Overheating, and possible bad temp sensors?

[ZukesMcGee]

What are the chances of both coolant temp sensors being bad. The one at the back of the block that controls the gauge, and the one at the front that goes to the ecu? These are sensors that came in a jdm motor I bought off of eBay. I've been chasing an overheating issue and am thinking the sensors may be bad. My coolant temp has read 240 degrees with no smoke and no boiling. Of course I know this is not a good temp so I'm trying whatever I can to figure it out. I replaced the thermostat, flushed the coolant, new radiator cap, headgaskets are good this has been verified. The reason I'm wondering if it could be the sensors is even though it's been quite hot, I've had no symptoms of an actual overheat.


I did just find the one of the coolant hoses coming from the heater core was collapsed. Could this be the issue? Thanks in advance to anyone who chimes in.

Edit: I forgot to mention it only overheats when driving close to freeway speeds. I can let it idle in my driveway with the ac on all day with no problem.

 

[JohnMc]

How are you getting that 240 degree temp number? How did you verify the headgaskets are good? Does the gauge read high when this is happening (although the gauge is basically turned into a mechanical idiot light).

 

[ZukesMcGee]

How are you getting that 240 degree temp number? How did you verify the headgaskets are good? Does the gauge read high when this is happening (although the gauge is basically turned into a mechanical idiot light).

I am reading the temps through an elm327 obd2 reader using dash command on my iPhone. Sometimes the gauge on the dash reads higher than normal but not until about the 240 mark on dashcommand. I checked the heads with a compression test and all were reading 205 or above. The oil is not milky and the coolant only smells like coolant. There is no white smoke or sweet smell from exhaust.

 

[JohnMc]

HG's don't always fail in dramatic fashion. Sometimes they fail in much more subtle ways.

I had a VR6 Jetta (180K miles) that failed similarly. No problems at all until you drove it pretty hard on a hot day, when the temp would spike up. No coolant use, no coolant leaking into the cylinder, no obvious smells or oil/water mixing, no white smoke. Just a very marginal leak that would only blow hot combustion gasses into the water jacket in certain situations.

My modified turbo Volvo has done the same thing in the past as well, it's a steel gasket (Cometic MLS) so if it fails it doesn't just blow out spectacularly, it jsut start leaking under stress. In that case, warm engine, no problem. 15 psi of boost, no problem. 20 psi of boost? It would spew coolant out of the expansion tank all over the engine compartment.

Autozone free-rents a tool you use with some tester liquid ($8 a bottle?) that sucks the air out of the cooling system through the liquid - it will tell you if there are combustion gasses present in the coolant or not. If it turns yellow, you have a leak somehow, cracked head/block or HG. It's a pretty cheap diagnosis.

If it's not the HG, then it sounds like a coolant flow issue. Generally speaking, when the car is going down the road at highway speeds the fan doesn't really need to do much work, the airflow through the radiator is sufficient just from air hitting the grille. Main culprit would be the thermostat not opening fully, but you already replaced that with no change in symptoms(?), so... internally clogged radiator? Or it could be an airflow issue because the radiator fins (or the A/C condenser fins) are clogged with bug/mud/bent fins.

 

[nissanh]

How is the fan clutch? How old is the radiator? Do you know whoever flushed the coolant system burped it in the right way? Also, check the temperatures at the top and bottom coolant lines on the engine with an IR thermometer.

Clean the AC condenser fins.

 

[ZukesMcGee]

How is the fan clutch? How old is the radiator? Do you know whoever flushed the coolant system burped it in the right way? Also, check the temperatures at the top and bottom coolant lines on the engine with an IR thermometer.

Clean the AC condenser fins.

Fan clutch is a brand new aisin as of today. I burped the coolant system by having the front elevated and the heat on set to face and feet while adding coolant slowly. Temperatures at the upper coolant hose read 190 right out of the engine. And the bottom coolant hose reads 170-180 or so. The radiator age is unknown to me but the top plastic is sort of browning so I'd assume it's well used.

One more thing. The temp is much more likely to read too high while driving with ac on.

 

[jgue467]

A brown plastic tank tells me it's Really old, perhaps original and could crack/leak at any time. Mine looked like that at 15 yrs when I replace it due to a crack. Perhaps it's partly plugged up with sediment/deposits - mine was. I'd suggest replacing it, for what little it costs to see if that clears up the high speed high temps. Oh thermo jiggle valve at 6 oclock? Luck. :cowboy:

 

[ZukesMcGee]

A brown plastic tank tells me it's Really old, perhaps original and could crack/leak at any time. Mine looked like that at 15 yrs when I replace it due to a crack. Perhaps it's partly plugged up with sediment/deposits - mine was. I'd suggest replacing it, for what little it costs to see if that clears up the high speed high temps. Oh thermo jiggle valve at 6 oclock? Luck. :cowboy:

I'll replace it for the peace of mind if nothing else.

Since the ir thermometer is reading 190 or so on the coolant hoses does that mean the temp sensors are the issue? These are sensors from a jdm motor of unknown origins, using the stock harness from my U.S. 4runner

 

[gamefreakgc]

What are the chances of both coolant temp sensors being bad. The one at the back of the block that controls the gauge, and the one at the front that goes to the ecu? These are sensors that came in a jdm motor I bought off of eBay. I've been chasing an overheating issue and am thinking the sensors may be bad. My coolant temp has read 240 degrees with no smoke and no boiling. Of course I know this is not a good temp so I'm trying whatever I can to figure it out. I replaced the thermostat, flushed the coolant, new radiator cap, headgaskets are good this has been verified. The reason I'm wondering if it could be the sensors is even though it's been quite hot, I've had no symptoms of an actual overheat.


I did just find the one of the coolant hoses coming from the heater core was collapsed. Could this be the issue? Thanks in advance to anyone who chimes in.

Edit: I forgot to mention it only overheats when driving close to freeway speeds. I can let it idle in my driveway with the ac on all day with no problem.

It sounds like your water pump isn't working. It should increase in speed but might be shot.

And yes, a collapsed hose is bad since it blocks coolant flow.

The sensors are cheap enough to replace anyway though right? Don't worry about the sensor to the gauge, just replace the one to the ECU (same one your scan reader is picking up).

 

[ZukesMcGee]

A new radiator appears to have solved the issue! Now I'm hovering between 190-195 on a 100 degree day with the a/c on full blast. Thanks everybody for the help.