300E Hesitation Off Line

My 1987 300E suddenly began hesitating and sluggish when accelerating from a stop like it is too rich then after a few seconds sort of bursts forth and runs fine. Does it in Drive (second gear start) or if started
in Low (first gear start) the same.
No vacuum leaks to shift modulator, changed fuel filter, ignition timing is fine and swapping ignition module didn't cure, runs okay on road, passed California smog despite this problem so catalytic converter must be okay. Good long-time local independent shop getting frustrated.
Any ideas?
Pete Cowper
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I'd suspect three possibilities:
Dirty fuel injectors which can be fixed by adding a couple of cans of fuel injector cleaner to the fuel.
or,
An engine manifold vacuum leak that allows a small quantity of air to dilute the air / fuel mixture at idle causing the engine to run lean - then, when you open the throttle, it runs too lean and stumbles until the fuel injection catches up to the condition. The leak isn't noticed above idle because it's small but constant and so becomes a smaller and smaller % of the total mixture as engine speed rises. Such a small leak can be found at idle by carefully applying some propane or carburetor cleaner to suspected sites like the fuel injector seals, intake manifold gaskets and vacuum hoses attached to the intake manifold.
or,
The engine's ignition "vacuum advance" is sticking. I put it in quotes because this modern engine probably no longer has a "vacuum advance" as in the bad old days of carburetors and ignition points etc., but it must have SOMETHING like it to advance the spark for acceleration and as the RPMs increase.
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Thanks for the suggestions.
Over the past year the shop has added fuel injector cleaner three times, and have replaced the valve seals to stop the plugs from oil fouling. The prior slight miss at idle was not cured by the oil seals, leading me to suspect some sort of lean miss. It has now moved from a slight miss at idle to the several seconds of sluggishness in accelerating away from a stop before the power seems to burst on.
The cars does feel like the "spark advance" is not working, which was checked by swapping in a good "ignition module" to assure the electrical/ignition system was not at fault.
We are trying to eliminate all possibilities before replacing the injectors. The injector seals were replaced earlier in trying to eliminate the slight miss at idle, as were several of the ignition cable ends.
Pete Cowper 1987 300E
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Well, given the additional information I'd first say that you've covered the usual bases and then some.
Fuel injectors can be professionally ultrasonically cleaned and tested for not much money so if it comes to that I suggest cleaning rather than replacement. A Google search for "fuel injector cleaning" will show what's involved.
Back to the lean miss. An apt description of what's slowly getting progressively worse.
I suggest a through check be made for a small vacuum leak; the most obvious site(s) being the intake manifold gasket(s) since the injectors' "o" rings were previously replaced. But EVERY intake air connection between the intake air metering device and the block is a suspect. I believe that's so because the whole engine bogs down, not just one cylinder. An exhaust gas analyzer can be inserted into the tailpipe to monitor the idling engine's emissions while propane is carefully applied to each suspect site. A vacuum leak will suck the gas into the engine, correct its lean mixture and reduce its emissions.
If nothing is found this way and everyone is convinced that the spark plug wires are OK then I'd suggest a compression test to determine if there's a burnt (probably intake) valve. Let's hope it doesn't come to this $$ level.
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Pete I had a problem with my 91 300e 245000miles It would start but was low on power sometimes couldn't move the car before stalling but would always start sometimes hardstarting. Anyway it was the ignition coil, the ohm values were good but it wouldn't deliver enough spark.

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