It's baaaack. M103 chuga-chuga at hot startup.

[S-Class Guru]

Well, we had some warm days, and the '91 300SE is back to it's old ways of idling rough for the first few seconds after a heat-soak.
Another new set of injectors and seals over the winter had no effect.

Condition:
cranks, runs, idles beautifully; except after it sits and heat-soaks for 1/2 to 2 hours. It cranks right up, but then chugs like it's running on 3 or 4cylinders for about 5 seconds. Then it smooths out and purrs. If you gas it during the chugging, it revs, while missing for a second or two, then clears right up.

This is the ONLY time this engine EVER misses a beat,
If cranked cold, it's perfect. If cranked hot after only sitting a few minutes, it's perfect.

History: I've owned the car for 16 years, it has done this for at least the last TEN YEARS during warm weather. In that time it has had all the usual tune-ups, a valve job, two sets of injectors. It has never really changed anything.
Just chuga-chuga-chuga for about 5 seconds after a heat-soak, then all is well.

Must be some sorta fuel issue, like vapor lock or leakage or boiling fuel when it sits and heat soaks?
I have resisted tampering with the fuel system, other than injectors/seals and checking the mixture and idle air settings. A couple years ago, I read up on the procedures here on the forum, and checked dwell, idle air, lambda, etc. Everything looked fine.

Is this a common ailment of the 103 engine? I hate to start digging into the fuel injection system after it's worked so well all these years, and the chuga has always been there - no better, no worse.
But it's starting to bug me.

I guess I'm hoping for someone to tell me "Yeah, they all do that when the Flux Capacitor goes out - replace it and be happy".

Any thoughts?


Tks,
DG