of pulling the vacuum hoses apart for that switch to get a better look we must have disturbed the vacuum connection that was before at least partially made with the broken connection stuck in the hose and butting up against the switch body. This resulted in a leak that is apparently robbing the system of enough vacuum so that the vacuum switch in the pump is now responding very sluggishly. I've ordered another vacuum switch (valve cover) and will replace all interconnecting hoses for both of those switches upon receipt. In the mean time, I'm going to jury-rig the broken connection so that is is glued back to the body of the switch and try to regain some of my lost vacuum.

Man, it's always something... Now I have no glowplug light, but I still seem to have preglow. I'll be troubleshooting that today as well, if my wife doesn't kill me first that is...

Thanks!!!

bullwinkle · #5 11-26-2005, 12:33 PM

I wouldn't worry too much about the glowplug light, it has no bearing whatsoever on the length of time the relay is on-just listen for the "thunk" of the relay turning off and the idiot lights brightening, I personally think the glow plug lamp is misleading, makes you think the job is done when it's not.

P.E.Haiges · #6 11-26-2005, 12:43 PM

KDH,

In your first post you mentioned PSI of vacuum and in your later post you mentioned "inches" of vacuum. These are two different units of measure. "Inches" refers to inches of mercury.

I think about 30 inches of mercury is the maximum amount of vacuum at sea level. This would be equilvalent to about 30 feet of water.

In other words, if you tool a column of mercury in a capilary tube and applied a perfect vacuum above it, atmospheric pressure would push the mercury ~ 30 inches high and never higher, the above test being done at sea level. At higher altitudes, the column would never be as high.

P E H

KDHBenz · #7 11-26-2005, 12:52 PM

I was reading 15 inches of Mercury as indicated on my Mity-Vac... So does this change everything? Do I have a weak pump?

As to the issue with the glowplug light, you're probably right bullwinkle, but the purist in me will eventually win out and force repair...

JimmyL · #8 11-26-2005, 12:56 PM

Quote:

Originally Posted by KDHBenz

I was reading 15 inches of Mercury as indicated on my Mity-Vac...

As to the issue with the glowplug light, you're probably right bullwinkle, but the purist in me will eventually win out and force repair...

FYI, often times the glow plug light not coming on is an indication that one or two of the glow plugs in inop. Just another thing to check....

KDHBenz · #9 11-26-2005, 01:11 PM

Thanks JimmyL... Having no real desire to be stuck up here in the frozen North without sufficient preglow, the plugs themselves will be the FIRST things I check. I have also read in prior posts that there is a transistor inside the glow plug relay assembly that can go bad and make the glowplug light inop while still having normal glowplug operation...

WannaWagon · #10 11-26-2005, 01:41 PM

Not to hijack this thread, but I'm hoping for an answer to these vacuum questions, too, so I'm hoping we'll all get back to that.

I recently had a sudden onset of hard brakes and slow engine shutoff, checked my vacuum output with a gauge, and got 10" of vacuum at idle when measured at one of the small lines that branches of from the big line between the vac pump and brake booster. Also, the vacuum does not increase with engine speed.

Does this indicate a bad vac pump? Or could it be a bad check valve somewhere, and where are those located?

Also, is it worth the trouble to rebuild the vac pump, or more sensible to just replace the whole pump?

KDHBenz · #11 11-26-2005, 01:50 PM

...we needed to get back on track...

Anyone...?

tangofox007 · #12 11-26-2005, 03:24 PM

With regard to the valves on the valve cover, since they only operate the EGR valve, I would disconnect and plug the supply line and see if that helps with the shut-down problem. If you have a vacuum tool (ie:Mityvac), try applying vacuum directly to the shut-off valve to confirm that the shutdown mechanism itself is not causing the problem.
And before you decide to overhaul your vacuum pump, check the vacuum level at the pump itself.

KDHBenz · #13 11-26-2005, 06:24 PM

Quote:

Originally Posted by tangofox007

With regard to the valves on the valve cover, since they only operate the EGR valve, I would disconnect and plug the supply line and see if that helps with the shut-down problem. If you have a vacuum tool (ie:Mityvac), try applying vacuum directly to the shut-off valve to confirm that the shutdown mechanism itself is not causing the problem.
And before you decide to overhaul your vacuum pump, check the vacuum level at the pump itself.

You are absolutely right tangofox007... Why in the !@#$% am I worrying about the !@#$% vacuum switches when their sole function is EGR?

So, instead of messing with it further and since I had long ago disabled via plate and BB the EGR anyway, I went ahead and eliminated all the EGR crap per the following (excellent) thread...

EGR elimination, 617 engine

The car now seems to shut down as it should, but I would still like to know where my vacuum pump stands just for grins. When I check it right at the pump, what should I get for a nominal reading? Remember, I had 15" at the brake booster...

Thanks to all!

tangofox007 · #14 11-26-2005, 06:44 PM

Quote:

Originally Posted by KDHBenz

When I check it right at the pump, what should I get for a nominal reading? Remember, I had 15" at the brake booster...

Mine pulls about 21 in Hg.

KDHBenz · #15 11-26-2005, 09:15 PM

What I can now say for sure, is that I was apparently leaking some MAJOR vacuum through the EGR junk. Since I've pulled all of that out and thereby significantly simplified that leg of the vacuum system and reduced the potential for leaks, not only does the engine kill as it should, but I've no longer any need to adjust the transmission vacuum modulator to improve shift quality. Where the quality of my upshifts and particularly my downshifts (very clunky and hard) was poor before, it has now returned to being as smooth as silk. So, while I will get back to checking vacuum right out of the pump, it is no longer the priority it was a few short hours ago...

I only wish I would've known what I know now before spending the 35 bucks on the first 3/2 switch...

Vacuum -- I HATE IT!

Thanks again to all!!!