Urgent, please help!
From your description of idle speed woes, it sounds like an acting out IAC belongs on your suspect list, as its responsible for maintaining engine idle speed. Maybe try disconnecting its electrical connector so the computer goes back to its pre-programmed control mode & see how it goes. If things are better, maybe remove & try cleaning the IAC with a non residual, plastic safe spray cleaner, like CRC, or Valvolene MAF sensor spray cleaner. Keep the electrical connector end elevated so solvents & removed crud don't run into the windings & corrupt the solenoid wire insulation. Cleaning may not last, it didn't for me on my idle speed woes, but was a good trouble shooting step.
If the CEL is lit, scan the computer for trouble codes & post All code clue Numbers.
Some thoughts for consideration, let us know how it goes.
Lots of things can cause your woes, so at this point I have more questions than suggestions. Sounds like the cooling system was neglected if the radiator drain is plugged, so putting the radiator on your suspect list isn't unreasonable, But lets think about this some more.
Was it doing this Before the water pump install???? Is the temp gauge indicating an over heat condition & if so is, or has the CEL been lit??? If so, do the upper or lower radiator hoses collapse when the engine is running since installing the new water pump????
Since the water pump has been replaced it means the cooling system has been drained & opened up, so how did the old coolant & things look inside???? Were the pump vanes ok, any rust or sludge in the pump area, or visible in the block???? Clues on the internal condition of those things can help decide what the condition the radiator & block cooling passages may be in & where the excessive pressure may be coming from.
You say the cooling system has been checked ok for gasses, so I assume you mean exhaust gases from a leaking or blown head gasket???
A faulty radiator cap that doesn't hold pressure can cause the coolant to boil when the engine is at operating temp, so has the radiator cap been tested, or replaced & if replaced, was the new cap tested to make sure its good????
Releasing the radiator cap lowers system pressure & can cause the coolant to boil when the engine is at operating temp, so we need some radiator cap input.
When you replaced the water pump, did you also replace the thermostat with OEM, or a like one with the air bleed valve design????
When refilling the cooling system, if you didn't use, or don't have an air bleed valve design thermostat, did you elevate the front end & run the engine with the front end elevated until the thermostat opened, so any trapped air could make its way past the thermostat & back to the radiator to be burped out by a properly functioning radiator cap????
After completely cooling, a coolant overflow tank level drop will tell if the system burped & topped itself off.
SO, we need some more details before suggesting condemning the radiator at this point.
Ok, good feedback & good idea to have used the OEM thermostat with the air bleed valve, as that'll help properly burp the cooling system after a refill.
Was it doing this over pressure thing Before the cooling system work????
If so, what condition was the old coolant in & how did the old water pump vanes look & how did the coolant passages in the block around the water pump look????
If the radiator is plugged, the lower radiator hose may be collapsing, so check that out.
If the radiator drain is plugged, it suggests the radiator at least has some sludge in it, but the condition of the old coolant, water pump vanes & what you did, or didn't find in the engine block after removing the old water pump, will give us important clues about the radiators possible condition.
Right now, with the info you've posted, its not clear to me where the over pressure is coming from.
Could be a leaking head gasket, rusted/sludged up block that's not properly conducting heat to the coolant, questionable radiator cap, plugged radiator, or some combination there-of, so we need answers to All of my questions to begin getting a better picture & yield some more clues about what may be going on with your cooling system.
Ok, good feedback & good idea to have used the OEM thermostat with the air bleed valve, as that'll help properly burp the cooling system after a refill.
Was it doing this over pressure thing Before the cooling system work????
If so, what condition was the old coolant in & how did the old water pump vanes look & how did the coolant passages in the block around the water pump look????
If the radiator is plugged, the lower radiator hose may be collapsing, so check that out.
If the radiator drain is plugged, it suggests the radiator at least has some sludge in it, but the condition of the old coolant, water pump vanes & what you did, or didn't find in the engine block after removing the old water pump, will give us important clues about the radiators possible condition.
Right now, with the info you've posted, its not clear to me where the over pressure is coming from.
Could be a leaking head gasket, rusted/sludged up block that's not properly conducting heat to the coolant, questionable radiator cap, plugged radiator, or some combination there-of, so we need answers to All of my questions to begin getting a better picture & yield some more clues about what may be going on with your cooling system.
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Have you tried flushing the cooling system to get as much crud out of it as possible???
With the temp coming down some at idle, it sounds like you have some coolant circulation, but not enough flow & the block passages are likely clogged up & have poor heat conduction from surface crud too.
Have you looked at the lower radiator hose to see if its collapsed when the engine is at fast idle???
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They market a "T" adapter kit that allows us to use a garden hose to back flush the heater, engine block & radiator, so I'd suggest trying that, to get as much loose muck out until the water coming out runs clear, then add their heavy duty de-scaler & run it as directed & back flush the system again until it runs clear & see how it goes. If that doesn't seem to improve flow enough to get things close to normal & you still suspect the radiator is clogged up, consider having it boiled out at a trusted cooling system/radiator shop, before replacing it. If you have to replace the radiator, consider online discount codes at Advance, I got a really nice replacement radiator for my 94 Taurus about 5 years ago @ 40% off, using codes here. http://www.retailmenot.com/view/advanceautoparts.com
Be sure to get the radiator cap tested & make sure its the specified pressure range.
Keep us posted on how the cooling system flush goes.
Remember most home water systems have about 60 psi, so don't position the garden hose valve wide open & pop something in the cooling system.








