Cold lower radiator hose

[Sarki]

Having an issue with my 95 T100 (4WD, 5 Speed) which has same 3.4 as my 02 4TR and hoping someone can help me figure this out.
Replaced timing belt, water pump, tensioner, etc as well as thermostat and upper/lower rad hoses.
Installed thermostat with jiggle valve at 6 o’clock position......all Toyota OEM parts.
Used Lyle funnel to refill coolant system (Toyota 50/50 mix) about 9qts or so.
Refilled radiator, ran engine until reaching normal operating temperature (temp gauge registering “middle” of gauge). This took about 10 minutes. Heat set at warmest temp with fan off.
Upper hose nice and warm but lower hose remains cold, not even warm.
Can reach down and feel the neck of the water outlet where the thermostat resides and cold to the touch, even after running for 30+ minutes.
Cooled down, removed radiator cap, started truck from cold and watched for water flow in radiator as it reached operating temp. Did not see any movement which I expect to see flowing once/if thermostat opens.
Again, lower hose and water outlet to thermostat remains cold to touch while upper hose heats up nicely.
If drive for 30 minutes truck doesn’t overheat. Temp gauge on dash stays just a slight bit past half way.
Heater working well.
Don’t think I have clogged radiator as it drains nicely & never noticed an issue before all this.
Removed and tested thermostat in hot water and it opens and closes right around 180-185 which seems good.
What’s the deal......air bound??
Sorry for long winded post.

 

[phattyduck]

Congrats on having a nice efficient cooling system. If the truck isn't overheating (probably worth verifying with and OBD-II device that can read live data), everything is working as intended.

The lower hose should always be cooler than the upper hose. Depending on weather, it might even be cold. The lower hose is the inlet to the engine/thermostat.

How's your radiator fan? Is it always loud? You might have a locked fan clutch working the fan harder than it needs to be...

-Charlie

 

[Sarki]

Congrats on having a nice efficient cooling system. If the truck isn't overheating (probably worth verifying with and OBD-II device that can read live data), everything is working as intended.

The lower hose should always be cooler than the upper hose. Depending on weather, it might even be cold. The lower hose is the inlet to the engine/thermostat.

How's your radiator fan? Is it always loud? You might have a locked fan clutch working the fan harder than it needs to be...

-Charlie

Thanks for response Charlie.
Definitely not overheating.
I really expected to find I had mistakenly not put the jiggle valve in the 6 o’clock position but lo & behold once I pulled the housing off it was exactly where it should be.
Just for clarification, I understand the upper hose will be hotter to the touch than the lower hose but the lower hose and water inlet housing for thermostat are dead cold even after fully warmed up and driving 30-45 minutes.....that surprised me.
I also expected to see some movement in the coolant in the radiator when running with the radiator cap off when fully warmed up.
Funny you mentioned the possibility of a locked fan clutch as my truck sounds like a freight train! Always wondered about that fan.
Having said that I don’t think that would be the cause of the lower hose staying cold.
Just seems like that hose and the water inlet housing should be at least warm after engine fully warmed up.

 

[phattyduck]

I also expected to see some movement in the coolant in the radiator when running with the radiator cap off when fully warmed up.
Funny you mentioned the possibility of a locked fan clutch as my truck sounds like a freight train! Always wondered about that fan.
Having said that I don’t think that would be the cause of the lower hose staying cold.
Just seems like that hose and the water inlet housing should be at least warm after engine fully warmed up.

Sounds like you have a locked up fan clutch then! That would force so much air through the radiator that the coolant would be fully cool by the time it reaches the bottom.

Replacing the fan clutch will also get you back 1-2 mpg and some power at the wheels while you are at it. It should only make loud fan noises right at startup (10-30 seconds) and in very hot conditions (AC on, low speed high load driving, etc.).

-Charlie

 

[Sarki]

Sounds reasonable and worth a shot.
I’ll button it back up, order and replace the fan clutch and see how that works out.
Thanks for your thoughts and advice.

 

[pluton]

How old is the radiator and is it plastic tank or metal tank?

 

[Dieselchessy]

Your rig sounds normal to me.

These engines have 2 coolant flow loops. One through the top end and one through the radiator.
When the thermostat is closed the water pump pumps the coolant through the top end and the stainless pipe that leads to the back of the engine and the heater core valving.
When the thermostat opens the water pump will pull through the radiator.

My rigs don’t tend to open the thermostat till I’ve driven 30 minutes plus. Even longer if it’s cold and I’m running the heater. If I just leave it idling in the shop I’m not sure the thermostat would ever open. You pretty much have to remove the thermostat to flush these things.

With thermostat closed the top hose will get hot and firm. The bottom hose will remain cool.

 

[Sarki]

How old is the radiator and is it plastic tank or metal tank?

Still running what appears to be the original radiator. Never had any issues as far as leaks or corrosion and coolant regularly swapped out with Toyota Red.
Hate to replace it just for sake of replacing as no chance of Strawberry milkshake since it’s a manual trans.

 

[Sarki]

Your rig sounds normal to me.

These engines have 2 coolant flow loops. One through the top end and one through the radiator.
When the thermostat is closed the water pump pumps the coolant through the top end and the stainless pipe that leads to the back of the engine and the heater core valving.
When the thermostat opens the water pump will pull through the radiator.

My rigs don’t tend to open the thermostat till I’ve driven 30 minutes plus. Even longer if it’s cold and I’m running the heater. If I just leave it idling in the shop I’m not sure the thermostat would ever open. You pretty much have to remove the thermostat to flush these things.

With thermostat closed the top hose will get hot and firm. The bottom hose will remain cool.

I almost get the feeling my thermostat isn’t opening even when gauge indicates normal operating temperature after 45 minutes of running.
If thermostat were opening wouldn’t I see coolant movement inside radiator if cap off.
The 2 reasons I thought I had an issue were as follows:
1) Lower hose and water inlet housing for thermostat remains cold even after running truck for an hour. Top hose hot as expected.
2) Don’t see any coolant movement in radiator when cap is left off and engine at normal operating temp, supposedly with thermostat open.

Ordered new fan clutch/fluid coupling since it is very loud and most likely locked up. Won’t arrive for another couple weeks, $87 through Amazon (Aisin branded unit).
Like suggested......maybe I’m worried about something that isn’t a problem. Just surprised when I grabbed lower hose after an hour and is was cold, not luke warm but cold.

 

[JMat]

I almost get the feeling my thermostat isn’t opening even when gauge indicates normal operating temperature after 45 minutes of running.
If thermostat were opening wouldn’t I see coolant movement inside radiator if cap off.
The 2 reasons I thought I had an issue were as follows:
1) Lower hose and water inlet housing for thermostat remains cold even after running truck for an hour. Top hose hot as expected.
2) Don’t see any coolant movement in radiator when cap is left off and engine at normal operating temp, supposedly with thermostat open.

Ordered new fan clutch/fluid coupling since it is very loud and most likely locked up. Won’t arrive for another couple weeks, $87 through Amazon (Aisin branded unit).
Like suggested......maybe I’m worried about something that isn’t a problem. Just surprised when I grabbed lower hose after an hour and is was cold, not luke warm but cold.

What kind of air temps are you having. Mine does the same thing from late fall to early spring. Once the temps get up in to the mid to high 50's the lower hose will warm up when I drive it.

 

[phattyduck]

The thermostat is not an on-off switch. Your thermostat is 'open' if the top hose and top radiator tank gets hot - it is just a small amount of coolant movement there though. That is especially true if the weather is cool/cold and the heater is on. As JMat said, you would probably experience something very different in the dog days of summer (and with an unlocked fan clutch...).

These trucks (and I assume the T100 too!) don't overheat in 120*F temps with the AC on max... there is plenty of heat rejecting capacity not needed in the winter / spring.

-Charlie

 

[Sarki]

Thanks for sharing thoughts on this.
Closed it back up after confirming no issues with thermostat. Refilled system with front end elevated on ramps while running with heat on.
After idling for 25 minutes Infrared gun indicated temp readings as follows.
155-top radiator tank
125-upper hose
70-lower hose
75-water inlet housing of thermostat
Heat blowing nice and hot, temp gauge dead center.
Like suggested, I may be looking for problem that doesn’t exist!
I’ll replace the fan clutch when it arrives in 2-3 weeks and all should be good, quiet and staying cool......hopefully!
Thanks to those who offered suggestions.