Hey yall. I recently replaced the o2 sensor, air filter, and spark plugs on a 89 toyota pickup that I recently purchased. Before replacing the parts it was running quite rough and would stall occasionally after running it for 20-30miles. After the replacement it runs MUCH MUCH better, and hasn't been stalling. However, after running it about 150 miles or so this weekend it started to lose power, idling rough, and began stalling while stopped. This started happening about 40 miles after a fill up. I also recently lost the gas cap. I'm planning on replacing the gas cap ASAP, but I'm wondering if the missing gas cap could lead to air pressure issues that could be causing these issues. Any ideas?

 

Might light the check engine light, not even sure about that, before OBD II years. but don't think it would case any kind of vacuum leak that would affect performance that much/

Maybe I'm wrong,,,,, put a new cap on it ,and tell us all what's what...

 

Postin' to every Toyota forum you can find?

No, a missing gas cap won't set a CEL. That's much (much!) more recent.

But it is a safety hazard. A lil' tap on your rear bumper will spray a fine mist of gasoline in the area of your exhaust. If your windshield was missing, would you plan on replacing it "ASAP"? A missing windshield won't burn you alive.

Your problem lies elsewhere. I'd start by checking the ignition timing.

 

Even the Toyotas programmed to read tank pressurization and set a CEL didn't develop drivability issues. A lot of drivers bitd bought aftermarket locking gas caps, and those often didn't make a great seal. I mean. good enough to keep thieves away and prevent water from entering, but not enough to pressurize the tank. But they didn't affect engine performance.