Sanity Check - Cooling issues

[CarlB]

Coming out of the thermostat housing (small gray box on the back of the engine) is a yellow / orange hose that runs to the expansion tank represented in blue. That line should be connected to the engine side of the thermostat to blead air. If it isn't there should be a small hole like 1/8 inch in the thermostat to blead air. I use the green coolant. The other coolants are made to last a long time and their anticorrosion inhibitors do not start working quickly. I use the green stuff because I seem to have my engine out frequently.    

[S1Steve]

Because MV8 and I like Schematics, this is from the Superformance build manual. 😀 Very similar to what Vlad posted…

[Silber]

1 hour ago, CarlB said:

Coming out of the thermostat housing (small gray box on the back of the engine) is a yellow / orange hose that runs to the expansion tank represented in blue. That line should be connected to the engine side of the thermostat to blead air. If it isn't there should be a small hole like 1/8 inch in the thermostat to blead air. I use the green coolant. The other coolants are made to last a long time and their anticorrosion inhibitors do not start working quickly. I use the green stuff because I seem to have my engine out frequently.    

 

The thermostat housing is on the front left side of the engine, and the small hose from that runs to the heater. The small line from the expansion tank runs to the manifold on the back of the engine. I made a new drawing with a different program that should make it more clear.

 

 

Looking at the other drawings, it appears that expansion tanks typically have a third hose for a breather. I only saw two lines coming from the tank, so I'll need to take a closer look tonight.

[Silber]

5 minutes ago, S1Steve said:

Because MV8 and I like Schematics, this is from the Superformance build manual. 😀 Very similar to what Vlad posted…

 

 

It also looks like the Zetecs have a different location for the thermostat, at the back of the engine instead of next to the water pump.

[IamScotticus]

13 hours ago, Silber said:

plan on doing a proper cleaning/flush of the system if problems persist.

If your coolant is 5 years old, change it.  Think of it like oil or brake fluid,  it needs refreshing at intervals too.

Edited Wednesday at 04:24 AM by IamScotticus

[MV8]

The bypass is for the #6 fittings. Apparently you are using the heater core as a bypass (separate loop on a duratec) without the oem circuit. I suggest rearranging to match the way ford intended.

Edited Tuesday at 11:18 PM by MV8

[panamericano]

2 hours ago, IamScotticus said:

Of your coolant is 5 years old, change it.  Think of it like oil or brake fluid,  it needs refreshing at intervals too.

I've always changed coolant once/year, if there is any aluminum vs iron in the system.  I read somewhere that one can get a reaction, talking about an aluminum engine, but I would include an aluminum radiator and iron engine.

[IamScotticus]

5 hours ago, panamericano said:

I've always changed coolant once/year, if there is any aluminum vs iron in the system.  I read somewhere that one can get a reaction, talking about an aluminum engine, but I would include an aluminum radiator and iron engine.

Exactly. Things like that. Modern OAT coolant has such good anti corrosive additives it can clear rust out of an Iron block. 

[MV8]

On my vehicles, I usually restrict the bypass to make the cooling system more efficient when hot and reduce the warm up time. Not on mazda 4 cylinder engines (B3,B6,BP, etc) though, since they usually have a steel pipe from the pump to the back of the head with an oring on the ends. The duratec just uses a long, molded hose.

 

After it is plumbed like the oem, note if your overheat and warm up are greatly affected by if the heater valve is open or closed.

Also, even if the expansion tank is half full not running, this arrangement may be circulating through the bleeder line in parallel to the heater, increasing flow cold/stat closed (we want just enough flow to prevent hot spots). The expansion tank probably needs to be connected to an upper or lower coolant pipe instead of the heater if it will have bleeder fittings. I'm not sure the bleeder is necessary with so much plumbing around it already at the same height. I would pinch it closed and test.

[Silber]

Not going to be able to get to working on it until Friday at the earliest, but here's the plan:

1. Prop up the rear of the car and allow engine to warm up enough to open the thermostat and run the fan. Purpose is to see if the system will burp and operate normally.
2. If Step 1 does not result in normal behavior, drain and flush the system while also changing the routing to bypass the heater completely. The fan switch for the heater doesn't work, and I don't drive the car in the winter due to salted roads, so removing the heater was already something I was considering.
3. Repeat Step 1 with the new routing and flushed system.

I'll report back here with my progress. 

[CarlB]

On 4/15/2025 at 12:41 PM, Silber said:

 

The thermostat housing is on the front left side of the engine, and the small hose from that runs to the heater. The small line from the expansion tank runs to the manifold on the back of the engine. I made a new drawing with a different program that should make it more clear.

 

 

Looking at the other drawings, it appears that expansion tanks typically have a third hose for a breather. I only saw two lines coming from the tank, so I'll need to take a closer look tonight.

I have a Zetec not a Duratec engine in my car. The thermostat housing is on the back where this drawing says manifold. I have been making assumptions that your plumbing is similar to the Zetec.  I am confused by the drawing. Generally, the thermostat is on the exit from the engine (hottest water) and it is returned to the top of the radiator. Where is the water pump? Why isn't there a hose into and out of the heater? The post from MV8 shows pictures of the thermostat and it's housing as well as the manifold. The picture suggests a larger hose than I would assume would be used for an air bleed. Is this the heater supply or return?  The drawings MV8 and Siber posted are the same. The Siber is easier to follow. Vovchandr had a good question, and I assume the water pump is on the side off the block. Those drawings and Vovchandr's Zetec drawing, show the typical flow path. Cool water out of the bottom hose going to the water pump and being pressurized as it enters the engine. The water flows through the engine and exits through the thermostat. All three of the drawings show an air bleed from the top of the thermostat back to the expansion tank. SiSteve has a drawing that shows an air bleed from the top of the radiator. That would be a big help if it had been included on your car. MV8 is correct about re-piping the by-pass. The drawing shows the radiator return going into the low-pressure side of the water pump. As the thermostat starts to open it will pull water through the heater and slow the engine from heating up. It should be going to the lower radiator hose.  

[Silber]

Looks like I'm going to be re-plumbing the system because I'm left scratching my head about this cooling system. I did not get the thermostat to open at all while the engine was running today.

 

I started off running the engine with the expansion tank off, rear of the car raised. I saw a bunch of bubbles in the expansion tank, but the thermostat did not open at 180 F. The radiator fan kicked on at 190 again, and I confirmed it is spinning correctly drawing air through the radiator. The lower coolant hose was cold when I shut the engine off at 210 F, at which point coolant started bubbling up out of the expansion tank. I put the cap back on, and found that some hot coolant flowed through the lower radiator hose. What?

 

Started the engine after a few minutes, 190 F, rear still raised and expansion tank cap on. Lower radiator hose cooled off with airflow from the radiator fan, and stayed cool when I shut the engine off at 210 F. I started to relieve pressure from the expansion tank cap, with my hand on the lower radiator hose, and as I was relieving pressure I could feel hot coolant flowing through the lower hose. I don't understand what is going on... is the thermostat opening only when I relieve pressure in the system?

 

Left the car for a few hours and tried one more time. Expansion tank cap on, car flat on the ground, the thermostat still did not open. I opened up the expansion tank while the engine was running at 200 F, no flow through the lower radiator hose, but I did notice some more bubbles, so I raised the rear of the car and let it burp for a few minutes. Put the expansion tank cap back on, shut off the engine, and no flow through the lower radiator hose. Relieved the pressure in the tank and felt the coolant flow again.

 

So my thoughts are that I've got a faulty thermostat, or this system is very difficult to burp/bleed and is really causing flow issues, or some other problem yet to be diagnosed.

 

I did, however, find that there is another bypass that I had missed that runs from the engine block just behind the thermostat housing to the manifold on the back of the engine. Everything on the right side of the diagram I posted gets properly hot, the upper radiator hose gets fairly warm, and the radiator and lower hose remain cold.

[Vovchandr]

Thermostat can be tested by throwing it into boiling water. 

 

Coolant system can be tested for thermostat issue by taking it out entirely. Depending on system might be an easy test. 

 

Certainly a conundrum 

[MV8]

Silber, I suspected you had #6 hose but didn't see it buried in there. Pinch the #3 hose (in my diagram) closed with a vice grip or repair/position the heater valve to close and retest. I believe eliminating heater flow is the quick fix to get you through summer.

Where is the indicating temp sensor located on the engine? Indicated temp, stat temp, and fan switch temp are usually not the same as they are in different locations.

Stats are rated at the temp where they should start opening/finish closing. You could not know if the stat was completely closed. Relieving system pressure creates differential pressure in the system resulting in flow that could briefly warm the lower hose.

I'd replace the stat as well, with a 180f and order two, so I have a spare at hand along with a replacement plastic manifold for the back for the head.

[wdb]

The line mechanic in me is whispering, "Customer states car is overheating. They just changed the thermostat." In which case the first thing I would look at would be the thermostat and its installation. Is the correct part? Is it installed correctly? Is there some mechanical issue that does not allow it to function as designed? That kind of stuff.

 

Also, +1 for tossing the thermostat in a pot of water on top of the stove.

Edited yesterday at 02:15 PM by wdb

[IamScotticus]

Thermostats and pressure caps are wear items that fail regularly. 

You can run an engine with without a thermostat, replacing it with a plate with a hole in the center. I guess a .5" diameter.  The warm up will take longer,  but you will have a driver while you sort through everything else.  Providing the radiator isn't clogged up.

 

Hoses are wear items too.  Modern materials are superior to the old EDPM rubber, which will dry out over time.

 

 

I know I pontificate on this subject,  but I do so in that the majority of my automotive failures have been cooling related catastrophes and I've learned everything the expensive way. Not much help on burping, I don't know your specific arrangement.  Generally, a well running system without problems should purge itself with aggressive driving.  Light throttling doesn't have enough push to flush well.

This is a good opportunity to refresh other items if they are over ten years old.

Edited yesterday at 04:13 PM by IamScotticus