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Signs water pump failing on 620?

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You have a clutch fan which can loose it's silicone fluid. The fluid provides a friction that turns the blades. Like an electric fan blowing on and another fan that is turned off. One spins the other.


If you can easily spin the fan blades then the fluid has leaked out. The engine will not spin the fan properly and it will tend to over heat.


If the fan can't be turned then the bearing in the clutch unit has seized. It won't over heat but will sound like an F-18 taking off.


The fan should be quite firm to turn, but not spin freely.



The pump doesn't go bad and will always push water, but the bearing or seal sure can fail. Loosen the belt and grip the plastic blades. If it wobbles side to side (not turning) or up and down the bearing is toast. It may make noise too. Remove the belt, noise gone? could be the pump or the alternator bearing. If the pump, get it replaced. If the bearing is bad enough the impeller will grind a hole in the back of the soft timing cover cavity and water will get into the timing chain cavity and into the oil.

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Find the rad over flow tube under the rad cap and run it into a plastic water bottle. See how much comes out, if any. Constant over flowing is probably compression entering the cooling system from a bad head gasket. The pressure exceeds the rad cap rating and it burps out. First try a new cap. You might even get the cap from a newer 620 that has a coolant recovery system. This rad cap allows fluid out into a container, but when the engine down, it siphons back in keeping the rad topped up. This is not a fix for a bad head gasket but notmally you only need to add coolant top up once a year.


Over heats because the rad is low, or runs too hot all the time? Running too hot all the time could be the thermostat. If any doubt, it isn't worth the bother to test them. It has to come out anyway so just replace it and the gasket. Spend at least $10 on a good one not the WallMart $2.99 one. Stock is 180F


No heat from the heater just means the heater valve is closed. Maybe the cable is loose or off the halve. Check that the heater hose from the head goes into the cab to the heater. If it has been looped down to the lower rad hose inlet to the water pump to by-pass a leaking heater, pinch this hose closed or cut and put large bolts in the hose ends and gear clamp in place.



Assuming it's not the above listed problems... over heating is either poor water flow or poor air flow through the rad.


Poor water flow...

Rev engine while watching the lower rad hose. Rad hoses rot from the inside and soften. If too soft the water pump suction will collapse it and choke flow.

Take rad cap off and look at the cooling tubes. Are they crusty with deposits? This is hard water scale from dissolved minerals. Take rad out and lay face down. Fill with CLR over night to dissolve deposits. Always use distilled water.

Fan belt tight? Does it squeak? Tighten or replace

Look on underside of water pump just behind the pulley. There is a weeping hole... is it wet? Seal is gone replace pump.


Poor air flow...

Shroud missing around fan. Shrouds increase the fans efficiency to move air.

Rad fins are bent or blocked with bugs, leaves or debris.

Grill blocked by fog lights or 'bush bar'.

Rad support has openings allowing air through around the rad. Seal these up so air has to go through rad.



Have cooling system pressure checked. System has to be able to pressurize. If not, there is a leak and it may now be obvious.

Try snugging up all hose clamps. Follow all hoses and metal lines, there is even one coming from the intake below the carb and the heater hoses inside the cab.

Always run anti freeze coolant mix. (after the problem is fixed)

Paint the rad flat black. Flat black radiates heat better than gloss paint.

Tune the engine up. Retarded ignition or lean mixtures will run hotter.

Remove all spark plugs. If one is much cleaner than the others this cylinder may be getting coolant into it.

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my 74 620 over heats on warmer days and my coolant is disappearing? no smoke from tail and no milk in the oil but i notices the heater isnt blowing hot. water pump?

Check the hose from motor to heater core, water will evaporate as it shoots out hitting hot surface, saw this couple weeks ago on a 521.

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the heater not blowing hot is the first sign of either being low on coolant or a plugged heater core or heater controles not hooked up or broken cable 

datsunmike nailed every aspect of possible problems other then a possible blocked passage somewhere in the block or head wich is 99 percent unlikley

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One other possible cause of coolant loss is a head gasket that's starting to fail or even a cracked block. Not all head gasket failures and cracked blocks show massive signs of coolant loss, but when you shut the engine off, and it starts to cool down, the contracting metals could allow enough room to have some coolant leak by the gasket and drop right into the cylinders. If it's just a minor leak, you might not even notice the water vapor getting pushed out of the tailpipe, because once the engine warms up, the metals expand and close up the gaps.


I'm not saying that it's common, but it does happen.


Late model GM V8's (early Vortec) are notorious for failing head gaskets and this is pretty common with them. I've owned a bunch of them and when coolant starts disappearing, I know it's a sign that the head gaskets are failing, and I put the truck up for sale

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There are 2 tools to help diagnose your loss of coolant.  

1.  Pressure tester that mounts on the radiator, in place of the cap.  You pump up the cooling system with essentially a small tire pump.  Adding 15 psi pressure will SHOW you where the coolant is going.  It'll either be spraying everywhere or running out your exhaust if you have a bad ahead gasket.  


2.  Block Tester is the name for a tool that tests your radiator for carbon monoxide.  A clear cylinder sits on the top of the radiator with the cap removed, and the presence of CO coming from the radiator will turn fluid in that tool from yellow to blue.  That's a dead give-away for a bad head gasket, despite a potential lack of any other signs.  Keep in mind that if you use this tool long enough, CO in the air will turn the fluid to blue slowly.  If you have a bad gasket, you'll know in 10 seconds.  Only test a warm engine.  


Napa stores have both of these tools.  You can likely borrow them.  


Whether you are overheating from loss of coolant or if the coolant loss is a result of overheating will also need to be determined.  That helps define your issue.  

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Sometimes its not the head gasket. Friend of mine had an 80s Corsica and he could not find out why his coolant was slowly getting low. No leaks, compression check okay, no white smoke, finally took it to his me mechanic friend and pointed it out that engine had a crack.

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