Weird Engine Problem
 

I have a weird problem on my 2002 mustang GT. I ended up blowing a spark plug out on my engine mid-race and had to replace the head on it. I did everything correctly with replacement on it but now my engine is acting weird. Weird as in now it won't cold start idle it just sits at its usual 760 rpm like when its warm and if i give it a little gas the rpms drop and come back up to its normal idle. Another thing my engine would hold its rpm just fine between shifting gears but now it just drops really quickly if im shifting from like 1st to 2nd or any gear for that matter. I have no check engine lights and can't seem to find the cause. HELP

Sounds IAC related, possibly.

Who told you you had to replace the entire cylinder head for a blown out plug?

I looked through a boroscope and and the bottom part of the spark plug hole was blown out it was about an inch and a half chunk

Weird Engine Problem
 

khankold, just a guess, but I'm thinking vacuum leak. Start the engine and spray all over it with carb cleaner or WD-40. If the idle "RISES" you found the culprit.

I'm with Buck Sargent, i would suggest having an fire extinguisher near by when you start spraying brake clean or carb cleaner around the manifold, just to be safe

Weird Engine Problem
 

Absolutely do NOT spray " brake cleaner" around anything that is hot. I only use "Carburetor Cleaner" or "WD-40," to check for vacuum leaks. Try not to spray either on hot headers. I have never had a issue with spray hitting the hot headers, but some common "safety" sense must prevail if you are new at this procedure. Oh yes, WJL is absolutely right about having a fire extinguisher at the ready.

yeah not a good idea to spray that around anything on your motor , take it from me i was doing same crap on my work van and bam ! lit up like a roman candle, had to rip my shirt off to get it put out, thought i was gonna have to stand bk and watch it burn