L300s have a circulation pump for the heater. It is powered by the cooling fan relay box near the battery. It is a small pump mounted in one of the heater hoses. The cooling system on the engine does not provide enough flow to adequately flow the heater and needs some help. Before condemning the heater core verify the pump is working properly.

 

The coolant pump was discontinued in 2003. Replacements are virtually non existent.The is a service bulletin stating that they are not essential and gives a list of parts to use in place of the pump.
On my 02 when the pump started to leak I believe I used an 90 degree elbow and a piece of heater hose to replace it.
Been fine for 2 years now. The heat output is normal.
As far as the heat problem, does the temperature gauge indicate the engine is reaching normal temperature ?
Do you have access to a scan tool to check actual temperature ?
I have used Bars Leak in countless cars over the decades. NEVER had a problem !!

By discontinued I mean they no longer were installed on the 03-05 's

 

I agree with Andy since he knows these cars and I DO NOT. I'm going by my past experiences with Mopars.

 

Not a fan of
"Fix in a can"
To fix for me
What I cannot see
I know somewhere
It's leaking in there
But a snake oil fix
is always a risk
Clog up the heater
In your Saturn beater
Might fix the leak
But drives some to tweak
Clog up the rad?
Incredibly sad
Stand clear of the aisle
Of fix in a can
UnclJohn he was wild
But these things he banned
Andy's a lifer
Wrenching since 2
now he's pied piper
And we all dig his tune
Light and airy like an impact gun
Whir whir whir nuts un spun
Remember slick 50?
Bonds Teflon to metal?
BS said FTC
By large fine was settled
We all use a few
But the ones I won't do
Have the first word as stop
And that's what I do
​​​to each their own.

I just thought we needed a little poetry around here

 

I like it, derf. You should make a sticky for that.

 

I agree, to each his own. There are a few "stop" products I have had excellent results with and no negative effects.
Bars Leak as I stated earlier, NEVER had a heater core plug up.
Lucas Power Steering Stop leak has worked miracles for me.
134a Super Seal has saved me hundreds of dollars. A/C worked fine for years after.\
If you havent tried them how can you judge ??

 

That is indeed why I ended my prose with to each their own.

To be clear, I am not bashing you.
I wonder if changes in formulation might have something to do with it. That stuff has been around since I was a kid. Don't know.

UnclJohn lived in Arizona and put 15W40 in his s cars. He did so because the temperature viscosity charts said he could and he wanted the maximum lubrication. I once asked him if, after running it a while, he had ever checked to see if it was still behaving as 15W40 because to get to that viscosity, you pretty much have to add additives. And when those additives start to break down, what the hell viscosity do you have at a given temperature? He was none too pleased. Never did answer me. Thought it was a fair question.

What does this have to do with what we're talking about?

To each their own

 

I did not know they quit using the circ pumps on the newer L300s. I only have the early cars and most have been 4 cylinder cars. I did buy a wrecked 03 L200 for parts. I a working on a nice 2000 LS1 for a grand daughter now, should run today!

 

Thanks for all of the follow-up input. I tend to agree that the fix in a can approach is not the way to go but I was not going to tear the engine apart looking for a potential problem without some evidence and as other have said, the coolant must be going somewhere. Part of the problem is the car is at my daughter's house and reports about such things as low coolant are sketchy and often long overdue.

I am glad to hear that I am not blind and that the car does not have a supplemental coolant pump. In the engine replacement process I had touched every coolant hose in the engine compartment and was wondering where it could be. There is a bent tube connection between two heater hoses on the back side of the engine that may be where the pump had been in earlier model years. If anyone needs the tube let me know as I have an extra (came with the replacement engine).

The latest update regarding heat is encouraging. Apparently the heat works normally at riving speeds and only drops off when idling. I am interpreting this to mean the hater core is not blocked but perhaps restricted, reducing flow at low engine speeds (water pump flow rates). The car generally doesn't get driven long distances so it probably never gets fully warmed up (thermostat open, and entire cooling system at operating temperature). I did run/drive it for a longer time period with the heat on high after installing the leak sealer and believe the entire cooling system was up to temperature before it was returned to my daughter/grand daughter.

The engine coolant temperature gauge behaves normally, rises to the point of thermostat opening and stays there but I have not accessed the actual coolant temperature.

 

If the coolant level is low, your heater will act like that. And the information about mostly short trips could also explain why it doesn't operate properly. Thermostat has to open in order to circulate the heated coolant through the engine. But you know that already.