Wont rev over 2500rpm after driving for a while and flashes up power light when missing and spluttering at 2500rpm 1990 300zx twin turbo auto

Is it an auto it sounds like the kick down switch usnt working there normally behind yhe accelerator pedal i think never had an auto merc been all manual

Sounds like a Rev limiter...

Possible that the memory was reset and so the ECM is also reset. You need to check this out with the Merc dealership so as to reset the ECM without further damage.

I suggest you check the fuel pressures and delivery volume before you replace any more unnecessary parts...

Don't spend money until you know what is wrong and that shouldn't be guessed but discovered by careful and systematic checking.

In addition check the plugs and spark power and quality

diesels are not known to be high revving engines to begin with, anyways, it sounds like your turbo is malfunctioning, get it inspected or repaired

The vehicle DOES have a rev limiter when in P or N. The whistle is the governor allowing air into the intake to detune the engine to linit the revs. The cabin heater light is for the AUX CABIN HEATER. It is diesel powered and will light when operating to maintain the set cabin temp. It has its own exhaust that comes out under the side of the vehicle. It can even run for a few minutes after the vehicle is turned off. That is normal.

A CLEVELAND ENGINE IN A FAIRMONT????HMMMMMMMMMMMM....SOUNDS LIKE THE PUMP IS GOING BAD. DO YOURSELF A FAVOR AND JUK THE FMX AND GET A GOOD USED OR REBUILT C-6 INSTEAD.

the oxygen sensor might be stopped up if the check engine light comes on go to auto zone and they might be able to dignose it

clogged fuel filter, or a short in your computer

I’m happy to assist further over the phone at https://www.6ya.com/expert/john_2c7bb9c157d0baa8

that maybe what the rev limiter is set for.That way you can not over rev causing engine damage.