The pressure in the cooling system of my 78 280z is very low (judging on the upper and lower radiator hoses) but, the car doesn't overheat i Have run the car for over an hour an the temperature doesn't even go much past the halfway point on the gauge. I just replaced the radiator, water pump, thermostat, and upper and lower radiator hoses. I have checked the belt and it is at the FSM specified tension and flushed the cooling system. Dose anyone have any ideas as to what could be the cause of this? Is the presure suposed to be that low?

Well, the pressure doesn't go higher than what your rad cap is rated to, which isn't too high

I wouldn't worry about the pressure as long as it's not over heating. and just around half way on the stock gauge is operating temp.

I don't know about that. The hoses on both of my cars would be 'tight' if you squeezed them after running the car for an hour. I would check all of the coolant hoses including the three small ones on the intake side and the one that t's into the lower water neck as well as the heater hoses including the ones under the dash, and the heater valve itself, and make sure that you have no seepage in those areas.

You didn't say if you replaced the cap. That might give you low pressure but I think it would want to over heat if it were a bad cap. Sometimes even a new cap could have a bad seal. New doesn't always mean that it will work.

I assumed that he changed the cap considering that he changed the radiator, etc. The EFI cars have those small coolant hoses I mentioned that are often overlooked and are a PITA to change.

i managed to to talk to an old nissan tech from the late 70s and early 80s he said to pull my thermostat again and boil it again see if its stuck shut from the flush or from goop.i checked all my hoses and there not leaking i even replaced some of them. thanks or the help.

Well, you can try that if you want but stuck closed, the car would overheat, stuck open it would be slow reaching operating temperature but it wouldn't be the cause of what you described. On another note, what thermostat are you using? OEM or aftermarket. I only ask because the OEM Nissan thermostat is better due the the larger opening.

i also changed the radiator cap

What about the coolant reservoir?

i am using an after market 180 degree thermostat, what about the coolant reservoir.

Cooling systems are closed systems, and pressurized only to raise the boiling point of the coolant. If the coolant isn't at it's boiling point - you won't have any pressure build up.

How warm in New Jersey is it now? Are we talking 70 degree days or 85 degree days?

The first thing to do - is pressure test the cooling system. If it holds 14 to 16 psi and the car isn't overheating - - I'd doubt you have a problem.

If you don't have a cooling system pressure tester - find a shop that does and ask them to check it.

FWIW,

Carl B.

Cooling systems are closed systems, and pressurized only to raise the boiling point of the coolant. If the coolant isn't at it's boiling point - you won't have any pressure build up.

How warm in New Jersey is it now? Are we talking 70 degree days or 85 degree days?

The first thing to do - is pressure test the cooling system. If it holds 14 to 16 psi and the car isn't overheating - - I'd doubt you have a problem.

If you don't have a cooling system pressure tester - find a shop that does and ask them to check it.

FWIW,

Carl B.

Everything the OP needs to know is in that post. Have the test done...and if it holds pressure, I agree that you have no problem.