Rough start

My 95 S420 starts fine cold and immediately after engine is turned off. However, if I start it 30minutes to an hour after running it, the engine is very rough for about 30 seconds. I have replaced distributor caps and rotors and checked plugs. I have also replaced the fuel filter. Any ideas would be welcome.

Originally posted by John Coleman
My 95 S420 starts fine cold and immediately after engine is turned off. However, if I start it 30minutes to an hour after running it, the engine is very rough for about 30 seconds. I have replaced distributor caps and rotors and checked plugs. I have also replaced the fuel filter. Any ideas would be welcome.

Sounds a lot like my 450SEL. I hazard a guess that the fuel accumulator may be bleeding off pressure.
Put on your diagnostic cap:
It wouldn't be anything in the ignition system if the problem is transient.
It would not relate to the filter if it is merely a starting problem.
Sounds like a temperature/time problem:
OK. What happens to a warm engine that does not happen to a cold engine?
Fuel vaporizes. So?
It empties out the fuel rail giving you a hard start where vapor has to "bleed out" before nice liquid is available for your engine.
Accumulator maintains pressure in your fuel lines.
What else?
Maybe you have a leaky injector that dribbles a pile of gas while sitting, causing that cylinder to not fire until the engine has cleared that one cylinder out.
I will post the cure to my car's *****y hot-starting. . . . .
Colin

Thanks Colin, your logic matches mine. My DeLorean had this problem, except it would not start when hot, after 30 seconds or an hour. It was the fuel accumulator leaking down. I replaced it and cured the problem. However, I don't think my car has an accumulator. It has two fuel-pumps, each with a check-valve. Maybe one of these is bad. I guess the first step will be to hook up a pressure guage to the rail port and see if the pressure is bleeding off. I also suspected one injector, but pulling all the plugs revealed no sign of one burning rich. I'll post my progress.

[vinamg]

This car is an EFI and doesn't have a fuel accumulator. I would check the fuel pressure regulator. It is located under the air cleaner on the stainless fuel line (toward the back). Run the car for a while and disconnect vacuum hose on fuel regulator, if fuel seeps that's your problem. Also, if you have a vacuum gauge you can check if the regulator holds vacuum.

Vin

Originally posted by vinamg
This car is an EFI and doesn't have a fuel accumulator
Vin

Well dang don't that make me feel stupid.
Anyways, I only go as far as L-Jetronic.
Let us know what you find, John, for
ejukashunal perpusses.
Colin : )

[loubapache]

There is a recent thread about this and I shared my experience (one of my car had the same warm start problems. No cold or hot start problems).

Colin's "Maybe you have a leaky injector that dribbles a pile of gas while sitting, causing that cylinder to not fire until the engine has cleared that one cylinder out. " is probably right on the money.

Try some Techron and keep trying.

[JP560]

John, Please be sure to post your cure. I too have exactly the same problem with an 87 560SEL. Cold starts great. Shut it off and start it again immediatly, starts no problem, Hot or cold. Come back in 30 minutes and you crank for 5-10 seconds and runs rough for 20. Unfortunately, I've already changed both check valves at the two fuel pumps, and today the fuel accumulator. I'm sorry to say ($75 later)-- No difference at all. I'll be checking in, for hopefully your success story, as I'm out of ideas. Good Luck JP