I am getting an over pressurized coolant system. (within minutes of cold start up)

This is what I have done currently to solve the issue prior to comming here:

Step 1: compression test (150 +/-1psi on all 4)

Step 2: changed thermostat (old one was newer but installed 30* crooked)

Step 3: replaced radiator as the old one would not drain (rust scale and sludge)

Step 4: burped system multiple times

Step 5: replaced head gasket (head decked, pressure tested, and inspected for any leaking in gasket area. No issue were found in gasket at all. Used MLS gasket)



Symptoms are the same after this. The car fires right up, but loping idle between 1000-2100. I managed to stop the loping duriong warm up by blocking off IACV completly. Once warm up is complete loping idle returns with spark advanceent.

The short time the idle is even I attempted to put on timing light and I cannot get timing within range even using full amount of rotation of the dizzy. I've double checked the cam and crank are 100% correct. (and balance shafts) So not sure what is causing this as yet.

The coolant system still super pressurizes in about 1-2 minutes of running. So far I am not seeing coolant in the oil yet, but the old stuff was pretty mixed in the end.

I have never seen this much pressure in a coolant system without a head gasket failure.

Does anyone have any thoughts? I cam currently thinking it has to be a cracked block, a sheared off water pump impeller or plugged up coolant passage in the block that is causing the coolant issues.

Hunting idle seems to be a bad IACV, and owner complains it was like that for a long time. (it has new vac lines for it, and all vac lines on the whole engine seem to have been replaced recently)

 

With the stat out, what kind of flow are you getting thru the coolant system...

"plugged up coolant passage in the block that is causing the coolant issues".

"Step 3: replaced radiator as the old one would not drain (rust scale and sludge)"

 

If your radiator was clogged up, chances are the rest of the cooling system is as well. Try backflushing the system. (Hook up a hose to the upper rad hose and flush it out.)