Under heating problem

CTUBS8732 · Apr 23, 2026

05 Tahoe 5.3 4x4 260k

So I recently blew a plastic core radiator and overheated a bit about 3/4 on the gauge. Replaced with a 4 row aluminum radiator and new water pump with tstat amd housing. (Replaced 2 thermostats since, thinking they were faulty)

My problem is after I put the new parts on, it will run at about 165-175 going 45-70 mph. If I idle it will get to temp. I do have bubbling in the resivoir and a constant flow of coolant going back in the resivoir from the radiator even on a cold start and stays doing it. Also I can hear the coolant flowing thru the heater core when I accelerate, like a swooshing sound.

I've searched and searched but can't find anything on it but stuck open tstat or fans constantly running. I dint have ac so my fans aren't even on.

Doubeleive · Apr 23, 2026

The return line on aftermarket radiators is sometimes too large, it needs to be restricted the factory return line is only about 1/8"

Marky Dissod · Apr 23, 2026

CTUBS8732 said:
My problem is after I put the new parts on, it will run at about 165F-175F going 45-70MpH. If I idle it will get to temp.

When towing, is this really a problem?

Doubeleive said:
The return line on aftermarket radiators is sometimes too large, it needs to be restricted the factory return line is only about 1/8"

Wonder why no one has ever considered installing a ball valve or some other variable restriction on that line? ...

CTUBS8732 said:
I do have bubbling in the resivoir and a constant flow of coolant going back in the resivoir from the radiator even on a cold start and stays doing it.
Also I can hear the coolant flowing thru the heater core when I accelerate, like a swooshing sound.

You may need to burp the air out of the system again.

Doubeleive · Apr 23, 2026

Marky Dissod said:
When towing, is this really a problem?

Wonder why no one has ever considered installing a ball valve or some other variable restriction on that line? ...

You may need to burp the air out of the system again.

eventually it throws a code or "can" throw a code. The engine is designed to run within certain parameters if that is off then it might be changing the air/spark/fuel ratio.
I would assume at some point one of those would lead to potential engine damage. If you want it to run cooler I would suggest a tune to allow for it just to be on the safe side.

Fless · Apr 23, 2026

See the thread below for this issue and potential solution, and note the additional link in post #12:

p0128 Engine temp below thermostat temperature/ strange O2 sensor values

Hello, I was just driving and suddenly the engine light came on. I read it at home and found that I had a P0128. This means the "engine temperature is below the thermostat temperature". In other words, the engine complains that it isn't warming up properly. Since the beginning I've had the...

www.tahoeyukonforum.com

Marky Dissod · Apr 23, 2026

Doubeleive said:
The engine is designed to run within certain (temperature) parameters, if that is off then it might be changing the air/spark/fuel ratio.

When an engine is run on the cool side of the intended temp window, AFR is definitely, always, enriched a wee bit, even in closed loop.
Major consequence is changing oil sooner / more often to deal with breakdown from fuel dilution over time.

LsHart · Apr 23, 2026

CTUBS8732 said:
05 Tahoe 5.3 4x4 260k

So I recently blew a plastic core radiator and overheated a bit about 3/4 on the gauge. Replaced with a 4 row aluminum radiator and new water pump with tstat amd housing. (Replaced 2 thermostats since, thinking they were faulty)

My problem is after I put the new parts on, it will run at about 165-175 going 45-70 mph. If I idle it will get to temp. I do have bubbling in the resivoir and a constant flow of coolant going back in the resivoir from the radiator even on a cold start and stays doing it. Also I can hear the coolant flowing thru the heater core when I accelerate, like a swooshing sound.

I've searched and searched but can't find anything on it but stuck open tstat or fans constantly running. I dint have ac so my fans aren't even on.

Air in the system. Or heater core is stopped up.do the fans ever come on when it's bubbling?

CTUBS8732 · Apr 24, 2026

Doubeleive said:
The return line on aftermarket radiators is sometimes too large, it needs to be restricted the factory return line is only about 1/8"

How would I go about trying this method?

Marky Dissod said:
You may need to burp the air out of the system again.

Ive tried jacking the front end up as high as my jack would go and let it run for even giving a few revs every now and then but it still has the bubbling.

Doubeleive said:
eventually it throws a code or "can" throw a code. The engine is designed to run within certain parameters if that is off then it might be changing the air/spark/fuel ratio.
I would assume at some point one of those would lead to potential engine damage. If you want it to run cooler I would suggest a tune to allow for it just to be on the safe side.

It has already thrown that code too for the thermostat. I just erased it with my scanner. Scanner is saying the temp thats on the dash.

I was thinking maybe the bubbling is exhaust gasses. Yall think I could've possibly cracked or warped my 706 heads? Just got done looking into heads and apparently if thats the case, I should switch to 862 heads.

Doubeleive · Apr 24, 2026

CTUBS8732 said:
How would I go about trying this method?

Ive tried jacking the front end up as high as my jack would go and let it run for even giving a few revs every now and then but it still has the bubbling.

It has already thrown that code too for the thermostat. I just erased it with my scanner. Scanner is saying the temp thats on the dash.

I was thinking maybe the bubbling is exhaust gasses. Yall think I could've possibly cracked or warped my 706 heads? Just got done looking into heads and apparently if thats the case, I should switch to 862 heads.

The head is not going to crack from running cool. When I had a aftermarket one for a couple months I clamped a smaller hose inside the larger return hose, whatever you use you have make sure it can't get thru the hose and into the system then it's going to get stuck somewhere else that it shouldn't be

Joseph Garcia · Apr 25, 2026

Doubeleive said:
The return line on aftermarket radiators is sometimes too large, it needs to be restricted the factory return line is only about 1/8"

"How would I go about trying this method?"

Someone posted about using a gun shell casing that worked perfectly as a restrictor.

Fless · Apr 25, 2026

CTUBS8732 said:
How would I go about trying this method?

GO TO THE LINK I POSTED IN POST #5 HERE AND READ THROUGH IT. Posting a problem, then ignoring the suggestions isn't going to give you any joy.

In that link, in Post #4, I made a suggestion. Try it. (Clue: it's in the second paragraph of that short post)

555hp · Apr 25, 2026

CTUBS8732 said:
05 Tahoe 5.3 4x4 260k

So I recently blew a plastic core radiator and overheated a bit about 3/4 on the gauge. Replaced with a 4 row aluminum radiator and new water pump with tstat amd housing. (Replaced 2 thermostats since, thinking they were faulty)

My problem is after I put the new parts on, it will run at about 165-175 going 45-70 mph. If I idle it will get to temp. I do have bubbling in the resivoir and a constant flow of coolant going back in the resivoir from the radiator even on a cold start and stays doing it. Also I can hear the coolant flowing thru the heater core when I accelerate, like a swooshing sound.

I've searched and searched but can't find anything on it but stuck open tstat or fans constantly running. I dint have ac so my fans aren't even on.

You need to get the air out (leave cap off untilbit comes up to temp and let it cool off, keep bottle filled). The second part is to add a restrictorvintobthe radiator to coolant bottle line. I used a 1/2 inch brass PEX cap (got mine off amazon( and drilled a 0.100 diameter hole in it). You can adjust the hole size a bit for your own local conditions (smaller in colder areas and larger for warmer).

Doubeleive · Apr 25, 2026

555hp said:
You need to get the air out (leave cap off untilbit comes up to temp and let it cool off, keep bottle filled). The second part is to add a restrictorvintobthe radiator to coolant bottle line. I used a 1/2 inch brass PEX cap (got mine off amazon( and drilled a 0.100 diameter hole in it). You can adjust the hole size a bit for your own local conditions (smaller in colder areas and larger for warmer).

not sure the theory of "running cool" is the result of trapped air, but would be the other way around "running hot"
with excessive flow to the reservoir it throws the system out of whack by allowing backflow from the engine instead of heating up
air traps water and there would be no flow or too little flow, resulting in excessive heat (complete opposite)
there have been diagrams posted before and I am sure it is because of the Y-pipe split from the reservoir to the water pump/hvac and psi pressure.
the cap is rated for 15 psi and I assume that heat creates expansion in the system allowing correct flow and lack of pressure from excessive flow prevents the pressure from being created. but I am part idiot so there's that.

Joseph Garcia · Apr 25, 2026

Doubeleive said:
not sure the theory of "running cool" is the result of trapped air, but would be the other way around "running hot"
with excessive flow to the reservoir it throws the system out of whack by allowing backflow from the engine instead of heating up
air traps water and there would be no flow or too little flow, resulting in excessive heat (complete opposite)
there have been diagrams posted before and I am sure it is because of the Y-pipe split from the reservoir to the water pump/hvac and psi pressure.
the cap is rated for 15 psi and I assume that heat creates expansion in the system allowing correct flow and lack of pressure from excessive flow prevents the pressure from being created. but I am part idiot so there's that.

If you are part idiot, then I must be a Blue Ribbon full idiot.

CTUBS8732 · Apr 25, 2026

@Fless I checked out the link. Im going to try the .45ACP method hopefully it works. If not im gonna try out @555hp method or @doubleleive. Definitely need this fixed. My gas mileage has gone to crap.
Yall think I can run a splice reducer back to bigger?

Doubeleive · Apr 25, 2026

CTUBS8732 said:
@Fless I checked out the link. Im going to try the .45ACP method hopefully it works. If not im gonna try out @555hp method or @doubleleive. Definitely need this fixed. My gas mileage has gone to crap.
Yall think I can run a splice reducer back to bigger?

whatever is practical will work

Mudsport96 · Apr 26, 2026

CTUBS8732 said:
@Fless I checked out the link. Im going to try the .45ACP method hopefully it works. If not im gonna try out @555hp method or @doubleleive. Definitely need this fixed. My gas mileage has gone to crap.
Yall think I can run a splice reducer back to bigger?

The. 45 acp brass worked for my Silverado after installing an aftermarket radiator.

Marky Dissod · Apr 26, 2026

CTUBS8732 said:
Yall think I can run a splice reducer back to bigger?

I do wonder about installing a ball valve in that line ...

Fless · Apr 26, 2026

You can just use pliers with soft jaws to temporarily restrict it somewhat to see what happens. Don't collapse the hose but just squeeze it a bit. Hose pinch pliers would be good, or even a small spring clamp.

LsHart · May 8, 2026

It really does not take long to burp air out of the system. If ur bubbling at temp and it's hot then there is an issue. But that being said if ur running a lower that normal temp thermostat that the computer can't compensate for the it will act like it's running hot.

Buddy had a camaro that the computer was not set for the lower temp thermostat and it wouldn't run right. Not sure if this helps but ur issue reminded me of his issue.

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