need some help

hey out there

i have an 82 mustang with a straight six. i am not trying to make it a track machine or anything i am just haveing some trouble with my engine.
i start it up and every thing is fine and then after i drive it for about 7-10 minutes at a steady rpm it start to sound like a diesel. and then if i try to accelerate from there i hear a louder knocking noise from the engine and it seems like there is a lack of power.
once i get into town and start driving in the city the diesel sound goes away and my car goes back to normal i have checked all my fliuds and every thing else i have no i dea what is wrong does anyone and any help would be greatly appreciated.

Hmmm. Almost sounds like its way overheating at high speeds. What does the temp gauge say?

no the guage says that it is running normal. i live in the country and it is like a 15 mile drive to get to town and i usually drive it about 70-75 ( if im not late to class) and the rpm are about 2700. the heat guage doesnt read hot. i am at a loss for what to do. i DONT have the money to be buying a new engine or a new car

it sounds like a vacuum leak in the egr circut. the egr adds inert exhast gasses to the combustion chamber at highway speeds. the knocking you hear is a lean condition that is happening when the egr valve opens or is supposed to be open. you might remove the egr valve and check the hole for excessive carbon, the vavle itself should have a leak if you suck on the nipple, you but should be able to make the plunger move by sucking on the nipple. you dont usually have to remove the egr vale to check to see if its moving. make sure you replace the gasket when you reinstall it.

thank you but might you tell me how to do all that. i do know about cars but not all the circit stuff. me and my friend just got done replacing a boss 302 with a 351W engine trnny and eguast. so i know my way around the hood. but i have no i dea what you are talking about. if you could would you explane how to do it