help with 240sx overheating problem

[nissanlifer]

I have a 1993 240sx with a serious overheating problem. When just driving normally around town the temperature gauge rises from normal operating temp to pegging the needle at 'H' in as little as 30 seconds! When I pull over to check it out, the radiator is boiling over! I put in a new thermostat, it still overheats. I put in a new water pump, no change. I put in a new radiator and cap, still no change. I took it to one of the most reputable shops in town, they kept it for a week, and then returned it to me saying they can't find any root cause. Finally, I take it to another shop in town, they do some tests, and find exhaust gases in the radiator. Their conclusion: blown head gasket. So I pull off the head. The gasket has no visible defects as far as I can tell. I took the head to a machine shop and they tell me its in excellent condition: no warpage, no cracks. However, and this may be the key symptom, when I remove the thermostat the car runs like a champ! But when I put the thermostat back in (i've tried 2 or 3 brand new ones), water flow appears restricted and it overheats again.

I'm stumped! Has anyone witnessed this behavior before?? Any help is appreciated. Thanks.

[240_Keyy]

that's wierd because it really does sound like a blown headgasket. I have seen BHG's that you actually couldnt tell where it was blown, but they were definitely messed up somewhere.

I had this similar problem with a couple of the swaps that I did, and it turned out to be that the coolant system just had some major bubbles in it, which allowed the heated water to flash boil and overheat, thus starting the cycle of quick overheating. Have you tried burping the system really well?

[pr240sx]

That my friend is air in the cooling systemBleed that sucker!

[nismojason]

I would agree with everyone else that it needs to be bleed, but then why would it work fine with thermostat out?

[nissanlifer]

So is the theory that an air bubble surrounds the thermostat and prevents it from opening??

The shop apparently pressure checked the system and checked for leaks. I've drained the system coolant and attempted to flush it out but I've never 'burped' or 'bled' the system. Recommended techniques?

[spider_slayer]

pretty easy...open the rad cap then loosen the bleeder screw till antifreeze starts to dribble out. tighten bleeder screw leave rad cap off and run the car ON A COLD DAY for a good 20-30 min with the heater on full blast. make sure the resovior tank is full. replace cap and top off tank again and go drive.

[240_Keyy]

spider_slayer speaks the truth

[nissanlifer]

Thanks to all.

I'll burp n bleed it and see what happens....

[tonypatthou]

Did burping the system really work.Let me know.

[tonypatthou]

After replacing the water pump, belts, and thermostat. I ran hot agin. Turned out the new thermostat was faulty.Replaced it and the car ran fine. I was very relieved.

[sdtouge]

obviously with no theermosatt the car wont over heat, it is always cold.

it is possible to have a blown hg. with out a warped head. you blocked could be messed up also...

[nissanlifer]

I pulled off the head. The head is in great condition. Head gasket has some visible damage though. I am going to replace it and see what happens. I'll post with the results.