bought an 89 RS 305TBI 2 years ago and had problems with overheating withing 3 weeks of owning it. i have won the battle until now. replaced rad, water pump, put in new 15" flex fan, ditched the electric, because i was sick of the toggle switch that i rigged because the relay bit the dust, flushed the system numerous times. i had the 160 thermostat in it and it would overheat so i just took it out and it worked for a good half a year without a prob but about a month ago i put a police chip in it and beefed up the hp a little, well last week i couldn't keep the car from boiling within 10 min. of idle. i ripped the air dam off of it racing it, so i know that could be a prob. but at idle i shouldn't be able to sterilize needles in the radiator fluid within that amount of time also i know that the timing is now off, compared to stock, because of the chip so that is also a poss. but i don't see it taking almost a month to effect it. the car is winning the battle. i am a great backyard mechanic but when it comes to cars with computers i am lost. oh the engine has 125K pure island miles on it. i know that is def. not good but i would like at least another 25K out of it, but right now it is sitting in my driveway laughing at me. the cars name is DIABLO, and i am sure i know how it got that name. could anyone tell me what to do because i am LOST, thanks a mil

 

your problem is that missing air dam! just went through the same thing with a guy right near me. he said i dont understand at idle it gets so hot even with dual fans, a fan switch and a 180 stat, its great when i drive but it heats up in traffic. well he didnt have a air dam so when the car was stopped it was pulling NO air into the radiator. he is now running a 195 stat still has the fan switch but no complaints at all since it doesnt go over 220 normally 210 all day.

 

You didn't say what you did to "beef up the HP a little" but I would say you have air trapped in the cooling system.

Remove the radiator cap and then run the engine up to operating temp. Keep an eye on the coolant level adding as needed. Once you are fully warmed up (the t-stat must be open) then you can replace the radiator cap while the engine is still running.