W115 rough idle resolved

[nachi11744]

Hello,
The poor/rough idle turned out to be an increasing air leak at the #1&2 intake manifold to ports junction on the underside. I found it on a cool, rainy morning when I turned off the engine after a cold start and a streak of liquid fuel appeared under the #2 intake runner and evaporated immediately. I guess due to the high ambient temps around here, anything fuel that leaks evaporates immediately leaving little or no telltale signs.

So, my conclusion on what was happening is:
1. When cold, there was a slight leak and that made cold drivability only slightly poor.
2. At *normal* temp and slow running, the manifold warmed up and sealed off the leak, so no rough idle.
3. High speed running caused the manifold to warp due to heat, causing a huge air leak with very poor, misfiring idle.

And the reason the manifold is warped around #1&2 ports is because the a/c compressor bracket is hung from the stud that holds the intake #1 runner to the head and a small brace is bolted from the back of the a/c bracket to the #2 intake stud and the vibration from the compressor is loosening the whole works.

My *repair* was liberal use of high temp silicone sealant around the intake/head gasket joint, removing it to renew the gasket seems to be devil's own job as the complete a/c compressor bracket, intake/exhaust manifold, downpipe, fan and many other little niggling things have to come off.

Thanks to all those who replied to my previous thread.
Have a good weekend and Season's Greetings to All.

[ctaylor738]

I did the silicone thing on my 280C and it lasted almost a week. Better order the gasket!