blown head gasket? serious gearheads take a look plz

Ok, i'm new to automotive forums and I bring with me a uniquely challenging problem for someone to try and assess.

Car idles fine. Little rougher than it should be though.
Leaving the cap off of the reservoir and not allowing pressure to build up results in the car idling without temperature problems, capping it and allowing pressure to build results in overheating. The catyletic(sp?) converter is getting excessively hot as well and may play into the problem.

While driving with the cap on the pressure builds too high and the relief valve shoots out water on acceleration. No water in the oil. Petroleum fumes coming from the reservoir. Check Engine light comes on upon re-cranking the car after it overheating then goes away after a few minutes. Have not hooked it up to a computer as this problem does not seem to me like one that can be diagnosed with a simple sensor malfunction.

Any help is appreciated, i'm a gearhead just more towards electronics.

got diagnostic codes
12 - Indicates a fault in the standby memory circuit.
23 - Indicates a fault in the charge temperature sensor circuit.
55 - end of codes

CODE 23 concerns me and i have no idea what a charge temperature sensor circuit

What are we talking about here, a Lincoln Town Car or a Chevrolet Cavalier? Some specifics would be helpful.

First of all, change the radiator cap. It's old and holding more pressure than stamped on the cap.

Sounds like you are blowing combustion gases through the cooling system. Read my thread titled "1991 Dodge Shadow Cooling System Issues" in the forum.

Let us know if you have any other questions or comments.

Code 23 - "Indicates that the throttle body temperature sensor (non-turbo)/(air) charge temperature sensor (turbo) input is below the minimum acceptable voltage, or the throttle body temperature sensor (non-turbo) is above the maximum acceptalbe voltage.

This sensor measures the temperature of the intake air as it passes through the throttle body. If this thing is not working, the computer will default to limp mode, causing a richer than normal mixture and possible cat-verter overheating. This will not directly effect the cooling system by itself.

Code 12: Problem with stand-by memory circuit. Direct battery input to controller disconnected within the last 50 ignition key-ons.

Disconnecting your battery will set this code. Have you disconnected your battery lately?

Ok, i'm new to automotive forums and I bring with me a uniquely challenging problem for someone to try and assess.

Car idles fine. Little rougher than it should be though.
Leaving the cap off of the reservoir and not allowing pressure to build up results in the car idling without temperature problems, capping it and allowing pressure to build results in overheating. The catyletic(sp?) converter is getting excessively hot as well and may play into the problem.

While driving with the cap on the pressure builds too high and the relief valve shoots out water on acceleration. No water in the oil. Petroleum fumes coming from the reservoir. Check Engine light comes on upon re-cranking the car after it overheating then goes away after a few minutes. Have not hooked it up to a computer as this problem does not seem to me like one that can be diagnosed with a simple sensor malfunction.

Any help is appreciated, i'm a gearhead just more towards electronics.

hello
is this engine a vm diesel? it sounds live you have a head gasket or cracked cylinder head. the reasion the engine is fine when the rad cap is off is due to the combustion gasses can escape when the cap is on the gasses cause what is best described as an air lock. snap on sell a great cylinder tester which is a blue liquid and glass vessel which is placed on the rad cap hole. this will be a cheep way to rule out or confirm gasket fault. if the engine is a vm diesel you could remove the water rail from the cylinder head and the aux drive belt must be removed important! jack the rear of the car so the cylinber head is level fill the engine with water untill the water is just covering the holes exposed in the water jacket then start the engine and look for air bubbles