Lean code

It could be virtually anything the system monitors. Being it is an 1988 it has the old control system. The codes it stores can be retrieved with a piece of wire or a paperclip.
Check at this link to see how to retrieve the codes by shorting the A and B terminals and reading the flashes. Once you know the code(s) you can post it here and we can try to help.

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

With a code that turns on the light but goes away when you add fuel it could be a lean condition or it could be something else. The code will help somewhat as it will tell what the ECM sees at least. A simple vacuum leak could cause this behavior but so could other things.

What is the actual code number, please? Very seldom is replacing an O2 going to do anything for a lean code. Lean means not enough fuel or excess air, like from a vacuum leak, low fuel pressure or similar. The O2 sensor just reports what it sees in the exhaust.

Okay, thank you. 44 means the engine is not getting enough fuel, that shows up as a lean code in the exhaust. Common causes are low fuel pressure from a weak pump or partly plugged fuel filter. On that truck the fuel pressure needs to be between 10 and 16 psi. Tested after the inline filter. Lower will set a lean code. Have you noticed any poor running or surges or is it just the light on?

Another common cause would be if the sensor wiring is damaged by grounding out say on the exhaust manifold or engine, that would drop the voltage the ECU sees and make it set the code. A good visual inspection of the sensor wiring from under the truck and through the harness would be the first step. Then a continuity check from the ECU to the sensor to verify wire integrity is good.

You will need a test gauge that can hook into the fuel line. Some auto parts stores will have them as loan/rent tools. Just be sure it has the fuel line adapter piece. The filter should be on the frame right under the drivers door. The engine side will have a threaded fitting where the adapter attaches. You will get fuel out while installing the adapter so be prepared. Depending on the maker of the pressure test kit it may connect in place of the fuel filter or it may connect at the engine side of the filter. I would buy a new filter and change it out either way, reason being, if you test it with the "replace the filter" adapter and it tests okay, is the filter plugged causing the problem? Or if it is the "after the filter" type, is the filter dropping the pressure? So changing the filter either way may help.

There are a few videos online about how to test that system.
Search- GMC, TBI fuel pressure testing and fuel filter replacement.

To cause this code the leak would have to be in front of the O2 sensor. So look for cracks or damage to the manifolds the manifold to pipe gaskets and damage to the pipes themselves. Normally you would hear the leak that would set this code. Plus the light would come on regardless of throttle position because it would be a constant leak.

They should not leak then. Is this a recent rebuild or anything like that? Or a replacement engine? Just trying to think through the problem.

The pattern you have where at the lower rpm it runs lean and then when you open the throttle it gets better really suggests a vacuum leak, it is pulling in air from somewhere other than the throttle body. The other component would be low fuel pressure or the pressure regulator in the injector is failing.

Does this truck still have the air system that sends fresh air into the exhaust? If it has the tube that enters into both exhaust manifolds and goes to a pump on the passenger side then it has it. That could cause this issue if the control valve is not operating correctly. I forgot that era had that system. It is supposed to only send air into the exhaust under certain circumstances, but if the valving was damaged it could send air into the exhaust and cause a lean code. (Around here most of them rotted off and got plugged)

If it has this system and you can still remove the inlet tubes and plug the ports or can remove and plug the hoses it would be an easy way to test if the system is the issue.

Okay, toss the air system. Pin hole in a heater hose would not.

Okay, 12 psi is a good number. The only thing left has to be a leak letting air into the system at partial cruise. Either vacuum or possibly the EGR is causing an issue. It operates at partial throttle. Disconnect the EGR and see if it still acts up. If not then there is something letting air through that system. Or it has a vacuum leak that the extra fuel hides as soon as you step on the gas. For that you might consider having a shop run a smoke test on it and see what shows up. Leaks can be in odd places.

Has this done this since the engine was installed or did it just start out of the blue?