I have a question. Can the Oxygen Sensor from a 1990 740 Volvo be the cause why the rpms will only go up to 1500? What about the throttle sensor?

I'm no expert on that subject, but those two components are easy to test. The FAQ tells you how to measure the voltage on the O2 sensor with a voltmeter. It's not that hard to do but some O2 sensors are difficult to stick the probe in to the connector. I just connect the ground to the car's frame.

The throttle position switch can be checked with the engine off. Some tiny alligator clips would probably make the job easier. Just pop off the connector and put your meter in "ohms" and attach to the 2 of the leads on the switch. Open and close the switch. You should see it switch from 0 to infinity every time you hear a click. Some of them have a wide open switch connection (non-turbo as I recall). Mine are turbo so the switch is only wired for closed throttle.

R Duke

oxygen sensor-no
throttle position switch-no