Cooling System Pressure

I am now experiencing the excessive pressure problem that another member of GM Forums was having. Normal operating temp, but on shutdown it blows through the cap and makes a big mess under the hood. Replaced cap with a new Stant 15PSI, and it still did it. So I blocked the caps exit with a nickel sized spot of RTV(not recommended)and it stopped, but even after letting it cool down removing the cap released a LOT of pressure. Today while driving 50mi from 3k to 1kft elevation, it began to leak(blow) out between the lower hose and plastic neck of the radiator. I R&R'd the hoses last month and failed to retighten the new s/s clamps, so it found it's way through this weak spot. However, the pressure must have been massive to break this fairly tight seal(it was snug). Has anyone had this happen to them? Any possible system details, relief valves that I don't know about?

I wonder since the hose was leaking you might be low on coolant or have an air pocket. Also check the small hose to the tank for blockage.

You did not say what year model you have but it really does not matter. The northstar engine is well known for head bolt threads stripping out. This allows compression gases to get into your cooling system, which causes tremendous pressure. Your radiator cap is designed to hold a specific amount of pressure (15 lbs. or so). The compression gases create more pressure than your cap, radiator or heater core can stand. If you keep your system blocked off you will create more problems. I would bet my life that you have a blown head gasket due to the bolt problem I mentioned. To repair this the motor will have to be removed the heads pulled off and timeserts installed. You can go to http://caddyinfo.netgetgoing.com/howto/nsrepair.htm and see what needs to be done. I know this is a Cadillac sight but they have the same northstar engine. Aurora has the 4.0 version. The cost for this ranges from 2500 to 4500. There are several tests for this. I think they are talked about on caddyinfo.com. Let me know if you need more info. Please email me when you find your problem.
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Well, it's all academic now. The drivers side tank fully cracked today on the freeway and went into full on, Northstar, limp home mode. It kicked in when the gauge was only on the last white mark(275?) and I only had to drive it another 1.5mi. to the next exit and parked it. Kind of cool watching it all work, and also very nerve wracking. It was 50 degrees out, and the gauge never topped out, so I think the plugs and wires will survive. Found a radiator shop 3mi from my house who just happend to have a new Delco(Harrison tanks) for $189! Lifetime warranty at no extra $. I couldn't believe it! He said that he buys GM overstock and doesn't mark them up to gain customer loyalty. He also said that, in his opinion, this was not a "problem child" radiator even though he often sells to local dealers when they need it. He was surprised to hear that this was my second in a year, and didn't know what could be causing any extra pressure in the system. I just hope the pressure didn't cause the crack. The current one may not be a Delco unit even though it was a dealer warranty job. So I'll find out Friday when I yank it out. If it happens again, I'm thinking of having a custom race/street shop in San Bernardino, CA make an all aluminum one for 5-600 bucks and be done with it. I was wrong. It didn't blow by the hose and neck, it was running down from the top.

I would have to have proof of a head problem before dropping that much cash. I know I have good compression in all 8. Wouldn't you lose compression if one or both were bad? A complete, brand new longblock is only $5250.

What we do at work is to hold the emissions sniffer above the rad filler hole to check for hydrocarbons.