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Suggestions For 620 Cooling System Mods


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I run a 79 620 with an L20B that has a stock cooling system and a mostly stock motor. It has always cooled the same from the 1st day I got her 5 years ago. Great in traffic for any period of time, can sit idling forever, but can't cruise above 70 for any extended period of time (20 minutes or more) with or without AC on before the temperature gauge starts to rise. The truck runs excellent. It will run at a stable temperature at 60-65 mph all day long with or without the AC. The timing is 12 BTDC per the manual. 50/50 fresh anti-freeze coolant mix. Has the original factory radiator/shroud/cooling fan. The original radiator is still good, but nevertheless I tried a chinese 3-row Aluminum which made only a very slight difference on the highway but didn't do as well as the OEM style in town driving so I switched back to the OEM style rad with shroud. I'm running a 160 t'stat because I'm in South Florida. I'm also running in 5th with an 82 280ZX 5-speed on the highway at about 3500 rpm. Anyone have some suggestions?

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What brand of T-Stat ? I have found over the years that a GENUINE GOOD BRAND thermostat is well worth the money ,,, all of these crappy little $3-$5 T-stat's that are no-name have either failed on me or fluctuate ,,,


I really like using the Dealership T-stat's or NAPA ones ,,, Others may be good ,,,


(1) Does it over-heat ? or just run a bit warmer ?

(two) How far does it normally run versus the difference you notice on the temp gauge above 70 ?

(3) Do you have ANY leaks or blockages in your system ?

(4) Did you back-flush out your Radiator or Block lately ?

(5) Consider the temp sender being bad ?

(6) Running leaner ? ( have you tuned your carb lately ? )


NOTE : Is there a blockage anywhere in the system ? including having radiator checked ?




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First thing is throw the thermostat away and buy a good $10 one and gasket. They are not worth the time to test and are cheap. It's very possible the thermostat isn't opening fully which is fine at idle and low speed but restricts flow at higher speeds and temperatures. If this does not help no harm done and it only cost you $10 for a new one.


Feel the bottom rad hose... is it soft? If so when revved up the suction from the water pump can cause the hose to collapse and restrict flow. Try revving the engine and see if it collapses. If so, get a new hose or put a coil of spring inside to prevent it squashing.


Very unlikely the shroud or fan is the cause as anything above 25mph there is enough air pushed through the rad just from vehicle speed. This would leave blockage of the rad tubes or poor circulation


Mitchell, is Dog your co-pilot?

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Hello, Thanks to All of you for your detailed replies!

I'm certain the vehicle is actually getting hot as the performance starts to drop, you can tell the motor is working harder. So I'll ease up on the speed for about 10 minutes and she'll start cooling down.

The plugs look great and have a nice brownish-tan color consistantly. I recently tuned her up and adjusted the valves.

The 160 Tropical thermostat was purchased from carpartswholesale as an expensive Beck Arnley brand... supposed to be good, i think? Prior to the BA 160 I had a Stant 180 in it and it didn't run much different, the only difference is in traffic the 160 BA stat runs cooler, but the 180 was fine in traffic also just ran a little warmer.

The lower hose is squeezable and is the original hose on the vehicle when I purchased it. Makes sense, so I'll replace it. Would someone have a brand recommendation, are they still available from Nissan? What about the generic accordion type hoses?

At this point, I have 3 radiators, the original, an OEM style replacement and a chinese 3-row aluminum. They've all produced similar results, except the chinese 3-row didn't do as well in traffic partly probably due to not being able to use the oem shroud.

I've never done a block flush. Is this something I can do?


The Dog goes everywhere with me and his name is Datsun!

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Something is failing.(duh)And with an aluminum radiator you should be able to "refridgerate" you motor this time of year even in Flori-duh.Test the T-stat as i had 6 bad in a row including name brands.Right now i have a factory 3-row that i have to block half of it off to get up heat if it's below 50 here.Other than that it sits right on the 180 mark.Aluminum extracts/rejects heat better than brass by 30%,so the behavior of your motor(heat-wise)indicates you have an issue that is putting abunch of heat into the motor.



Are you getting full timing advance?

Have you tested for exhaust in the coolant?

Your sympton of hotter in city indicates an airflow issue.Fan clutch good?

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