Summers here and I'm having a hard time getting My 1988 300TE to start when hot.

OK, in Feb I had a valve job done and head gasket replaced and a few other parts. The shop told me that my fuel pressure regulator was not so good so I replaced it. The problem is the car starts ok in the mornings and it will start right up after you shut it off. But if is sets for 5 min or more it has a hard time starting backup. and when it starts it acts like it is out of gas and has a ruff idle but then smooths out after 30 sec or so. The problem get worst as it gets hotter out.

Try this

Turn key On and wait for 15 secs,
Turn off and back on and wait for a couple more secs and then try start.
Post results

I had the identical problem in my 89 300TE. I went through the intake system and fixed all the vacuum leaks. This helped a bunch and I did a write-up in the DIY section as it's not exactly obvious for amateurs like myself. I also recommend checking the condition of the cap and rotor (unlikely though).

I had heard that fuel leak-down is likely to cause this, but since injectors and associated parts are expensive I opted to get a fuel pressure tester and run a series of leak-down tests. Turned out there was zero leakdown, and I saved a bundle on the injectors, fuel pressure regulator, etc. It also reassured me that the fuel pressures were to spec.

And while the problem isn't 100% resolved, I'd say fixing the vacuum leaks improved it by about 75%

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89 300te 222,222
92 300e 190,000

Since the problem is at its worst when hot and after being stopped for a while, I would suggest it could possibly be the fuel accumulator (assuming no fuel pressure leak-down elsewhere). If the accumulator is faulty, fuel system pressure is not maintained after stopping and the higher temperatures allow fuel vapour bubbles to form in the fuel lines making for difficult starts.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.

I have followed this subject closely as my M103 is exactly the same and it is a common complaint.

A little bit of throttle when starting works every time. This is even what the Owner's Manual suggests for a warm engine.

I have checked all the associated fuel system parts and recently replaced the accumulator. Nothing has changed the warm start condition however I have greatly improved the response and feel by adjustment of the EHA and by having the engine in excellent tune.

IMHO it is unquestionably to do with old injectors. They don't last forever and 100k miles is the suggested life. They have done a good job already for 15+ years!! One or two leaking-down a drop or two creates the richness that causes the start-and-die. This explains the instant resart or the success of the use of throttle (more air).

It is further explained by the fact that the engine actually continues to heat up after shut-down when the cooling system is not working. This additional heat puts additional strain on the tired injector.

The benefit is that they are not teribly expensive for an M103 and can be changed DIY.

It is the only thing that makes sense although I have not changed mine as yet.

89 300CE
153k kms

At 160,000kms changed the injectors.

Fuel accumulator has been changed prior and EHA adjusted.

Whilst idling and general pick up seems smoother (still checking fuel consumption) there does not appear to be any change to the start / die / start when warm issue.

Not sure what more can be done as otherwise it runs perfectly.

89 300CE
160k kms.