96 s-10 4.3, the truck idle's fine. Runs under a load fine, but when I hold the throttle steady I get a miss.. It will run like that until it sets code po300.. it has new wire's, plugs,cap rotor.

Injectors may be clogged causing lean fuel misfire under load.

I'm not sure what you mean by "under load", maybe with a/c on, etc. Also is it really an engine misfire or a rough idle. If it really is a misfire, a scantool at your local parts store should be able to read a code for that. Sometimes a rough idle could be due to just a cracked or disconnected vacuum hose or dirty throttle body. You didn't say whether it runs ok at highway speeds. Sometimes a rough idle can be cleared up a lot with throttle body service or injector flush. Sorry, this is the best I can do with information provided.

tune up,fuel filter,air filter,clean throttle body.

You should check you MAF sensor. clean your throttle body and change your spark plugs. shouldn't be anything if you say there is NO vacuum leak.would recommend searching your PCV valve for a vacuum leak

this is what i found about code f85




QUESTION: 1991 Sedan Deville, 4.9 port injection
Car runs great until outside temp reaches 92 deg. or above. the car will start to cut out under load (take off or hill. if i pull over the car will idle fine rev the engine to a steady high rpm and it will cut out. i had a code e44 (o2 sensor rich)so i replaced o2 sensor with no effect. the other code received was f85 (Throttle body service required). I have checked all connectors and hoses attached to the throttle body. all ok. looking inside throttle body there ar two tubes under the flaps one is totaly restricted with a black buildup and the other partly blocked. Not sure what these tubes do and if that could cause engine to cut out when hot and under load. any ideas?

ANSWER: You found your problem which is this black residue which when hot turns to a jelly substance and blocks the idle air passage. You need to clean all that residue from the throttle body and do not forget to spray the small hole in the side of the throttle body just behind the throttle plate.

check fuel pressure regulator and fuel pressure. should be between 45-50 psi and hold steady.. shut off car and let sit 5 min if pressure drops more than 10 lbs one of the two is bad...

Check engine light on? Any codes? Missed before and after the cap was changed? Or is the cap new so you assume its good. I have had caps bad out of the box.

Idle speed control valve located on your throttle body. It has the engine searching for idle, internal plunger is not backing down. This will cause the check engine light to light . if you wont, i think they still offer free check engine light checks at auto zone, i now they still do around here

sounds like its breaking down, check your ignition system, test your leads, check your plugs, check your ignition coils etc etc etc.

try replacing the throttle position sensor