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L18 fluctuating temp


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Just did another engine swap from the l20 I bought that burnt though all it's oil in 100 miles(people didn't like driving behind me haha) to an l18 with dual su carbs, drove it home from granit falls to lynnwood and the temp gauge( after market and stock) would fluctuate from normal running temp around 185 and just drop to 140, then it would creep up to around 210 then drop back to normal and then repeat this cycle, never seen my temp go from normal all the way down to that low of a temp then get real hot and then go normal again, any suggestions, oh and the thermostat is new, but I know oven new ones fail

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like smoke said

also try running it w/o the cap on there for a while so it bleeds out.


I myself alwasy drill a 1/8 hole as a bypass.


But really if my car was doing this I know soemthing is wrong. Since you put a new stat in there it must have been doing it before. thats why you changed it.


ps also the ck the bottom hose to make sure its not soft and collapseing when you rev up the motor.




Big Datsun crew up north maybe somebody will help you

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Put the new stat in because motor didn't have one in it when I bought it, bottom hose has a spring in it so it's not collapsing, gonna see if bleeding it will help and maybe a better thermostat as well since it could be bad even though it's new, wouldn't be the first time I got a bad new one

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Sounds like your head gasket is starting to go. My truck did the same thing a few weeks before the gasket finally went. A easy way to check is to start the engine cold with cap on, and see how long it takes for the cooling system to build pressure. If pressure starts building before coming to operating temp, combustion gasses are most likely getting into the cooling system and causing the air pocket at the thermostat.

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Well, the best would be a Japanese-made OEM stat, but failing that, a Stant is OK if you have to go aftermarket.  At least I haven't had issues with them.  I don't know many brands.  Just don't get a "fail-safe" one because common opinion (and anecdotal evidence) suggest they don't work as advertised and usually fail somewhere between 2 and 365 days after installation.

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Factory ones are around $15 so to get reasonably close quality after market expect to spend at least half that and more is better. This isn't a god damn cigarette lighter where you could use a match if it fucks up... this is the thermostat that regulates the engine temp and can leave you stranded with a blown head gasket. Spend the money on quality.

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The amount of money for a product unfortunately does not coincide wit the quality if there is a gimmick like K&N filters (oil and air), E3 sparkplugs, AMS oil you get the picture I have recently begun buying nearly all of my parts for my and my wifes daily driver at the dealer and have not looked back (not saying no aftermarket parts are good but I would just rather know for sure I am getting good stuff) I just did waterpump thermostat and hoses on my 06 325I all OEM costed about 600 bucks (it is an electric water pump) and uses pentafrost coolant $$$ but I know if I drive up to Michigan to see family I wont have any problems.

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The amount of money for a product unfortunately does not coincide wit the quality ....


Absolutely, I agree, but if nothing else to go on, the cheap $2.99 should be avoided. NAPA and most auto parts houses have Stant and others that are 'brand names'. I was making the observation that the original part is about twice what good quality parts are worth, roughly. You can't go wrong with the Nissan ones but I wouldn't go less than half elsewhere.

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I am also going to strongly suggest you get a Nissan thermostat.   Remember, an aftermarket auto parts compamy is just trying to profit off a part sale.  A car company wants to protect its reputation, and is less likely to risk engine damage, or having a dealer do a warrenty claim, to save a few cents on a part.

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Is it possible that the impeller has become loose on the shaft in the water pump



No it would leak water for sure. water pumps only really go bad when the bearing fails and looses the seal.

I have seen impellers rotted almost off inside but that due to not running anti freeze. or changin it out.



this is simple if on a cool day the motor still gets hot with no stat then the head gasket is going to go or a really plugged radiator.


try another stat but make sure its bleeded. on one can try another cap also. but its a reaching guess!

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I just thought it might be worth looking into after the t-stat is replaced with a quality one if it is still happening. Keep in mind he said it is going up and down, it is clearly a intermittent circulation problem that would be caused by t-stat, collapsing hoses (he verified this is not happening) or water pump. If the temp drops way below operating temp it has to be the thermostat if it is fluctuating above operating temp with driving it could still be partially blocked rad. An inferred temp gun going across the fins will locate cold spots (pugged tubes) quickly. 

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