M103 idle vac? - PeachParts Mercedes-Benz Forum

Hit Man X · #1 01-04-2008, 12:30 PM

Anyone tell me how much vac this motor is supposed to pull on hot idle in park?

I resealed mine and my gauge still isn't reading correct in the vehicle, so I'm not sure if I have a vac leak or the gauge is just off.

S-Class Guru · #2 01-04-2008, 03:31 PM

Hey, HitMan.
My '91 pulls 16 inches mercury on my mighty vac under idle conditions you described (warm and approx 700 rpm). This seems a bit low, but I sure can't find any vacuum leaks, and this number is constant and repeatable..
I can't remember what the dash vacuum guage in the car reads at warm idle. I'll check it when the wife gets home.

DG

Hit Man X · #3 01-04-2008, 05:22 PM

I was getting around 15-16 at idle when I checked earlier. Maybe my gauge is off or is normal. It had been over six months since I drove the car so I can't recall where the blasted needle pointed. I tore the top end apart to reseal and fix ever leak. Every piece of rubber I can find has been replaced.

Eagerly awaiting your reply!

S-Class Guru · #4 01-05-2008, 11:15 AM

Okay, I checked it out.
Mine is a '91 300SE, so should be same setup.
At warm idle in neutral, approx 750 rpm, the vacuum gauge is
almost pegged, less than 1/10 inch from the peg, maybe 0.050".
If I increase rpm just a tad, say 850, the needle slides over and hits the peg.
As I said, mine runs about 16 on the mighty vac, so I can assume that the gauge pegs about 17 " mercury?

Cheers,
DG

Hit Man X · #5 01-05-2008, 08:22 PM

Well I found a large leak today on mine. The Y fitting from the manifold tit to the climate and gauge hose fell off after just touching it!

So, I fixed that and picked up nearly 2" of vac at idle

Now I have around 17.5" at idle, very steady too. I'm getting pretty excited now. When I would blip my throttle it seemed to take right around 20" to peg my gauge.

17.5-18" is just about on the needle as you describe.

I have a few more connectors to check here probably tomorrow. Hopefully I'll get this resolved. Yet, it's 100% livable even if this is as good as it gets. Then next week comes precat gutting and new flex discs with center bearing.

Xsbank · #6 01-05-2008, 09:12 PM

Where are you measuring the vacuum from and what do you mean "pegged?" At the left side of the dash gauge or the right side?

S-Class Guru · #7 01-06-2008, 09:57 AM

Yeah, by "pegged" I meant all the way to the left, or full vacuum.
There is actually a little post, or peg there for the needle to hit.
We are comparing that guage with a hand-held shop guage connected to one of the vacuum ports.

This came from and old term, when we would try to "peg" the speedometer on '57 Chevys.
Another bit or trivia: the seemingly crude term "balls to the wall" is actually a term from WWll. It means to push the throttles on a B17 all the way forward to the firewall. The throttles had handles or balls on them. Thus the term for "full throttle" became "balls to the wall".

Hitman, a couple of comments:
I asume you are working on the 300SE.
You mentioned pre-cats. Mine has little bulges on the pipes just below the exhaust manifod, they look like pre-cats. However, upon inspection when I had the pipe off one day, they are not; they are straight thru. They look kinda like an old glasspack, with little holes in the pipe to let exhaust circulate into the outer chamber. Must be a sound damper device.
If you have a calif car, maybe your is different.

Also, check the vacuum line 3-way T, that sits forward of the brake booster, and feeds vacuum to the climate control. I used little squeeze clamps to sung it up.

DG

Hit Man X · #8 01-06-2008, 01:08 PM

Yes, pegged means at the needle.

Full left is vac, to the right is zero vac. I am measuring the vac at the intake manifold, right off the tit there.

So they're not precats? Sounds good to me, less work to do.

Will do on the Y connector. I'm going to recheck a few fuel injector cups that do not appear to be 100% seated. Can't hurt.

S-Class Guru · #9 01-06-2008, 07:48 PM

Keep us posted if you find more vacuum. I think possible minor vacuum leaks may be the root cause for my warm-start stumble and chugging; I have sure tried everything else!

DG

Xsbank · #10 01-06-2008, 08:47 PM

My vacuum gauge has about 1/4 to 1/5th of the way to go to the peg so it looks like I have a leak also... well, when it stops raining...

Xsbank · #11 01-07-2008, 03:45 PM

Found a vacuum hose that was cut and leaking - cut the end off with scissors and stuck it back on but no change to the gauge.

At idle and you step on the brakes, the gauge moves to the right, but when I take my foot off, the gauge doesn't move right back - anyone know what gives? Is the vacuum vented to the air when you remove your foot? Why won't the gauge move again?

S-Class Guru · #12 01-07-2008, 04:30 PM

You lucky Devil; I kept Old Pearl today, and it was sitting in the garage, still warm after an errand.
I checked the vacuum/brake thing.
At warm idle in Park, the Vacuum needle is 95% to the left, or almost full vacuum on the gauge. When I step on the brake, nothing happens - the gauge does not move. I can push really hard - nothing. When I let off the pedal, no vacuum change, it holds continually at 95%.

Sounds like you may have a brake booster issue?
If it's leaking a bit when depressed, it may not recover quickly, or may not recover completely until you blip the throttle to obtain really strong vacuum.

DG