Boxer Medic

24v seems a bit excessive! Hope this is not repeating stuff everyone knows by heart...if so, Sorry! 4 wire coils, (IIRC, have the transistor built in the coil pack) have a common 12v feed, a controlled ground for each coil and a timing signal from the ECU. If only 3 wire coil pack it will have an igniter to control it. 4 wire COPs have a 12v supply, transistor controlled ground, a transistor base control from ECU and a ign conformation to the ECU. 3 wire COPs is same except for the ign conformation.

The T-pins are at Walmart for about $2.50

Here is some excellent starting problem troubleshooting videos.

Voltmeter works great. Could be something simple like this...

This should give some direction and help in diagnosis

Yeah, its hard to fully grasp initially, for me anyway but he does a good job of explaining and showing the how to test and troubleshoot. Let's see how the drive turns out.

I know this thread is a few weeks old and since I'm not sure if it has been resolved I thought this video might help. Similar symptoms, different vehicle but it may help.

The only way I know to verify a head gasket leak into the cooling system with near 100% accuracy is with a exhaust gas analyzer. If there are hydrocarbons in the cooling system, there is only one way they could get there.

This might help out some. It has helped me learn by watching it be done methodically and competently.

Check for bubbles in the over flow tank, my 97 legacy outback ej25 did a lot of the same things when my HG went bad. Random overheating, loss of coolant, poor heat from heater and bubbles in coolant overflow bottle. If your friend has a hydrocarbon tester you can warm up car and "sniff" for HC's in the radiator and/or overflow bottle. If *any* hc's are detected your HG are bad. There should be zero HC's in the cooling system.

Good place to start.

According to http://www.troublecodes.net/subaru/ that code 24 under 1988 and later it says faulty IAC valve or circuit. Since it is a new IAC that leave's the fault in the electronic circuit. Here is a list of videos that show how to test IAC circuits:

There are three or four different code 24 definitions based on year of vehicle. Try disconnecting battery for a minute or two. It *may* fix the problem, if that does not work I would focus on verifying the wiring to the IAC. Check for power and make sure the ground is less then 100 millivolts with the circuit powered up, key on engine off or running. There is a chance the air passages are physically restricted also. What year, model and engine is your car.

What is the code? Eg. Circuit fault or position error? Check power and ground at sensor. I'm still learning Subaru's, I know the GM linear egr valves must have the battery disconnected for the computer to set the new normal closed valve voltage. Would not hurt anything and, best of all, its free and easy.