HI,

There is only an operational test for the FPR because it's mechanical. It only passes fuel back to the gas tank above a certain pressure around 43 psi or what ever is printed in bars on the outside.

Specifications go like this per each part number configuration. Or at least that is the "game" they are playing to jack prices all over the place for a very simple, $12 to $20 manufacturing cost priced device, for the last 30+ years.
Mercedes, "Built like no other Car in the World," is such a lie! It and its parts are only priced that way!
No car brands mentioned here. It's just the normal practices.
A Naturally aspirated engine: 3.0 bar (43.5 psi)
A Turbocharged engine: 3.75: 3.95 bar (54.4: 57.3 psi)
A bar is one atmosphere or approx 14.7 psi. I call it 15! Stoichiometric combustion is 1 part gasoline in 15. 14:1
You go up in altitude say a 1000 feet, you lose almost 4 degrees in temperature.
That's close to metric 39 inches or a meter. One millimeter, that's .039 thousandths of an inch. Whoops, almost a 4 again!
3/8" is .375 add a .015 you have 10 mm or .390 again! Fractional (1/64 of what) thinking started all this chaos. Remember the old days of the stock market. Computers changed that, so there is hope to lose fractions altogether.
The world is not as complicated as they want you to think it has to be.


To see these readings you need to connect a dial gage to fitting on the fuel rail that leads to the FPR. Luckily on yours, the 1993 there is a factory one.
Some yard hunters snag the rails off these cars and fit then to earlier cars. I added a Schrader fitting to an '86 car years ago. I get them from a refrigeration supplies house.

This way you can watch it while it's running and after the shut down. You want see if it holds some pressure for about 10 to 30 minutes. This reduces vapor bubbles that can develop during a heat soak when the car is parked.
Things to remember is the ECU fires the pump up on each start cycle and at even zero pressure there can still be a volume filling the rail. So the pump should be able to refill the line very quickily unless there is a total drain down by either the FPR, a fuel injector (4 of them) or the main fuel pumps external check valve mentioned in another persons posting.

Another thing that changes with heat is the metal of the intake manifold. It is a very expansive hunk of metal when compared to old cast iron manifolds.
As with any air seepage passed the AMM, like vacuum hoses or a bad gasket, is more likely to happen when things are expanded from heat.
The heat thins down the density of air volume, the engine is going to start with a leaner mixture due to a report from the ECT sensor.
Similar to a low fuel volume or loss of rest pressure/volume.

The time frame you speak of doesn't seem to far off to me for a time to time, but not if a continuously mind bending issue.

The ECU pre primes the fuel rail, for a second or so, every time as the switch passes through position two to roll the engine in position three.
It got figured out over the years by the engineers hearing complaints of cranking that they decided if it was needed when cold, why not every-time for simplicity.
There is a warm start up mode or really it's just a "warming up" running mode section built into the programing.
These cars operate to change pluse width on the injectors only. There is no longer a cold start injector to dump raw fuel anymore.
The old Carburetors had squirt pumps operated by humans avoiding tune ups!
The Emissions restrictions closed down those "avenues of practice" of pumping excessive Hydrocarbons in the air.

The engine can run fine after the engine starts with the fuel management system dialed up. It still needs to have control of mixture and needs feed back coming forth.
This is when there are many unknowns creep in that can raise "temporary havoc" with normal smooth starts.
The systems are so fast it can cover up discrepancies unless components go really bad. This is what you are hoping for....a quick fix... being cheaper than this or that. That's normal for us 240 owners.

It would be nice to know, from you, how long of a time the engine was stopped to achieve what is considered a "warm start" and not a "hot start" too!

Laboratories work with new engines under repeatable controlled conditions.
We now, have what they don't get to tinker with!
We have to find out what got aged to provide its symptoms to make it different!

Check valves on fuel pumps don't get engine heat but can where out their springs. I had only one over the years, so it's Very rare.
If you have that problem it won't start nicely cold or hot, given enough time to sit. It takes time to leak down the lines and filter volume.

After seeing the insides of Art's FPR necropsies, I can understand why they age and wear.
They can lose rest pressure two ways. The seat or the diaphragm.
Failure is a slow process, "starting," of course, at any time.(:-)

A FPR is considered to be one of those things that have to be changed to see what happens.
Sort of like a NEW intake manifold gasket might do.

What is nice about these two items is that both are easy to do work on.

I think I would start with a simple change of the intake manifold gasket.
The intake manifold gasket is a lot cheaper than a FPR and either one will eventually go bye bye!

Like me! (:-) Good luck!

Phil