Lately, my 99 has been acting up when I start it up. The idle is irregular, as if I was giving it gas every other second.

It can go on like this for at least 2-3 minutes, but if I hit the throttle hard, it stops.

Dunno if it's related, but I also have an issue where I get a CEL at least everyday telling me it'S running rich (P0172). I reset this all the time, but it's getting old.

What pisses me of is that last summer, I changed my o2 sensor (the one closests to the tranny), figuring it would cure this, but now it still says it's running rich, yet doesn't tell me the sensor is bad.

The sensor I bought is an application-specific Denso sensor I got from a car parts store online, and it cost like 225$, so it's not like I bought a generic cheap ass model.

Any help (for either problem, or both if they're related?)

 

Lets say you have been driving for a while, and you have come to a stop (maybe a traffic light). What does your truck idle at then? Is it still irregular?

The fact that tapping the throttle settles the idle makes me think its your TPS. Usually that would throw a TPS circuit code, so it may be another issue, but TPS malfunction would describe the irregular idle only corrected by tapping the gas. Might be more than one thing though....

 

If I drive the least bit, the idle is perfectly smooth when I get to a traffic light.

 

That makes it a bit less likely to be the TPS.....

you tried the basics? like checking/cleaning the MAF, airfilter, new plugs?

 

x2 on cleaning the MAF, changing plugs, etc...i think its something small like that

 

i would clean/replace that stuff (worst case scenario it doesn't fix it, but it probably needs to be done), then I would explore things like Idle Air Control Valve, TPS (if you can find a buddy to swap with you, although after your response, it sounds less likely), injectors, etc... thats just me though, and I'm no mechanic