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HELP! Datsun 620 Cooling Problems. NEED ADVICE!!


wolfmandu

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So. I have live in Altadena, CA. I have a 1978 Datsun 620 pickup. 5-spd with the 2.0 engine. Initially the truck was getting very hot (gauge would crawl to the top and I would have to pull it over and let it cool) when I would get on the freeway and go over 45-50 miles an hour. In stop and go traffic it was fine at lower speeds.  I put in a new fan-clutch, thermostat and most recently I pulled the thermostat, flushed the engine out with a hose about 15 times and put in a new 3-row aluminum radiator. The truck had been sitting with the original owner since 1994 so I was hoping the radiator would end up being the problem. I have sunk every extra penny I have in to this truck and am totally in love with it. It will be with me the rest of my days. I don't know if the only problem could be with the head as I think I have trouble-shooted everything else I can think of that might have to do with it getting too hot. I have never done head work on a vehicle and am not even sure what to look for. If there are any Datsun lovers in my area I am hoping that someone might even be willing to take a look at it with me as I'm doing all the work myself. My 620 is a true piece of 70's art and turns head whenever she's out and about. Even got a matching Datsun pickup-bed trailer that I tow behind her and am planning on driving the duo up north with my son but I have to figure out this problem first. Any and all advice will be appreciated. I barely turn the key and she fires over, without hesitation every time. Also, when I first bought her I pulled the dipstick and after having sat for so many years, the oil was a beautiful honey color. I NEED HELP. Thanks in advance to all you Datsun Heads.

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Two main things cause overheating at cruise speed:

 

1.  lean fuel mixture which many times is caused by vacuum leaks, not carb adjustments.  Vac hoses, carb bolts, intake bolts, all can cause this.

 

2.  Low or retarded timing settings.  A bad vacuum unit on your distributor that doesn't hold vacuum can cause both issues.  Make sure timing advances to at least 34 degrees total at 4000 rpm with the vac unit disconnected.  Make sure the vac unit rotates the breaker plate when you suck on the line, and it should hold indefinitely when you put your tongue over the hole.  Worn points or a sloppy distributor will cause you to lose advance over 3000 rpms.  

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Get an IR (infra red) gun and shoot the thermostat housing when reading hot to confirm it is over heating, and it's not the gauge or sender.

 

 

At 40MPH forward motion is enough push air through to cool so not likely the fan but the belt may be loose and the pump slipping not pumping properly.

 

 

Rev it well up and look at the lower rad hose. Do you have a soft hose that collapses from water pump suction? Rad hoses deteriorate from the inside out and may look fine on the outside.

 

 

Are you constantly adding coolant? You may have a head gasket the is in the process of blowing. Compression will get into the cooling system and over pressurize it. The rad cap will burp out water. Air does not cool an engine.

 

Are your heater hoses joined together to by pass the heater??? Big no no. Put a plug in line to prevent coolant flow.

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Thank you very much for your replies. I should also say that yesterday, after installing the new radiator, I filled it up with water and started the engine. I watched the gauge go from cool to hot in under 4 minutes. Never even got that hot  before, just sitting there, started from cold and at low idle. I am thoroughly perplexed.It's almost as if, now, after installing new fan clutch/water pump, new thermostat and new radiator, it's running hotter than ever before. At least before it didn't get that hot until I took it on the freeway at a higher speed. Any and all advice welcome.

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Get an IR (infra red) gun and shoot the thermostat housing when reading hot to confirm it is over heating, and it's not the gauge or sender.

 

 

At 40MPH forward motion is enough push air through to cool so not likely the fan but the belt may be loose and the pump slipping not pumping properly.

 

 

Rev it well up and look at the lower rad hose. Do you have a soft hose that collapses from water pump suction? Rad hoses deteriorate from the inside out and may look fine on the outside.

 

 

Are you constantly adding coolant? You may have a head gasket the is in the process of blowing. Compression will get into the cooling system and over pressurize it. The rad cap will burp out water. Air does not cool an engine.

 

Are your heater hoses joined together to by pass the heater??? Big no no. Put a plug in line to prevent coolant flow.

 

 

Number one on my list. Confirm you have a problem.

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Try an infrared thermometer measure the temp at the thermostat housing and hoses . You're assuming the temp gauge is working correctly. I was getting ready to replace the voltage regulator when I checked both connections on the temp gauge and the fuel gauge and found they were corroded.

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