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Overheating at highway speeds


DatsunKaua'i

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So I got my 620 running and it purrrs!! Only problem is that at highway speeds (over 40) it starts to get hot and if I stay at 40 or above it goes into the “red zone”. At idle it does not over heat at all. I have an electric fan as well as stock fan, both work. Also new thermostat and fresh coolant.. maybe tome for a new radiator?? Anyone got one for sale!!?? Thx 

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More or less in descending order from cost nothing to check, to replacing something....

 

 

Give the engine a good rev while looking at the lower rad hose. Water pump suction can collapse a soft lower rad hose. Replace hose or add a coiled wire spring inside to hold it's shape.

 

Fan belt properly tensioned?

 

Do you have a fan shroud on? It helps the stock fan's efficiency.

 

At night, hold a light source on the engine side and shine through the cooling fins. Are they plugged with bugs, dirt, leaves?

 

Is the electric fan pushing air or pulling against the stock fan?? Obvious but worth checking.

 

Can you spin the clutch fan easily with your finger? It should only move with firm finger pressure on it and should not spin freely when given a fast push. If the silicon oil has leaked out the fan will not spin fast enough. 

 

 Can air 'leak' around the rad or above the rad support just under the hood when closed? Any obvious openings will let air through rather than having to go through the rad.

 

The '78's static timing should be 12 degrees BTDC. Rev the engine and the timing should advance up to 20? ish degrees.  Pull the distributor's cap off, and vacuum line off the carb's port. Suck on the hose and observe that the rotor turns clockwise about an inch and holds till you release the suction. (it's assumed the vacuum advance is working)

 

All engines run slightly warmer when working and may rise slightly on the highway. Your gauge may be faulty and nothing is wrong. To check, take an infra red (IR) 'gun' with you on a ride. Pull over and 'shoot' the thermostat housing. It should be withing 5 to 8 degrees of the thermostat rating.

 

Lower the rad coolant so that the tops of the vertical rad tubes are exposed. Are they clean and open, or crusty with hard water scale deposits? I removed my rad and set it front down with cap on and filled with CLR and let sit for a day. I fizzed and removed all the scale. Always use distilled water to top up. To reduce top ups get a coolant recovery reservoir from a newer vehicle and the special rad cap for this system. This will reduce top ups to maybe once a year. Highly recommend it if your truck is an older model.

 

 Replace the thermostat with an (at least) $10 unit rated for 180F. New is no guarantee that it works correctly. Yes you can test the one you have but it has to come out anyway and you need a new gasket, so for all the time and bother in the kitchen with  the stove, pot of water and a thermometer,  just slip a new one in and done. If it doesn't fix the problem it can be assumed that the old one is OK and save it as a spare for this one.

 

 

 

 

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The first thing I always do when checking "overheating" problems is to verify the gauge reading. Since the OEM gauge is electric, it relies on solid grounds and power delivery to read accurately.

 

Use a meat thermometer in the radiator fill neck to see how hot it gets while just idling in the driveway. Check that against the gauge. You can also ditch the electric gauge in favor of a true mechanical gauge, at least until you get the problem solved.

 

Is the radiator stock?

 

Is the electric fan moving the air the right direction?

 

This is me checking oil pressure against a stock Toyota gauge.

 

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Edited by Stoffregen Motorsports
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The stock gauge has a 'run range' and no numbers. If given gauges with numbers the vast majority of owners would have no idea what the changing numbers might mean and panic, so to sooth them they are told by the factory... "there, there...if needle is in this range, you're ok to drive it". Same with oil pressure gauges. It can drop 10-15 PSI when hotter, than on cold start up. Then there's the 'alarming' low oil pressure at idle. No wonder a low oil pressure light is set for about 8 PSI.

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I like to see the needle about in the middle of the 'run range' maybe up to 60%. As it isn't numbered I have no idea what the actual temperature is but this is where it runs when running best with a 180F thermostat. At about 70% the electric fans come on. Only way to know is shoot with an IR gun. Perhaps you gauge is faulty.

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Found a pin hole in the heater line going into the fire wall. Also my hoses seem limp(not stiff) haha, since the pinhole fix temp hasn’t gone up as high as it had been but I haven’t driven at 3000 rpm for more than 20 mins, and it still get up to about 70%.

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1 hour ago, DatsunKaua'i said:

Found a pin hole in the heater line going into the fire wall. Also my hoses seem limp(not stiff) haha, since the pinhole fix temp hasn’t gone up as high as it had been but I haven’t driven at 3000 rpm for more than 20 mins, and it still get up to about 70%.

 

Borrow an IR gun to be sure your gauge is telling the truth and you are running over hot.

 

REV IT UP WHILE LOOKING AT THE LOWER HOSE!!! Does it collapse??? Simple yes or no. If it does, it could be a soft hose or over suction from a part blocked rad. The pump is straining to pull water through it.

 

Drain a gallon of coolant out and LOOK!!! Are the tops of the tubes inside blocked with scale???? Again yes or no. I already described how to clean the hard water scale out and prevent this from happening.

 

What about the clutch fan did you check this like I said? All these cost you nothing to do. It's hard to move forward until the obvious causes are eliminated. 

 

 

 

A pin hole won't cause over heating unless the water level lowers.

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  • 2 weeks later...

Clutch fan fine, rad hose seems limp but does not collapse. Drained rad definitely blocked with scale flushed entire system. Still overheating. Note* I did not use CLR stuff yet. new find also radiator is stock and the engine was replaced with a L20 it's a 73 so stock would have been L16 if I'm not mistaken. 

Edited by DatsunKaua'i
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