Blown HG or Stuck Thermostat?
 

Well, me and my tuner did a little more tuning and had a problem. Let me shed some light on my setup before I explain. I am running 28 psi dropping to 25 psi after 7500 rpm on the 3mm HG from IP and ARP headstuds on 110 octane fuel. It's putting down nearly 600 whp and I've been this way for almost 2 years now without a problem. We decided to up the Microbit on the AEM to get a little more fuel at the top and so we had to retune it on the street. We almost had it done and made a pull and hit 12.4 AF at around 5500 rpm and timing was around 7 degrees for a brief second and the rest of the pull was in the 11's AF as the timing slowly climbed to 17.5 degrees at the top, around 8800 to 9000 rpm. The highest knock volts we saw throughout the run was around 1.9 or 2.0 which we believe is very good and conservative. Well, low and behold, we noticed that my engine temps started to climb after the last run and kept climbing up to about 239-240 degrees F and we shut the car down and checked things out. The car was still running just fine except getting hot and loosing coolant out of the overflow. What we can't figure out is if the thermostat is just stuck shut or the HG is very slightly blown. The lower radiator hose stays completely cold while the upper hose gets hot and feels pressurized. We decided to let the car set and cool down for a while and then started it up. Temps came up to normal and the fans came on but the lower hose stayed cold with the top hose getting hot again and pressurized. We checked the exhaust for coolant smell and the oil filler cap for creamy residue but nothing was showing a sign of a blown HG. We then took it out for a run to see what would happen and boosted it up to 25 psi to see if a cylinder would push more coolant and overflow the tank but it didn't and the engine just got hot again with the lower radiator hose stone cold again. We can't determine if a blown HG would cause this or a stuck thermostat. Does anyone have any experience with something like this? Mind you, I do leave the car set during the winter for up to 4 months at a time without starting it so maybe it prematurally damaged the thermostat doing that. It's a 2002 model with only around 14k miles on it.

Any help is greatly appreciated to determine the problem before I pull the head. I'm hoping it's just the thermostat messed up but before I start working on it I'd like to get some insite on the possible problems. Could it be a blown HG or a bad thermostat?

Sounds like the thermostat to me. They're so cheap and easy to change, I would have swapped it out before even thinking about the head gasket.

That's what I'm thinking too but I'm not too sure which direction the coolant flows from the pump. Could it just be that the HG is blown but there is a huge air bubble in the system that is keeping the coolant from running through the whole system and to the lower hose? It makes no sense because you would think that if we boosted on it again, it would push more coolant out of the overflow but it didn't.

Air bubbles tend to work their way up to the upper radiator hose pretty quickly. You would have to have almost no coolant in the car at all for the lower hose to be empty. If your upper radiator hose is hot, and the lower is stone cold, then you have coolant not flowing. A head gasket can't cause that. (unless you somehow manage to put a headgasket on that blocks the coolant passages, obviously) Change the thermostat.

Good deal and that makes sense. We will change the thermostat and see what happens.

Thanks

BTW, it really didn't lose that much coolant either. If you look in the radiator, it is still a little above the fins.

Is the thermostat on the upper or lower hose and is it pretty easy to change?

Coolant tester.
Sounds like the thermostat to me, but this is a easy way to test for cylinder leaks.

i would deffinately say the thermostat isnt opening up. Let just hope you didnt pop the headgasket when it was running hot and shot coolant out of the overflow. Does the overflow work right at the moment? for instance when the car cools does it draw the coolant back into the radiator? when i blew my headgasket that wasnt working properly.

Definitely change that thermostat and bleed the system to get all the air out and then check back in with us.

It only got up to 240 degrees F so I don't think that is hot enough to hurt the HG but who knows.

I haven't refilled the Overflow tank so I'm not too sure if it is working like you say.

I'll let you guys know how it goes.

Thanks for that info on the leakdown tester, I may try that too if the thermostat doesn't fix it.

yeah 240 isnt that high but I think mine blew because it overheated and i know it didnt get much hotter then 240ish. Then again thats what i THINK blew it but im still not certain. My symptoms were air leaking from combustion chamber into the cylinders causing air pockets and cold heat. If i took the rad cap off even when cold it would burst out. I really think and hope its just the t-stat in your case though.

I left mine set for 4 months this past winter and before I started it, I thought I'd check the coolant and it had pressure behind the cap but no fluid came out. I just figured it was just very well sealed and had absolutely no leaks. Could this be what you are referring to?