This situation as it's been presented is indeed a bit different than most of the "rough idle" posts that have been listed at this site.
- Short duration @ startup
- Hot or cold
- Reoccurs when warm at stoplight
Take any of these individually, go thru the archives and you can come up the the "most likely" pretty fast, but all 3 together......
This is a tough one.
If it were me, I'd rule out plug wires/dist. cap/rotor, a common cause of rough idle in 103 motors. The short duration you describe cancels that out for me.
I'd suspect fuel injectors were it not for the reoccurrence at stop lights once warm. One would expect an overall rough running condition if that were the case. Wouldn't they?
In other posts here, there's been mention of a vacuum line that runs underneath the intake manifold on 103 motors. It might not hurt to take a look. Also, it might not be a bad idea to examine your motor mounts. They can cause shakiness at idle. I'm not sure if things settel out after 3-8 secs in the event of bad mounts?
Let us know what you discover.
1991 300-SEL - Model 126
M103 - SOHC
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