Engine Help diagnose random overheating??

TomOsiris

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May 19, 2019
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I have been having a random overheating/coolant issue since I've gotten the car out this year from sitting over the winter.

Stock bottom end HCI setup, 180 tstat, dual electric fans, sve radiator, coolant temp sensor is new, stock temp gauge

I noticed the car was overheating the first start of the year, which I attributed to a bad tstat and replaced, which seemed to work for a while.
However I had a friend following me the other day who told me my coolant overflow bottle was spilling out on to the ground, even though the car was running very cool 180 deg.
That cap will only open and allow overflow at very high pressure, and without very hot coolant there shouldnt be enough pressure in the system to open the cap.
To me, this would signal a head gasket issue, as it would be dumping combustion pressure into the cooling system, causing the rad cap to pressure release into the overflow,
However, my mechanic did a pressure test on the cooling system, and it held overnight without losing a lb of pressure, you would think a blown gasket would leak both ways?
This is an NA car so I doubt the heads are lifting or stretching the head bolts, it only makes around 300 hp.

After replacing the tstat, and pressure testing the coolant system I decided to see if I could replicate the issue so I drove the car for a week or 2 with no issues.
However last night when I pulled into my driveway, the car was badly overheating extremely quickly.
I had been monitoring the temps the whole ride home and it was running very cool on the stock gauge, but (luckily) as soon as I got to my house it was VERY hot. 250+ temp
This had happened over the course of one or two minutes at most.
I shut it down and let it cool for a bit, started again and it was still rapidly overheating, at idle the needle was climbing rapidly.

It was behaving exactly like a stuck closed tstat, however the tstat was already wide open the whole ride home and the car never went under temp to close the tstat.
I opened the radiator cap which was under high pressure but not enough to open the overflow valve, and the coolant that shot out was NOT 250+ temp as the gauge was showing.
The hoses and fan shroud were warm but not hot, it seemed as if the coolant in the engine was extremely hot, but in the radiator was not.
It was overheating so rapidly that I assumed it had to be either closed tstat, or the water pump not spinning, so I removed and inspected it, there is nothing wrong with the pump that I could tell.
I tested the tstat which was brand new anyways, and it operated correctly in the boiling water test.

I put the same water pump and thermostat back on today and test drove the car pretty hard and it did NOT overheat.

My experience with head gaskets leads me to believe that combustion is slowly leaking into the cooling system and creating an air pocket at the high point at the tstat, which would cause it to close because it is not submerged in hot fluid.

My mechanic is convinced a blown gasket would show in the the coolant pressure test, but has not offered a different idea for the symptoms.
We have not compression tested the cylinders, because previous owner seized the plugs into the heads, there has never been a misfire or spark related issue however.

Also, I used a test kit to test for exhaust gasses in the radiator, the blue fluid turned slightly blue green but did not turn yellow.

I would not prefer to not tear this engine down during driving season for obvious reasons.
I think its obvious that I should do a compression test, however these plugs are so stuck there is serious risk of damaging the threads or snapping them off in the head, as confirmed by my mechanic.
We are hesitant to do anything that could result in needing to pull the heads.

The car runs amazing, idles smooth, power throughout the rpms, driven hard often. It does not skip a beat or blow smoke out the exhaust at startup.
Help me decide what to do next to diagnose and correct this very intermittent and frustrating issue.
 
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TomOsiris

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May 19, 2019
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Change the radiator cap.
Your mechanic can put some dye in the coolant and test for head gasket leaks.
Radiator cap is brand new and functioning as it should. The question is why is there excessive pressure in the cooling system at some times, but not others?
Also not sure what dye would help, its not leaking coolant anywhere.
Could you have air trapped in the system?
It is definitely behaving as if there is an air pocket trapped at the thermostat, that is the only reason I can think of why a thermostat would close on an already hot engine. However, I am inclined to believe it is not "air", but a pocket of compression/combustion that is sneaking past the head gasket. The pressure test we did should eliminate any other way of air getting into the system.
 

General karthief

wonder how much it would cost to ship you a pair
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If it passed the pressure test I would look at another reason for the overheating like air pocket.
Did the new thermostat have a small hole in the body and do you have it at the top?
 

limp

wrap a little cheese around it and its a done
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Radiator cap is brand new and functioning as it should. The question is why is there excessive pressure in the cooling system at some times, but not others?
Also not sure what dye would help, its not leaking coolant anywhere.

It is definitely behaving as if there is an air pocket trapped at the thermostat, that is the only reason I can think of why a thermostat would close on an already hot engine. However, I am inclined to believe it is not "air", but a pocket of compression/combustion that is sneaking past the head gasket. The pressure test we did should eliminate any other way of air getting into the system.
Had the same thing happen on a Subaru, which were known for their bad head gaskets....
 

TomOsiris

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May 19, 2019
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If it passed the pressure test I would look at another reason for the overheating like air pocket.
Did the new thermostat have a small hole in the body and do you have it at the top?
There is no hole in the thermostat, but I have heard of people drilling one and setting at the top as you described.
I will try that next, thanks for the response, I'm open to ideas.
Had the same thing happen on a Subaru, which were known for their bad head gaskets....
yea this is how my old 5.4 F-350 also acted with a blown head gasket, remedy was to remove the tstat and drill a hole in the rad cap to not allow it to pressurize.
Drove it like that for 2 years before it finally gave out completely, with no heat! Winters were not fun needless to say.
I am hoping thats not the case here, but with this car its a different story and I would obviously pull the heads if I was 100% sure, but don't want to do that until I rule out everything else.
 

7991LXnSHO

wanna catch the space herp
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Radiator cap is brand new and functioning as it should. The question is why is there excessive pressure in the cooling system at some times, but not others?
Also not sure what dye would help, its not leaking coolant anywhere.

It is definitely behaving as if there is an air pocket trapped at the thermostat, that is the only reason I can think of why a thermostat would close on an already hot engine. However, I am inclined to believe it is not "air", but a pocket of compression/combustion that is sneaking past the head gasket. The pressure test we did should eliminate any other way of air getting into the system.
How about a cylinder pressure leak down test, being sure to check for coolant bubbles?
 

TomOsiris

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We have not compression tested the cylinders, because previous owner seized the plugs into the heads
^^^
How about a cylinder pressure leak down test, being sure to check for coolant bubbles?
This would certainly be next on my list, if I could get the damn plugs out without taking the threads of the head out with them!
They are seized so tightly that my mechanic was unwilling to continue trying to take them out, for risk of being responsible for further damage
 

7991LXnSHO

wanna catch the space herp
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^^^

This would certainly be next on my list, if I could get the damn plugs out without taking the threads of the head out with them!
They are seized so tightly that my mechanic was unwilling to continue trying to take them out, for risk of being responsible for further damage
I missed that line while reviewing your posts. It sounds like a good time for Liquid Wrench penetrating oil, and after many applications, one the thermal shock freezing products.
 
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7991LXnSHO

wanna catch the space herp
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No worries, I typed alot
I think I'm gonna have to give this a shot, couldn't hurt at this point.
Thanks for the input
It would help if I could proofread before posting. :doh: Here is the “of” I left out. of

I figure you already are applying heat each time you drive, so freezing seems safer than a torch with AL heads and limited space.
Timecerts might be in the works if it does not work.
 

KRUISR

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Apr 16, 2015
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Help me decide what to do next to diagnose and correct this very intermittent and frustrating issue.
Here is what I would try. With rad cap off, start engine and let warm up and watch the t-stat open (coolant will suck down in the rad and flow will begin coming out of the rad tubes into the tank with the rad cap. Let it run this way for at least 5 minutes, adding coolant in small increments to keep level at least half full in the tank. At some point the fluid level rise in the tank and will overflow if you don't put cap back on - put the cap back on. This is how I have bled my system whenever it has been drained/opened and refilled. I will let it continue to run for a while still making sure the cooling fans come on (if they haven't already). I will top up the overflow bottle to the fill line and shut the car down. Over the next few times I use the car I will check the overflow bottle is at level.

However I had a friend following me the other day who told me my coolant overflow bottle was spilling out on to the ground, even though the car was running very cool 180 deg.
That cap will only open and allow overflow at very high pressure, and without very hot coolant there shouldn't be enough pressure in the system to open the cap.
Overflow bottle does not pressurize, it is open to atmosphere and will spew onto the ground if overfilled.
 

General karthief

wonder how much it would cost to ship you a pair
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Before you pull the heads try jacking up the front of the car as high as possible, take the radiator cap off, squeeze the lower hose many times, start the engine and let it run. When it starts to warm up squeeze the lower hose several times, let the engine run, continuing to squeeze the lower hose until the hose gets hot.
The idea is to get the top of the radiator higher than the engine and squeezing the hose should push the air bubble out to the radiator.
Lower the car and continue to monitor the coolant level for a couple heat cycles.
At some point you need to address the spark plug issues, the above advice using liquid wrench between a few heat cycles is good, I would not be afraid of using a long breaker bar or even an impact on them, when you get them out use anti-sieze on the threads of the new plugs.
 

90sickfox

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The overflow regulates the fluid level inside the radiator. If fluid is pushed out of the bottle it'll make the radiator low putting an air bubble in the system when the system tries to pull coolant and instead gets air.

First thing to check is for the air dam under the core support and the panels that are supposed to be on the sides of the radiator opening between the core support and the bumper crash support. There should also be one on each side running back around the sway bar along the bottom of the front frame rails. These things are very important if on and off highway speeds. The fans also have to be set right and move enough air...in the right direction. Electric fans have been the cause of many mustang overheating issues. If the car runs hot and pushes fluid out of the overflow, and doesn't get topped back off with coolant, it'll make an air pocket that will block the coolant in the engine. That coolant will start boiling hot but can't go anywhere. Coolant in radiator will stay just warm while the water pump just pushes against the air that's stuck at the thermostat.

Just FYI... if the fan is spinning while you are driving at highway speeds it can actually cut off flow through the radiator....especially of you don't have the air dams to direct air through the radiator. Air will flow to the area of least resistance. At highway speeds air can go around the radiator instead of through it.

An awesome way to check if it's the fans is to install a factory clutch style fan on it with a factory fan shroud.

Engine coolant temps have a sweet spot for efficiency. My car likes to run at 202° with cam, heads, and turbo. I run a 192° thermostat like factory. I run Contour fans controlled by mspnp2.
 

TomOsiris

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May 19, 2019
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Overflow bottle does not pressurize, it is open to atmosphere and will spew onto the ground if overfilled.
What I meant is that the RADIATOR cap was opened by pressure, which is the only way that fluid can get into the overflow bottle, the rad cap seals below the overflow hole so with sufficient pressure it will open and allow flow into the overflow bottle
Before you pull the heads try jacking up the front of the car as high as possible, take the radiator cap off, squeeze the lower hose many times, start the engine and let it run. When it starts to warm up squeeze the lower hose several times, let the engine run, continuing to squeeze the lower hose until the hose gets hot.
The idea is to get the top of the radiator higher than the engine and squeezing the hose should push the air bubble out to the radiator.
Lower the car and continue to monitor the coolant level for a couple heat cycles.
At some point you need to address the spark plug issues, the above advice using liquid wrench between a few heat cycles is good, I would not be afraid of using a long breaker bar or even an impact on them, when you get them out use anti-sieze on the threads of the new plugs.
I am going to try a few new techniques such as this to ensure there is no air trapped in the cooling system, thanks for the response.
Spark plugs ugghh I know I have to do something about them I just really hate having the car down during driving season, winter is long enough as it is..
The overflow regulates the fluid level inside the radiator. If fluid is pushed out of the bottle it'll make the radiator low putting an air bubble in the system when the system tries to pull coolant and instead gets air.

First thing to check is for the air dam under the core support and the panels that are supposed to be on the sides of the radiator opening between the core support and the bumper crash support. There should also be one on each side running back around the sway bar along the bottom of the front frame rails. These things are very important if on and off highway speeds. The fans also have to be set right and move enough air...in the right direction. Electric fans have been the cause of many mustang overheating issues. If the car runs hot and pushes fluid out of the overflow, and doesn't get topped back off with coolant, it'll make an air pocket that will block the coolant in the engine. That coolant will start boiling hot but can't go anywhere. Coolant in radiator will stay just warm while the water pump just pushes against the air that's stuck at the thermostat.

Just FYI... if the fan is spinning while you are driving at highway speeds it can actually cut off flow through the radiator....especially of you don't have the air dams to direct air through the radiator. Air will flow to the area of least resistance. At highway speeds air can go around the radiator instead of through it.

An awesome way to check if it's the fans is to install a factory clutch style fan on it with a factory fan shroud.

Engine coolant temps have a sweet spot for efficiency. My car likes to run at 202° with cam, heads, and turbo. I run a 192° thermostat like factory. I run Contour fans controlled by mspnp2.
So I think part of my problem may be that my overflow bottle is too small, this was suggested by my mechanic.
It doesnt have any sort of fill lines hot/cold etc either.
Your point about pulling air instead of coolant from the bottle is certainly something to think about.
I should probably mention this car has NEVER overheated at highway speeds, the air dams are in place, and the electric fans move plenty of air to keep the car running cool.
This overheating happened at idle in my driveway, and was acting like either a stopped water pump or a closed thermostat, I confirmed the pump was working so I am sure the thermostat was closed due to being trapped in a pocket of air.
I am going to try all of the suggested filling/bleeding techniques above and probably get a larger overflow bottle as well.
Thanks for you response, appreciated.
 

TomOsiris

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May 19, 2019
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Do you have a spill free funnel for filling the radiator? I find that helps tremendously. In fact, I haven’t had any air in a cooling system since I started using one.
No I do not, I have heard mixed things about these as far as if they actually help anything or just make it easier, I was suggested by my mechanic to let the coolant "find it's level" by overflowing but this seems to me like that would just be introducing air into the system? i.e. pushing coolant out but not pulling it back in? I may end up grabbing one of those funnels though, couldnt hurt at this point.
Thanks for you response
 

KRUISR

5 Year Member
Apr 16, 2015
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I should probably mention this car has NEVER overheated at highway speeds, the air dams are in place, and the electric fans move plenty of air to keep the car running cool.
If driving at highway speeds I have never had my fans come on. Air moving through rad is enough to keep things cool. Where is you temp sensor that controls the fans located (intake, t-stat housing, heater hose, lower rad hose, ...)? What is the temperature that they trigger at?
 

TomOsiris

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May 19, 2019
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If driving at highway speeds I have never had my fans come on. Air moving through rad is enough to keep things cool. Where is you temp sensor that controls the fans located (intake, t-stat housing, heater hose, lower rad hose, ...)? What is the temperature that they trigger at?
The fans are hooked to a sensor that threads into the port at the bottom of the radiator, and I have a knob hooked up to adjust what temp they kick on at, I have it turned all the way down so I would guess they come on at like 180-190? I should also mention that this is a drift car as well as a general cruiser so cooling is more intensive than normal driving, lots of 1st and 2nd gear donuts, figure 8s etc, situations with extended periods of high rpm without a lot of speed or airflow over the radiator.