W114 Vacuum Leaks but No Idle Issues...

[350SL4spd]

Hey Guys,

My '73 280C appears to have no vacuum anywhere: Tranny shifts hard, power locks don't work, folding seat backs don't hold, etc.

However, the care idles fine & accelerates great. Which if I'm not mistaken would imply that there no major vacuum "leaks"? Also, the power brakes seem to be fine.

Any ideas what I could be looking at? Weak/dead vacuum pump thingie on the front of the engine? Where else should I look first?

Thanks!

[Mike D]

The body stuff and transmission work separately from the brakes. The brakes are supplied directly from the front of the manifold, through the vacuum assist pump, to the booster. A minor leak may be concealed by the action of the assist pump. The assist pump has no effect on engine vacuum as it has check valves built into the pump.

The transmission and body vacuum is supplied from a fitting on the rear of the intake. This is engine vacuum only. This goes straight to the vacuum modulator and tees off to supply the body by way of the vacuum reservoir under the right front fender. Check for a bad fitting on the manifold, the rubber line rotted where it makes the bend or the line running back from the reservoir along the firewall entering the cabin beside the brake booster.

A quick check to find if you have a leaking diaphragm inside the car (trunk or fuel tank locks, A/C heater switch diaphragms, seat locks, etc) is to plug the line running into the cab. I believe it will be a yellow tubing. Check the modulator by blocking its feed line and see if the cab stuff works.

If you've just recently switched on the heat then I'd bet on one of the air flap diaphragms being bad. A leak here wouldn't have too much effect on the idle because the vacuum has already been sent to the reservoir and also due to the restriction of the long vacuum line runs.

[350SL4spd]

Wow. Excellent response Mike. Looks like I have some work to do.

I'll be getting back to you shortly about those parts.

[kraakevik]

To add to Mike's comprehensive summary I would also look at the vacuum check valves (blue/black, green/black on later models) near the firewall under the hood. These prevent the vacuum in the reservoir and other devices from bleeding back into the intake circuit--you should be able to suck on them one way but not the other.

The large check valves in the line to the brake booster and in the vacuum pump itself occasionally go bad. The diaphragm in the vacuum pump frequently fails, completely compromising vacuum in the crankcase--if that's the case you'll see oil smoke on the overrun as oil mist is drawn into the intake and burned.

These vacuum leaks are often tough to find--good luck


Tim Kraakevik
kraakevik@voyager.net