BrianM

My friend has a 1991 500sl with a 5.0L engine
It starts fine cold, but when warm it takes about 30seconds to start. when started it runs ok

Ausmbtech

If you have a LH fuel injection (electronic injection), not KE injection (big air flap under air filter) then it would most likely be a leaking fuel pressure regulator causing your hard hot start issue.

BrianM

Mine has a round flap about 3" under the air filters, is that what you mean by big flap? If so what might it be then?

Brian

Ausmbtech

I'd recommend you get someone familiar with Bosch KE injection to carry on with diagnosis for you vehicle due to the relative complexities of the system for someone with limited experience.

I believe it would be fuel pressure related but unlike current fuel injection, educated guessing isn't recommended

benz-tech

needs a MAF or a crank position sensor........jk. cis-e is no fun, you need to find an old-school tech to look at it. i (in this rare case) would shy away from a dealer.

BrianM

I will check these out.
I might also mention that when has been running and warm it will start right away right after shutdown. the hard starting is after it sits for 1/2 hour or say up to 2 hours. We have tried releasing the pressure in the gas tank before starting, but no different.

Ausmbtech

It'll be a fuel pressure related problem, could be the accumulator next to the fuel pump but, as I said before, it really needs to be diagnosed by someone who knows KE-jetronic very well and who has the appropriate equipment.