Need trouble shooting help
 

My 1995 S420 starts easily from cold. But if I drive only a short distance and park for a short time, it doesn’t want to start and run. It will crank and sometimes backfire. If it does start, it will not stay running. If it sits for several hours, it fires right up again. If the car is driven for some distance and parked, the problem does not occur. It only happens when started from cold, driven a short distance and then parked for a short time. Any ideas where to start looking for the problem? Car runs perfectly in ever other respect.

Sounds like engine is flooded--try holding throttle open when cranking.

Flooding is caused by worn fuel injectors.

Thanks, that is a logical answer that fits the problem like a glove. The car has 115K and is in really great shape in nealry every respect. But when I replaced the plug wires not long ago, they were pretty thread bare. I suspect the same might be true of the injectors. Unrelated, but similar in age, was the heat/AC blower when I recently relplaced the original blower regulator -- with help from this forum, by the way.

Question: How can I tell if the injectors are worn? How difficult to replace?

Thanks for a great response.

I don't think that holding the throttle open increases the air to the engine. You have to take the air filter off and hold the air plate on the injector module down with something other than a body part, in case of backfire. FWIW

I don't think a 95 has mechanical injection, no air plate, AFAIK. Anyway, I do not buy the worn injector theory. Why do they only act worn under certain conditions? I don't have the answer however.

Old school
 

I still like the flooded engine theory. I’m old-school, and the engine acts exactly like a flooded engine would act. High tech certainly can throw a monkey wrench into such a theory, but consider this. Since the car is only driven a short distance, the engine doesn’t build up much heat. If it only sits a few minutes, the flooded cylinders don’t have time for the fuel to dissipate. If the car sits for a long time, say two hours, it starts just fine. Also, when the car is driven for a reasonably long time and distance so that it heats everything up, it will start again without a problem, I theorize, because the fully heated engine/compartment, etc. dissipates any extra fuel pretty quickly. I admit, it’s an old-school theory, but it fits.

Anyway, how can I tell if the injectors are worn? I don’t want to launch into an expensive replacement if it isn’t needed. The car has been to two competent mechanics, one a Mercedes specialist, who have not found any fault codes or other tech problems that would account for the strange behavior.

And you've checked the fuel pressure regulator? Those injectors would never be worn out at that mileage. You wouldn't replace them anyway, way too expensive. You can get them cleaned, refurbished and tested for way less than the cost of one new injector at places like this - http://cruzinperformance.com/ But I would make sure the FPR is OK first.

Thanks deanyel, I’ll check that next. Going back to flooded engine theory, the next time I experienced the problem, I held the gas pedal down, and the engine started. It ran rough for a few minutes then smoothed out and ran fine. Could not put it in gear without it stalling until engine smoothed out at idle. I agree about the injectors.