Where to Start Looking
 

"... it has a pretty bad vaccum leak that causes it to idle poorly when it's warm."

This was part of a description for a 1978 280CE convertible on ebay this week. It hits the nail on the head with a problem that has been creeping into my life. My 280E with a M110 engine runs great when cold, but once warm, it runs very roughly. So much so, that it is embarrasing to be sitting a light with the car behind me wondering "what's with that car in front?". The whole car rocks 'n rolls at idle, like it's running on three cylinders. Timing's OK, Plugs, cap and rotor are OK.

So where do I start looking for this vacuum leak and how? Is it likely to be off the manifold for the intake adjustment? I have what I thought is a minor leak in the system that controls the door locks. Could this be related?

All suggestions are welcomed. Thanks, ALex

Sorry, I asked first then searched.
 

My apologies. There is good information on a recent thread about this. I should have searched first and asked later. Alex

It would be a good idea to buy a Mityvac so you can actually see how much vacuum you're pulling. You may also have an injection problem - leaky fuel distributor, bad injector, or regulator, or etc.

You may have a bad EGR valve or it is opening at the wrong time - that will induce a massive vacuum leak. Do a search - lots of posts on how to test for that.

You can see the effect of the cabin vacuum leak by removing the connection on the back of the intake manifold, and plugging it. You should carefully examine all the rubber connectors and plastic tubes on the engine, especially the ones that get hot. Spray some engine cleaner or penetrating oil (use a spray nozzle with a tube so you're precise) around the intake manifold gaskets and injectors, and see if the idle changes momentarily.