Low Vacuum Reading

[toms]

Group:

This post relates to a 71 280sl m130e engine.

I have historically had a low vacuum reading at normal (800rpm) idle. I generally get a steady 13-14 on the vacuum gauge, and I suspect this should be more like 17-18. I have excellent compression and leak down stats (175 across all cylinders and 8% leakdown) so I suspect a vacuum leak. I tested the brake booster-no leak. I turned the idle air screw all the way in and could not choke off the car-in other words, the car kept running at a very low rpm with no idle air. This sounds like a vacuum leak somewhere in the intake manifold area, but I am not sure.

Any ideas about how I can isolate the vacuum leak? Any other ideas as to what could cause the low vacuum readings? I have ordered a new intake manifold gasket and nuts in case I go so far as to replace that gasket, but this may be more of a winter project.

[ctaylor738]

You should definitely be seeing more vacuum at idle.

The time-honored technique is to use a spray can of carb cleaner with a plastic tube - like Valvoline - and with the engine running, spray the likely suspects. If you hit the leak, you will hear a slight increase in RPM or smoother running. The intake manifold is easy to check.

The high-tech method is blow white smoke into a spark plug hole with a special machine and see where it comes out.

If the car otherwise runs and idles well, you may be suffering from a myriad of small leaks at the manifold, vacuum connectors, injectors, injection plumbing, etc. The most effective way to deal with this is to replace everything that could possibly leak.

You might also have a leak in the cabin vacuum/door locks.

[toms]

Hi Chuck,

I will give the low tech approach a try. If I can't identify the source of vacuum leak, I will head to the dealer/independent and ask them to blow smoke up my...well, ya know...spark plug hole.