2000 Mitsubishi Mirage 1.8L P0170 Code: Symptoms, Diagnosis, and Cost-Effective Fixes

Hi everyone, I'm new to this forum and hoping for some expert advice! I've been dealing with a persistent P0170 code on my 2000 Mitsubishi Mirage 1.8L, and I'd like to understand what's really going on. Background: Over two years ago, the car passed its pollution test. Shortly after, it started running poorly — initially with severe hesitation when accelerating from a stop. The check engine light came on before I got home. Upon inspection, one spark plug wire had come loose. This has happened before, but never triggered a code at that time; I suspect this time the driving duration was too long. I cleared the P0170 code initially, but it returned after just a few drives. Since the code is generic and not specific to any one component, I assumed it might be related to an oxygen sensor or fuel mixture issues. Given that my car has over 100k miles, rust typical of the Rust Belt, and is due for a timing belt replacement, I initially planned to sell it. However, with gas prices now exceeding $4 per gallon, I’ve decided to keep it — especially since it gets significantly better fuel economy than my truck. Symptoms: - Severe hesitation during acceleration from a stop (initially) - Over time, this improved but was replaced by erratic idling and the car sometimes dying when sitting at a light - While driving, performance is fine with no power loss under full throttle I adjusted the idle speed to around 1000 RPM, which reduced the dying issue — though it didn’t eliminate it. The factory specification for idle is 700 RPM ±100. Recently, I took the car in for an oil change and had a professional scan it with a real OBD2 scanner. They reported a flagged MAF sensor and idle speed issue. I replaced the air filter and cleaned the MAF using MAF cleaner. After driving, idling improved significantly — the car no longer died nearly as often. However, the P0170 code returned immediately after. When I reset the idle back to 700 RPM (spec), the light came back on again. The idle now fluctuates between 600 and 800 RPM, with a regular pattern — it rises and falls at intervals. When it drops below a certain point, the car sometimes dies. Fuel economy has dropped from about 28 MPG to around 22 MPG. I’m hopeful that lowering the idle might improve this, but I'm not sure. Before spending $165 on an aftermarket MAF or over $300 (which is a significant investment) for an OEM sensor — which may or may not resolve the issue — I’d like to know if there are any other cost-effective solutions. My car is worth at most $1,500, so budget is very important. I love this car because it’s simple and reliable — it was my mom's vehicle — so parting with it would be hard. Any suggestions or troubleshooting steps would be greatly appreciated! Thanks in advance!