I have a 1992 ford mustang lx 5.0 5 speed. my idle at operating temp stays high, then gradually goes down to norm, even while driving i have replaced egr valve & sensor tp sensor valve idle air control all of the vaccum lines, that i could find what else can i do ?

In general the best and safest method is to use the timing marks on the crankshaft pulley and the camshaft pulleys. If no work has been done on the engine a simple way is to listen to the engine as it is running. An engine that is out of time when at idle and being revved will run unevenly, have rough acceleration and possibly a lack of power. If it is really far out of timing it will make lots of nasty noises as the valves hit the pistons.
If your setting up the engine after major work, like a timing belt change or a head gasket replacement, You MUST use the timing marks on the crank and camshaft pulleys to avoid doing serious damage.

Double check and make sure you have enough coolant in your system. Additionally why you are at it put in a new thermostat preferably the lowest temp allowable by manufactorer. A cool running engine is an efficent engine

The cheapest would be a cold air kit (probably less than $200.00). It is easy to install, but won't quite get you there. The next is a better exhaust system (probably under $200.00). The other easiest modification is a performance chip or engine tuner, but they can be upwards of $300.00. A high performance coil, spark plugs & wires can help as well. The bad news is that each of these will make a minimal improvement by themself. Together they make a grand improvement. Everything is expensive.

Check your fuses to see if one is blown first. Thats a simple fix.

the ac pressure depends on the gas in use
r314 runs at high pressures
overheating at slow speeds indicate an inefficient coolant fan and the high ac pressures indicate that the condenser fan also is not operational
if it has a viscous fan hub , replace the fan hub
if it has electric fans check / the fans, fan relay , fuses , coolant temp sensor

replace the lift shocks on the gate

Check that the water pump drive belt is tight
Check that the cylinder head is not blowing.
Check the impleor on the water pump is not eroded
Check theair flow through the radiator is ok.
Check electric cooling fans operates if fitted.
Are you driving in to low a gear?
Are the brakes draging?
Check to see if you have some contaninated fuel
Check that the correct ignition time is ocurring at all engine speeds.

Best of luck Cliff UK

Hello and welcome to FixYa!

All EFI (electronic fuel injection) engines idles high during start ups. It serves like an auto choke to regulate the vacuum supply and most of all to lessen the wear and tear of the engine. Please drive the car only when it reaches the normal operating temp.

Hope this helps and thank you for using FixYa! Have a good one!

Have you tried to replace the thermostat?

disconnect the battary for aprox 1 hr this will clear the computor of bugs then reconect drive for about 30mins and problem should be solved .
good luck