06-18-10, 09:32 PM
so car has full coolant level. waterpump is working. radiator is not clogged. no line blockages. so why does leaky headgasket cause overheating? how does combustion gases entering the coolant system cause the temp gauge to rise?
A leaky HG causes the temps to rise because it pumps air (exhaust gases) into the cooling system. Air does not transfer heat as well as coolant and when the air reaches the ECT sensor it heats up quickly and the temp climbs. Once coolant sloshes back through and covers the sensor, the temp comes back down.
06-19-10, 11:31 AM
combustion gas in sealed coolant system means coolant has to be pushed out? gas is much higher pressure than coolant system. so engine overheats or at least gauge says it is hot. sensor is in coolant crossover casting. seems many stories of overheating involves constantly adding coolant. some of that is due to leaks such as cracked radiator, poor sealing radiator cap and leaky hoses. is the high system pressure pushing coolant out where it can?
The exhaust gases get routed to the surge tank via the purge line and vented when the system gets above 16-18 psi via the surge tank pressure relief valve in the cap.
Combustion gases directly cause overheating by overheating the coolant, and indirectly cause overheating by overpressurizing the cooling system, thereby forcing coolant out, which makes matters even worse. As Ranger said, the gases do get routed to the surge tank and are vented, but they can't get vented fast enough. With the first HG failure I experienced, I tried to keep driving the car around town for short trips, with the cap on, as I diagnosed the problem. Well, the pressure built up enough to crack one of the radiator side tanks, so I had to buy a new radiator before I could continue my journey, ending up in a HG failure diagnosis.
The second time I had a N* HG fail, I knew enough to leave the cap on loose - just one click - for short trips. The exhaust gas smell coming from the surge tank was very obvious - no need for a block test for me - I knew what I had, and thanks to Cad Forums, I knew what I had to do - drop that cradle and insert the block.
06-19-10, 04:55 PM
my radiator hose blew off about 6 months before the gasket really let go and started putting a lot of coolant into the cylinders. i found a caddy to fix/buy but it is 30 miles from my house. don't feel like a $125 tow to start the job. since i have not driven it yet, i don't know how fast it overheats or at what speed? maybe i could drive real slow on side roads?
Sure - just leave the cap on, but loose, and stop along the way to let it cool off and refill the water it is blowing out of the surge tank. You won't do the engine any more harm - limp mode will take car of that, but personally, I would not want to drive in limp mode any further than necessary, so just stop a few times to get it home.