My 1992 3.9 will start and idle very rough for a few seconds and die. Giving it throttle will also kill the engine. I’ve replaced plugs wires coil cap and rotor. Ive also noticed the tach has Stopped working.

 

Due to the age, the tach may just be failing; or it may be screaming that your crank sensor is failed.

Before I paniced too much, again due to age ...

1) Disconnect the battery negative cable.

2) Using a 10mm or 3/8" socket (Don't remember which!), remove the connector to the SBEC on the passenger fender.

3) Reinstall the connector.

4) Repeat 2 and 3 a total of 3 or 4 times.

5) Reconnect the battery negative.

6) See if that helped any. On both counts.

( I THINK the tach is just generated from the ECU, but it may be referencing the crank sensor for deciding to generate the tach signal. which is 2x per revolution.)

After that, it's time to check the TPS, the MAP, and the IAC all three. Then check and see if the exhaust is plugged by pulling the upstream O2 and seeing if it starts and runs. But don't rush to that; the sensors are easier to check.

(With age, you MAY just want to replace them; but that's your money not mine, so up to you. Remember, it's 28 model years old now.)

RwP