Porsche 944 turbo won't start, have replaced DME and fuel pump, starter turns, engine won't catch

I would jump the fuel pump relay first. If it starts replace the relay. Or fix it. Bad diode in it. $50 compared to 50 cents. Done many of them.

Not much left unchanged is there and no doubt you have done compression testing on all 4 cylinders as well. (If not you need to). I assume it is cranking over normally with the starter motor but not firing.

I would try one of the Porsche Owners Forums for this as one of the other 944 owners there should be able to help..... Like this one..................

Porsche 924 944 968 Technical Forum Pelican Parts Technical BBS

try this u tube link
911 Porsche Carrera DME Relay and ECU DIY

Pull the injectors and have them tested for electronic continuity and spray pattern as the needle may be rusted up or the coils in the injectors corroded away for the long period of no use. Regardless of what engine you have if you have spark at the correct time --compression --fuel then you must have go in the engine . Continuing to run will depend on the make and design as the other variables come into play.

that sounds like a fuel pump or fuel line blocked bro. good luck. doubt the dme

get an alternator test

It sounds like you don't have sufficient fuel.
Do you hear the fuel pump turned on when you put the key on?
If it is not, need to check the fuel pump relay.
Jump wire at the fuel pump relay to to turn it on to see if Fuel pump is working
Need to check if the fuel delivered to fuel distributor in spec.
Jump wire your relay 60s to check how much gas delivered to the fuel distributor, by disconnecting the gas input at fuel distributor and catching fuel in a calibrated glass container. The gas quantity ml divided by 60 sec to have a flow ml/s. If this is not meeting spec then change the fuel pump. Good luck.

If the fuel pressure regulator is stuck there will be to much pressure to the injectors and the injectors won't open, and thus no start. Since you relived the pressure by lossening the cap the engine ran.

You will not be able to hear the fuel pump on that model. First you have to determine if you get fuel and have spark. If it is a fuel problem, replace the DME relay first. Very common. They act up and fail without warning. Part is cheap and located in the fuse box. Next would be the fuel pump.
If it is spark related it may be the Flywheel Reference Sensors. There are two and the cost about $100 each.

Good Luck.