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l28ET Thermostat wont open!


SDgoods

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L28ET in 73 240z.   Aluminum radiator, aftermarket overflow. 

 

So ever since ive had this car, intermittently the thermostat wouldn't open and i'd have to let it cool down, (after it got hot),and eventually it would open and everything would be normal.

 

Now EVERYTIME I drive it, the thermostat upon initial warm up will not open, pressure builds in the system, top radiator hose gets rock hard and radiator is cold.  I can see it coming on as the temp gauge will hit operating temperature, then start to actually get colder.  

 

I replaced the thermostat yesterday and no change.  I thought I read on here a couple weeks back that there's another valve in the system that bypasses water away from the thermostat?  althought I cant find the thread.

 

Not sure what to do at this point! 

 

Thanks in advance!

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The top radiator hose is on the down-stream side of the thermostat, where pressure would be lower if the T-stat was closed and the pump was pumping.  Your top hose would not be overly pressurized.  With no flow I think that the temp gauge would still get hotter if the engine was over-heating, which it should do if the T-stat doesn't open.  There's a small whole in the T-stat that allows coolant to pass through.  There's something off in your theory of what's happening.  Might not have anything to do with the thermostat, that's why changing it had no effect.

Edited by NewZed
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The top radiator hose is on the down-stream side of the thermostat, where pressure would be lower if the T-stat was closed and the pump was pumping.  Your top hose would not be overly pressurized.  With no flow I think that the temp gauge would still get hotter if the engine was over-heating, which it should do if the T-stat doesn't open.  There's a small whole in the T-stat that allows coolant to pass through.  There's something off in your theory of what's happening.  Might not have anything to do with the thermostat, that's why changing it had no effect.

 

It might not be the thermostats fault, but it certainly is failing to open.    How else would the radiator and upper hose be dead cold?    Now that im thinking about the positiion of the water pump near the radiator inlet, Are you saying  the water pump is pushing the coolant up through the radiator always, and once the thermostat opens it then continues flowing through to the top of the engine?  I always thought about it reverse of that.

 

Some guys on a facebook thread I started are talking about pump cavitation from me bypassing the  rear heater hose fitting  back to the front near the lower rad hose.

Edited by SDgoods
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The pump pulls from the bottom hose and pushes the water through the block, up in to the head, through the thermostat and in to the top hose.  There's a diagram, with arrows, in the FSM (of course), in the appropriately named chapter.

 

Thats what I originally thought, thanks.  So it seems as thought because its not opening, the pressure is building up on the other side of the system IE the lower hose and pushing up against the back of the thermostat, or something along those lines.

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Some guys on a facebook thread I started are talking about pump cavitation from me bypassing the  rear heater hose fitting  back to the front near the lower rad hose.

Uh, don't do that. Plug the holes and watch your problems go away.

 

I noted when I run the car with my heater on full hot it does similar things. That is not a good way to "eliminate" the heater. You just open a 15mm shunt for hot water from the back of the engine to bypass directly to the inlet.

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Uh, don't do that. Plug the holes and watch your problems go away.

 

I noted when I run the car with my heater on full hot it does similar things. That is not a good way to "eliminate" the heater. You just open a 15mm shunt for hot water from the back of the engine to bypass directly to the inlet.

 

Don't by pass them or don't plug the fittings?  Theyre bypassed right now.   

 

Even with no thermostat, major pressure has been building up in the system, to the point that it explodes out when I take off the cap.  The Water was SUPER frothy and brownish, although not sure if its oil or not.    I think im going to replace the head gasket and associated parts either way, but im going to flush out the radiator and block first. 

 

What do ya think?

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Read the cooling thread. Don't loop the hoses, PLUG THE HOLES!

 

Facebook...ugh... Do they even know cavitation before thermostat....screw it I'm not writing it all over again. Good luck. Read/search. This is covered.

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