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Troubleshooting Cooling Issues

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I have an '89 4.0L Auto w/AC, original "closed" cooling system


What I have done so far:

replaced water pump, lower radiator hose, lower hose from firewall to factory "t" connector, thermostat.


Aux fan does not come on at 190 like it should, but does operate with A/C on, so while that is a problem, its not causing the main problem.


System maintains pressure fairly well. only loses 1 or 2 lbs after a minute or so. Not sure if that is enough to worry about.


The only thing I can think of to try is replace fan clutch or the reservoir bottle. (since the system seems to maintain pressure this is the less likely source i think)


I have bled the system by removing the coolant temp sensor on the top of the engine with the front end angled down, and also again by removing the upper hose from the t-stat housing. So shouldn't be any air pocket problems.


If anyone can offer any other suggestions, or common problems they've had, that would be greatly appreciated!

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According to the FSM the aux fan is supposed to cut on at 190. It will get as hot as I let it. I usuallý turn it off amd let it cool down once it redlines. And it does it at all speeds, but faster when its sitting still.so yes its overheating.

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What year FSM do you have? Odd that the fan should come on before it gets to its operating temp. :hmm:


Do you have access to an inferred temp gun? They are handy for tracking down where the hot-spots are. What is the condition of your radiator?


Does it act any different now than it did before you replaced anything?

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you can tell if you need a new fan clutch by how its spinning. and they don't usually cause over heating in all situations.


honestly, i've seen way too many times where a t stat is bad, and you get a bad one from the parts store. personally, i'd try another t stat.


and i've found when filling that closed system, fill it to the top to bleed it out, let it run, keep filling, then you know there's no air, it will make a mess cuz it all can't stay in. but it works.

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First of all.......you have a Pressure tank, not a "reservoir bottle"


This is apt to leak thru the plastic cap, and easy to replace.


I'll second the over the counter thermostat being bad.


You also have a 20 year old radiator, and for the less than $100 for a replacement, I would go that route :roll:


If you have the '89 FSM, check page 7-30 for the fan clutch test.


And you are correct, with AC, the auxiliary cooling fan kicks on at 190*.


You are filling the cooling system by the book, but the way I've filled them, is thru the upper radiator hose, taken off the thermostat housing, fill the radiator / block, and when the anti freeze comes out the thermostat housing, the block is burped. Then attach the upper hose, fill the pressure tank 1/2 full, run till hot, shut it off and let it cool, and it will suck any additional fluid out of the pressure tank. Do this 2-3 times, and the system if full. Also note that the cooling system takes about 3 gallons to fill.

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Sorry for the delay in responding! I start a thread and then abandon it for 2 days... :shake:


No I haven't got an infrared thermometer to test for hot spots that way. Guess its time to invest in one of those. That will def help me track down stopped up passages in the radiator.


I'll pull the t-stat too and make sure its operating properly.


And yes I know its a pressure tank, sorry about improper term there :doh: It doesn't leak, the system maintains pressure fairly well. I may see if I can pick up a new cap though. Local parts stores don't carry the whole tank, but I may be able to get the cap in the section with all the "help!" products.


I was told if the fan clutch was bad, the fan would spin pretty freely. It doesn't, but I also don't notice the fan speed varying any at different temps.


I'm going to replace all parts of the cooling system eventually, just trying to nail down the biggest problem first so I can get it back on the road. (After I get the interior back in it anyway)

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Replaced coolant recovery tank cap, thermostat (double checked that I put it in right side up), coolant temp switch, fan clutch, bled system and still overheating. Only link left in the chain is the radiator, I think. Looks like I can pick up one off ebay for about 60-70 shipped. So I'll try that route next. And my aux fan relay will be in this evening so hopefully the fan will kick on properly now.

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