My 89 corvette is hard to start when cold, idles very rough. replaced the IAC and Maf sensors and had the throttle body cleaned.replaced spark plugs, cables and distributor. had timing checked. now it idles very high when cold and not easy to start as in the initial problem. help, this problem has cost me a 1000.00 so far.

Do you have any trouble codes?

check for vacuum leaks!

the iac sensor is along the side of throttle body , did you give the throttle plate area a good cleaning out this is where the air path is for the IAC sensor , a lot of idle problems have been fixed with a can of throttle body cleaner and few good squirts of the stuff down the hole

Check vacuum lines. Replace as needed. Clean the throttle body, inside of it. Use brake clean and a tooth brush. Clean the back side of the throttle plate and where it setts in the throttle. Clean the MAF sensor, just spray this down on both sides. Don't use a brush on this. You could break it. Once done cleaning, leave the MAF off. Start the car, it will take a second or two. Rev the engine up and spray brake clean into the throttle, not so much to kill the engine. Do this two or three times. Then put it back together.

the iac code has nothing to do with the maf . the maf is your mass air flow sensor and it is in the air tube going to the throttle body. the iac is your idle air control valve that is in the throttle body next to the throttle position sensor

clean throttle body with carb cleaner

remove neg. cable from battery so the ECM will reset this may work. make sure the IAC is air tight. if this doen't fix your problem have someone check the IAC valve if it's bad the part runs $300.00 pluse

PO506 --- Idle Air Control System RPM lower then expected

Clean Throttle Plate
Check for any air/vacuum leaks --

Ducting from air filter to throttle,loose or a vacuum leak
Throttle Body Leaking
MAF Sensor
IAC Valve gasket
Egr Gasket
PCV Seal

Hopefully you have installed new spark plugs since 2005
Also replaced the front (2) O2 Sensors at 100,000 miles
Cleaned MAF Sensor once or twice a year

try cleaning the IAC (idle air control) sensor and check the hoses going to the MAF (mass air flow) sensor, i have found both causing me problems with erratic idle. the IAC was totally gunked up with carbon on the pintle and i had found that the screws holding it down were loose ( but only clean this with throttle body cleaner, not carb cleaner it might damage the sensor, and replace the vacuum hose to the MAF sensor, i found mine was cracked and old and brittle, a car that has been run poorly for a long period of time might also cause a problem with an oxygen sensor and this would lead to poor fuel economy also, try to clean that up or just replace it. Also have the transmission fluid serviced, i have seen this problem lead me to think i had an engine idle problem or rough perforamance when accelerating. can you rate this!

replace the coolant temp sensor, not the coolant temp sending unit

I had the same exact problem on my 2004 Colorado (3.5L). Cleaned the MAF sensor (with MAF sensor cleaner) and throttle body/plate - cleared it right up.