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Complete crap heat


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I have an 84 720, and the heat is total balls. your lucky to get any actual warmth from it.


The fan blows strong, so no worries there

The metal tubes in and out of the heater core, inside the cab, are warm but not hot by any means. 

The actual blend door is working properly, and i suspect the valve for incoming coolant into the heater core is working, because the in&out lines are about the same temp


Probably the most peculiar thing is that i can drive it all day long, down the highway on the hottest day in the summer and the radiator will have no pressure in it, i can take the cap right off, without any overflow, and stick my fingers in there, and the coolant is lukewarm. IT NEVER LOOSES COOLANT and has a fresh headgasket, but the heat sucked before.  I suspect this is related to my issue.


Cooling system is burped, changed fresh coolant. only thing i can think of is that the thermostat is stuck open? 


Any ideas or similar experiences?

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The heater valve in my 720 gets stuck also. If I accidentally move the temperature off of HOT, the cable doesn't have enough stiffness to push it back.. I generally have to wait until I'm stopped, lean over, and push the valve actuator fully open with my hand. After that, it'll cook me out of the cab with the fan on full blast.

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The heater valve in my 720 gets stuck also. If I accidentally move the temperature off of HOT, the cable doesn't have enough stiffness to push it back.. I generally have to wait until I'm stopped, lean over, and push the valve actuator fully open with my hand


this happen to one 510 i owned from then on I leave the HEAT ON and use the flap vent to control it.So the valve dont wear out

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Always change the thermostat if there is a heating or cooling problem. Spend over $10 though and get a good name brand. This will save you time an bull shit in the long run.


The heater valve is next. It can simply be manually opened for the winter.


Make sure the air mix gate is working and can be fully shut off to stop cold air mixing with the hot


Part drain the system or do this while changing the thermostat... Remove the two heater hose ends from the side of the block and the under side of the intake, make sure the heater valve is in the open position and use a garden hose to flush the core of sediment. Now flush the opposite direction. Do several times until only clear water comes out. Connect up, add some more antifreeze.

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Ok, thanks for all the replies. There is a thermostat, but I haven't replaced it so that'l be my first go. 


Justin, temp gauge is barely moved at all, like... barely above the lowest point on the gauge


The valve needs to be Pushed to be open, is it possible that it itself is broken? 


Mike, is there any procedure to burp the heater core?


again thanks for the replies, good to know I can go somewhere if I'm stumped!  :thumbup:

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No. It may not be completely free of air when refilled but as soon as started the coolant flow from the pump is enough to force any air through.



When I got my '74 710 it would barely read on the gauge.  It closed when cold but the seal was all warped. Probably the original 38 year old (then) thermostat. Changed it and it even ran better when the temp was right.



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"I have never tested a thermostst. Not worth the time or bother doing it. You have to drain and replace the gasket anyway, so slip a good new one in."


Yes, they are better, higher quality ones than you get at a chain auto parts store selling fart cannons and spoilers for Hondas

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I seen to remember about $13 for a dealer thermostat for my L20B or maybe NAPA. I can't imagine trying to save $5 at WallMart on a fram thermostat that has to keep you and the engine warm and not fail and boil over summer or winter leaving you stuck or worse a warped head in another city.

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I have had no problem getting parts that are still available for my 521 at two different Nissan dealers.  By no problem, I expect most items to have to be special ordered.  A car dealer would need a warehouse the size of a Costco store to keep everything available for 45 year old vehicles in stock.

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datzenmike is right on the money here.


Checking a tstat is a waste of time, pull it and drop a new brand name part in. I worked a service counter in a 12 bay independent repair shop for years. Sold hundreds of tstat jobs. I disagree with the notion that a dealer thermostat is inherently better than a quality aftermarket part. Definitely better than house brand, though. This is not to say no dealer parts are better, many are.

A part as simple as a tstat is likely to be built by an aftermarket part maker anyway and boxed as OEM for the manufacturer. For instance, 15yrs ago, we had a situation that proved Saturn brake pads from the dealer parts counter in a GM box were IDENTICAL to AC Delco pads for that car. GM bought AC Delco's pads to manufacture with. There is a long "pissed off customer being filled with BS by a dealer" story behind that one.

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