The mystery of the Lost Coolant

Also, has anyone bypassed the coolant lines to the throttle body?
I accidentally made a hole in one of them with my hook tool getting them off.
I could shorten it by 10mm or so to get rid of the damage and it would still go on and work perfectly, but it occurred to me just now that as it virtually never gets below freezing here they don't seem to serve any useful purpose.
I thought it might stop the IAC valve freezing at idle as it does have small passages with high velocity air passing through, but I looked through the service manual and can see no reference to it needing to be heated.
I know the performance gains from not having a heated throttle body would be negligible, but these things need every help they can get!! 

I'll take a bet with you, bypass it and you'll find the engine stays at the "cold idle" speed longer or possibly all the time.

But just make sure it's a bypass and not simply block it off.
I once got given a car by a mate that was sick of it always loosing water, it would heat up, over pressure and come out.
The heater had died and one of the mechanics at work simply removed the heater and blocked it off.
After looking at it, I unblocked it and fitted a hose instead and it never had that problem again.
That's a tiny hose and probably won't have the same problem, but why risk it.

Quite a few of the vehicles on the roads now have the thermostat on the return (cool) side of the loop, and these depend on circulation of warm water through the heater core for the thermostat to open, both the M13 (Gen 3) and I believe the K15 (Gen 4) are like this, the G series (Gen 2 & early Gen 3) don't.

fordem wrote:

I'll take a bet with you, bypass it and you'll find the engine stays at the "cold idle" speed longer or possibly all the time.

I had wondered that, but the IAC circuit I don't think has any temperature measuring capability. The ECU already has that info from the coolant temp sensor, seems pointless measuring the same thing twice just for the idle speed. The incoming air temp is measured at the MAF sensor which is before the throttle body, so shouldn't affect that either. I believe some other vehicles do have a mechanical (I'm guessing Bi-Metallic Strip) system in the throttle body to manually drop the idle speed when warm, in which case you are correct, the idle would never drop to "warm" RPM. 
Also, I believe the idle speed is increased when A/C is turned on, which it wouldn't do if it was a completely mechanical system.

I haven't touched the heater system, just looping the throttle body line from the thermostat housing back to the block, bypassing the throttle body.

I've basically got all it back together apart from the lower rad hose, so will find out what happens I guess. I just thought I'd give it a shot, doesn't sound like anybody has tried it. 

Many vehicles have a "wax based" cold idle mechanism in the throttle body and typically these have that coolant circuit that you are considering bypassing - I've never looked at the M13 to see if they are built this way, but I'm willing to bet that they are, simply because of that coolant loop.

Moving on, the IAC is not a "completely" mechanical system, the idle speed is controlled by the ECU, the ECU controls the IAC valve which controls the air bypassing the throttle plate and so controls the idle speed.

Typically, the a/c compressor, the HVAC fan, the powersteering pump and any one of several electrical loads will trigger the ECU into increasing the idle speed.