Flatch

I've got a 1994 base model pickup w/ 22re for 11 years. Has 155,000 miles. I drive like grandpaw, so its still on the first clutch and timing chain. clutch feels solid too. but anyways. I've been getting code 25 lately, but only on the hottest days. It seems to run perfectly so far as i can tell, but it does idle very low on hot days. Like when its more than 92 degrees outside. It idles very quietly, can barely hear it, and i'm guessing its idling very slow. I don't have a tach.

it all started 2-3 years ago. it was hot outside. about 15 minutes down the road, the CEL came on. but it was running perfectly. so i ignored it for about a year, found out it was code 13 & 25. I pulled the efi fuse, reset the cel, and it went away for about 2 years, until recently. the cel came on after 5 minutes of driving. i reset the cel. it stayed off until another hot day, then came on after about 25 minutes of driving. lately i've only been getting code 25.

so i can't figure it out, because it seems to run normally so far as i can tell. all i know is, it doesn't like hot weather, and it idles very low, and quietly on those days, but it always has done that for 11 years. it has full power, doesn't miss, isn't rough running, and doesn't shut off when idling slow.

last winter i had all new tailpipe from the cab back, and a muffler installed. there's a cover over the exhaust header thats rusty, and used to rattle until it rusted so much that one of the bolts isn't holding it on anymore. thats about it.

any ideas?