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With this image, I am at a loss to figure out how coolant can move from top to bottom as Peva said. If all the hot coolant was flowing from the top hose and then through the radiator, it would not be able to reach the thermal sensor in the t-stat.
The heater core loop is open/running whenever the engine is running. It returns to the engine through the thermostat housing (that's what you always hear generically referred to as the thermostat bypass) - it may be that the thermostat sensing element gets directly bathed by that flow until the thermostat opens, or maybe all the time - I forget.

I tell you what - the guy who could explain that exact thing is FireM (Bob Day). He has posted threads on it more than once on the 300M Enthusiasts Club forums, and maybe even once or twice here. I had the same question as you, and he set me straight on it, but it's been a few years.
 

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I looked at the image that FireM posted. However, I'm still skeptical b/c it doesn't look like it came from the FSM.

Now, I'm going to look for a disc that has the data about coolant flow. (One that I inherited from the dealership when they went to online-only FSM info.)
 

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It's figure 8 on page 7-22 in the '02 FSM.
 

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I haven't been able to locate LH yet, but I did look at an LX system which also had the 3.5L engine. Seems you are correct about flow! I'm apparently losing a lot of that old grey matter. Good riddance I say!
 

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(Starting in late '99 or early '00, a heat exchanger (not shown) was added in line with the reservoir supply hose (item 1) to heat the blow-by gases in the PCV hose to keep volatiles from condensing out and clogging the PCV hose and valve.)
41328
 

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Yes, that's the image that FireM had in his post. Seems it is correct after all.

I discovered a '99 LH FSM and the same image was there too. Good work Peva! I stand "Corrected." Page 7 - 2 in the '99 FSM.
 

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Fair enough.

On the fill and bleed, ..... If you don't create a scenario where you somehow force additional coolant into the overflow section of the reservoir, ......then sucking new excess coolant from the overflow section when it cools back down - resulting in displacement of the last bit of air that would have remained in the ststem if you didn't start out with additional coolant in the overflow section).

.........HAVING SAID ALL OF THAT, I'm not saying that air in the system is the problem. 🤪
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Paying close attention to what you are saying, I think I’m getting picture of what is needed. The system initially needs excess coolant for a final purge using the natural heating/cooling cycles. The FSM states that the overflow is normally empty. Either way, there is hardly any coolant in the overflow of the car in question. I suspect that is a problem. Maybe not THE problem. But that must be addressed first.
Quite often going back and rehashing the basics can clear up a matter.

Also, the FSM mentions during engine operation, the high pressure RADIATOR tank runs full. The low pressure outlet tank fluid drops. “If this level drops below the top of the transmission oil cooler, aeration will occur drawing air into the water pump resulting in”..... all sorts of bad things. Section 7-7 of the 2003 FSM. I really don’t know exactly what they are talking about. (Incidentally, I have dissected an old Intrepid radiator to try and understand that concept stated in the FSM. ).
 

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Just sayin'.................
 

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One other thing the FSM likely says is for the bleeder to be open while filling with coolant. It helps a lot to have a small diameter tube attached to the bleeder and routed over the front bumper into an empty coolant bottle. This will help you save a few pints of coolant, as a lot of coolant is expelled while purging the system of air bubbles. Naturally, none of this procedure should be done with a running engine and definitely never while hot.
 

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...Also, the FSM mentions during engine operation, the high pressure RADIATOR tank runs full. The low pressure outlet tank fluid drops. “If this level drops below the top of the transmission oil cooler, aeration will occur drawing air into the water pump resulting in”..... all sorts of bad things. Section 7-7 of the 2003 FSM. I really don’t know exactly what they are talking about. (Incidentally, I have dissected an old Intrepid radiator to try and understand that concept stated in the FSM. ).
Here is my take on what they are saying there (quote of full paragraph with my comments in brackets [ ] ):

The FSM said:
Low coolant level in a cross flow radiator [cross flow - not vertical flow - in our case from passenger side to driver's side] will equalize in both tanks [meaning both end tanks of Mr. Radiator: Passenger side tank where coolant enters from the engine, being pushed by the water pump, and driver's side tank where coolant leaves the radiator and goes back to the engine] with engine off [so it's pointing out the obvious there that with engine off and therefore no coolant flow thru the radiator, the coolant settles out to the same level in the two end tanks]. With engine running and at operating temperature, the high pressure inlet tank runs full and the low pressure outlet tank drops [by "high pressure inlet tank" and "low pressure outlet tank" there, they mean high pressure and low pressure due to coolant flow thru the radiator tubes - not anything to do with system pressure compared to ambient air pressure, but everything to do with pressure drop from passenger side to driver's side due to coolant flow rate and resistance to flow from the narrow tubes]. If this level [the driver's side tank level] drops below the top of the transmission oil cooler [which is at approximately the same level as the entrance to the lower radiator hose], aeration will occur [because now air, not coolant, is at the entrance of the lower radiator hose] drawing air into the water pump resulting in the following: . . .
What they're emphasizing here is the importance of properly bleeding the system and overfilling the reservoir when filling to ensure that all air gets out of the system, which can only happen if there is make-up coolant in the overflow section of the reservoir (pressure bottle). If there is no (or minimum) air in the system, then there is no way the driver's side tank coolant level drops low enough to cause aeration when the engine and water pump are at high rpm and flow thru the radiator and pressure drop across the tubes are high.

(It is possible that when they say "aeration" and "air" in that paragraph that they are also referring to cavitation and coolant vapor due to the same pressure drop across the tubes. Likelihood of cavitation due to the tube pressure drop could be increased by low system coolant level causing higher coolant temperature (closer to the boiling point). It could actually be a combination of aeration/air and cavitation/coolant vapor, and they didn't want to get to technical, so they just said "aeration" and "air".)
 

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One other thing the FSM likely says is for the bleeder to be open while filling with coolant. It helps a lot to have a small diameter tube attached to the bleeder and routed over the front bumper into an empty coolant bottle. This will help you save a few pints of coolant, as a lot of coolant is expelled while purging the system of air bubbles. Naturally, none of this procedure should be done with a running engine and definitely never while hot.
You will not get pints of coolant out of the bleeder if done correctly. The tube will avoid spilling a small amount of coolant that comes out with the sputtering of the last of the bubbles. You have the bleeder cracked open as you fill and overfill only until the bubbles stop coming out, then you close the bleeder, and when overfilled to the right level in the funnel, you remove the reservoir overflow hose clamp.

BTW - some say that you can get a few more bubbles out if you bleed it with the car on an incline with the nose pointing uphill, which implies that bubbles tend to get trapped more towards the rear of the engine. But, if you do a decent job and use the overfill method, any remaining bubbles will purge themselves with running the engine at road speed for a few minutes to trap the air in the reservoir followed by one or two warm up/cool down cycles to displace any additional removed air with coolant in the overflow compartment (that is there because of the overfill) being pulled back into the engine.
 

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Has anyone tried Water Wetter?


Included in the description..."Coolant additive with a unique agent that lowers temperatures by reducing and eliminating bubbles or vapor barrier that form on hot metal surfaces"
I looked at using it a few years ago. IIRC, the problem is that you only get its full effects with no antifreeze. To prevent coolant from freezing, you need to add antifreeze. The benefits of Water Wetter are reduced proprotionally with the amount of antifreeze you have to use for your area. So not ideal or possibly impractical for year-around use in colder climates. If you need 50/50, then what's the point? Otherwise, probably OK.
 

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Discussion Starter #35 (Edited)
Thanks for replies so far everyone. Here is latest update:

I purchased the filler kit Ronbo recommended (thanks Ronbo).

Replaced the reservoir.

Put 1 one bottle of Water Wetter (my brother said it worked for his car, so saw no harm for only $11).

Filled to top of reservoir.

Bled system until no bubbles.

Test drove car but stopped after a few miles because smoke coming from under hood, but determined it was from coolant that had leaked on cat converter and steaming.

Upper rad hose hot, lower rad hose and radiator cold.

Test for bad impeller recommended by peva indicated water pump good (thanks peva).

Needle moved to half during entire test drive, but after pulling in drive way and letting car sit for about a minute, needle moved past half...had to turn on heater to get needle to move below half.

Suggestions?

I also, submitted post on battery drain problem I'm having with the vehicle...any help appreciated.
 

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Did the fans start up? I read that the radiator and lower hose were cool while the upper was hot.

I'd be looking to get another radiator. Sounds like your new one is not working.

There are several rows to the radiator. None of them were hot?
 

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The radiator fans do work correctly. I can only access the radiator by sticking my hand through where the fans are. Key in Off position. It’s a small area I can feel but yes cold to the touch in the upper area and mid area of the rad. Incidentally, it’s a two row rad. Engine idling today in the yard, the temp. gauge comes up to the normal mark and stays there.
 

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Peva, the coolant system was bled and filled according to the method you posted. We used the tall funnel as the OP pointed out and the overflow tank was filled about 40%.

(I’m looking at the posts using my phone so reviewing this is a tad difficult ).
Someone mentioned a mechanical blockage. I don’t think that’s the case because it will idle at normal temp. and the fans blow warm air. So coolant must be flowing to some degree. Could a Part Pick tag stuck in a hose be causing a partial blockage? Yes. I’ve seen that before.
 

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I have also seen plastic slag partially or even in some cases totally blocking the inlet or outlet.
 

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Discussion Starter #40
The old reservoir had a dark film on the inside, so would flushing the system possibly solve the overheating issue, and if so, recommended products to do that?
 
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