I have an '87 pickup with a 22re. For most of its life It's been getting about 10L/100 km (roughly 24 mpg) Over the last few months this had decreased significantly. I've replaced the cap, rotor, wires, plugs. I suspect my throttle position sensor may be bad, but is there anywhere else I should be looking for problems? Or is it possible that the motor is just getting tired (380000 km)?

 

o2 sensor

 

Have you looked into the fuel filter? If your injectors are getting clogged you could send them off to witchhunter to be cleaned.

The link to the FSM is in my sig, use that to check out your TPS if you suspect it.

 

Winter fuel additives give you a nice ~10%-20% drop in fuel mileage.
Ethanol also makes these trucks run worse.

 

Thanks for the sugestions!!
I've recently replaced and set the TPS, as well as cleaned the throttle body, and my mileage has not improved
Another problem I'm having (which I'm hoping is related to the poor mileage) is a high idle (1200-1400 rpm) when the engine is warm. It idles perfect (750 rpm) when it is first started.

 

Not any mention of fuel injector aditives, it seems that should be of some help.

 

Your idle should be at 850rpm or whatever when the engine is warm. Set the idle properly, check the timing as well.

 

Idle and timing are perfect, and I use fuel injector cleaner often.
I just did a compression test and found that I have a cylinder with only 50% compression. I'm gonna assume that's the problem here