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Old 02-25-2015, 09:42 AM   #1
Gerald81
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Default Mystifying Coolant Problem [Update: Solved(?)]

I normally monitor oil and coolant temperature via Defi gauges. Normally, both temperatures be at about 30C on a cold start, and will rise together and stabilize at about 85C (185F) under normal daily driving.

Recently, I observed that the coolant temperature would take a much longer time to heat up. When oil temperature is already at 70 and rising to 80, the coolant temperature is still only up to 40ish. After prolonged driving, oil temperature will reach between 80C to 85C as per normal, but coolant temperature will vary between 60C to 70C. At traffic stops it will gradually drop to 60C or even lower until I move off.

The only time I can get the coolant temperature to rise higher than 70C is through keeping consistently high RPMs (between 3500 to 4500), in which case it will rise up to 90C. Oil will follow suit to about 90C as well. But once I back off or when at a traffic stop, the coolant drops all the way back to 60C to 70C again.

The OEM dash gauge seems to indicate that coolant temperature is rising at a normal pace from a cold start as before, and stays at its typical position at slightly above the second mark throughout my drives.

Also, ever since the coolant temperatures have gone wonky, the coolant level in the expansion tank is way up beyond normal levels usually observed after turning off the engine following a drive (where it used to end up slightly above the HIGH mark, it is now nearly at the filler cap). Furthermore, the coolant level in the expansion tank has remained at the same high level even after the engine has cooled off for several hours. Previously, it would drop back down to lower than the HIGH mark after the engine has cooled down.

The coolant looks normal but seems to be swirling. The engine oil also looks normal with no signs of oil loss.

Couple of other information:
During the engine rebuild, I switched the water pump from the stamped open impeller design to a closed impeller design. But this problem also surfaced recently, and not immediately after the rebuild. I also have a Trust oil cooler installed (with 70C thermostat) which I have been using prior to the rebuild with no issues. I am using STI 1.3bar radiator caps on the upper reservoir and the radiator. This has been my setup previously prior to the rebuild with no issues as well.

Anyone has any ideas on what could be going on with my coolant? Low temps are nice but this is a little worrying.
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Last edited by Gerald81; 01-09-2017 at 11:06 AM. Reason: Update
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Old 02-25-2015, 10:03 AM   #2
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sounds like an issue with your t-stat sticking open
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Old 02-25-2015, 10:08 AM   #3
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Thanks, I thought it could be the thermostat as well. But will this also cause the coolant in the expansion tank to fail to return after the engine has cooled down?
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Old 02-25-2015, 10:42 AM   #4
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i just re-read your post, and saw you are using 2 STi rad caps. You only need 1 on the coolant header tank by your turbo. The one on the radiator should be rated higher pressure. Notice in stock setups that the rad caps with the ears are on the coolant header tank..designed to blow off to the overflow. The cap with no ears is on the radiator itself. I'm pretty sure that is your issue with the failure to return the coolant.
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Old 02-25-2015, 11:49 AM   #5
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Alright, I read that too on other threads, but seems strange that I had been using this setup for so long without issue prior to this. So sounds like the thermostat and radiator cap both failed at the same time?
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Old 02-25-2015, 11:57 AM   #6
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Btw, don't the stock thermostats stay closed when they fail? So the coolant temperature should increase instead of decrease, right?
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Old 02-25-2015, 12:26 PM   #7
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i doubt both the tstat and rad cap failed at the same time. You have to troubleshoot and investigate both separately. Unfortunately checking the t-stat makes a mess.
FWIW, i've seen stock t-stats on other makes that fail partially open. You didn't mention if you were running stock or aftermarket in your original post. Your symptoms seem to point to a tstat issue or an air pocket issue if it wasn't burped and refilled properly. I assumed you burped it properly.
Good luck..let us know what you find!
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Old 02-25-2015, 01:22 PM   #8
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Replace the t-stat with an oem one. They aren't that much. Fill and properly bleed the system and you should be good to go. Suby t-stats tend to fail open or partially open. However they don't fail all that often. Might not be a bad idea to do a system flush while you're there.


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Old 02-25-2015, 05:37 PM   #9
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mine was doing the same thing with a failed t-stat.
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Old 02-26-2015, 07:39 AM   #10
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Ok so with the car having cooled overnight, and expansion/overflow tank still full, I tried opening the upper coolant reservoir cap and the radiator cap, and coolant started spewing out. I quickly screwed the caps back on.

After driving for about 10 minutes, I saw that as usual, the oil temperature was normal at about 80C, and the coolant temperature was at about 60C. However this time I noticed that the OEM dash gauge was starting to rise from its normal resting point at just above the 1/3 mark, and was heading towards the 2/3 mark. I quickly parked the car and turned off the engine, thinking I may have introduced some air into the cooling system. I checked the expansion tank and sure enough there were bubbles coming up to the surface.

I had a shop look at the car, and as expected, it was found that the Defi water temperature gauge was not showing a consistent reading to the ECU output. I had the lower radiator cap replaced with a stock 137kPA piece, while retaining the STI 1.3 bar cap on the upper reservoir. Air was burped from the system and we turned on the engine again and this time, the reading on the Defi gauge started to rise quickly to match what the ECU was showing.

On my drive back home the temperatures on both oil and water gauges seemed to behave normally. The OEM dash gauge also did not rise beyond its usual resting place. However, upon reaching home, I turned off the engine and checked the expansion tank, and noticed it was bubbling. It was slightly different that what was observed before, where the bubbles were visible on the surface and popping after 1 or 2 seconds of surfacing. This time it was coming up almost rhythmically, and popped immediately on reaching the surface, always at the same area. In fact I did not even notice the bubbles at first, and only saw that the surface of the coolant kept getting displaced, which prompted me to look more carefully.

Now am worried if this could be a head gasket issue? Or maybe still some air left in the system?
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Old 02-26-2015, 09:19 AM   #11
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could be still some air in the system. If the coolant isn't being siphoned back to the coolant header tank when completely cool, check your hoses from the overflow tank to the header tank. A crack in the hose between the tanks may be the problem.

if that idea is exhausted, test those bubbles with a block test kit to determine a headgasket issue.
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Old 02-26-2015, 10:54 AM   #12
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I tried burping more air from the system. Opened the upper reservoir cap but it was more or less full to begin with (except for some volume of coolant that squirted out when I opened it). Topped it off anyhow, and capped it back. Started the engine with heater on full.

Got the engine up to temperature (>90C water temp) and checked the expansion tank. It was still bubbling, this time it seemed more bubbly. I took a video which I am in the process of uploading, and will share the link shortly.

EDIT:
Not sure if relevant, but the bubbling did start even BEFORE the engine reached operating temps and the fans kicked in.
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Old 02-26-2015, 11:08 AM   #13
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Wouldn't coolant start circulating before the engine reached op temps?
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Old 02-26-2015, 11:11 AM   #14
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Link to video:

After turning off the engine, there was an occasional bubble for a couple of minutes before it stopped altogether.
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Old 02-26-2015, 03:35 PM   #15
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change your thermostat and go from there. I know you don't want to drain your coolant but it has to be done. If your head gasket is leaking, it will probably be because it overheated because of the bad t-stat.

Last time I did it, i also did not want to waste coolant so i used these gearwrench hose pinch off pliers on the lower rad hose. Only had to top up about 1-2 liters instead of the 8+ liters my koyo holds.



Bubbles are never a good sign...

also you basically replaced your rad cap with the same thing. 1.3bar = 130kPa so you just increased your lower rad pressure by ~1psi.
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Old 02-27-2015, 09:10 AM   #16
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Another update - stranger things have happened.

So after I left the engine to cool overnight, I checked the coolant level in the overflow tank this morning, and it was HIGHER than when I just turned off the hot engine yesterday. Normally, I would expect the coolant level to fall as the system loses pressure when the engine cools down, but this is the exact opposite.

Anyway, on my drive to the shop, same issue occurred. Defi water temp was showing low readings between 60C (140F) to 70C (158F), but OEM dash gauge was creeping up. I turned on the heater at full blast and it never moved much above halfway and certainly did not reach the 2/3 mark, and would occasionally drop back to just above the 1/3 mark. But this is certainly not normal and I expect the coolant temp to have been between 100C to 110C (210F to 230F) when the needles moves up from its usual resting spot. Throughout all this, the oil temperature never rose much higher than 90C (194F). I would have thought if the coolant was boiling hot at >100C, the oil temperature would at least be in the high 90C+ range or beyond.

Once at the shop, for some reason, after giving it a few good stabs on the gas pedal while in neutral, the Defi water temp started to rise rapidly and matched the ECU readings. Took the car out for a test drive with light boost (<6 PSI) and checked the overflow tank - no bubbles whatsoever. Went another round with slightly more boost (~9 PSI) and higher rpms, and saw that the coolant in the overflow tank was bubbling (similar to the video I posted above).

Brought the car back to the shop and did a cylinder leak down test. Results are below:

Cylinder 1:


Cylinder 2:


Cylinder 3:


Cylinder 4:


Generally, they all looked ok to me, although cylinders 2 and 4 seemed to have slightly higher leakage compared to 1 and 3. Could this mean the head gasket on 2 and 4 is shot? There were no bubbles observed in the upper reservoir throughout the entire leak down test.

The car is now being left overnight awaiting a thermostat change as well as further procedures to verify if the Defi water temperature sensor is working correctly.

Meanwhile, what do you guys think? Small head gasket leak? Air in the system still?

EDIT: The leak down test was zeroed in the middle of the "SET" area, which I suppose is 5%. So should I read the leakage as X% minus 5%?

Last edited by Gerald81; 02-27-2015 at 10:19 AM.
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Old 02-27-2015, 09:22 AM   #17
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where is the defi sensor located?

curious as to why you went with a 70c oil cooler tstat? that's too low imho: you need the oil to get above the boiling point of water to drive out moisture.
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Old 02-27-2015, 09:59 AM   #18
Gerald81
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The Defi water temp sensor is located on the upper radiator hose (the plug is integrated in the hose - it's a Blitz radiator hose).

The thermostat on the oil cooler came with the oil cooler from Trust so I just stuck to it. Yeah the oil temp is a bit low and stabilises around 80C to 85C on normal driving, but never had any issue with it so far.
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Old 02-27-2015, 12:19 PM   #19
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did you remove the oem oil/water heat exchanger? your oil could be cooling your coolant!
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Old 02-28-2015, 04:32 AM   #20
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I don't think the oil is cooling the coolant.

Anyway, the guys at the shop found that the cooling system was pressurized even after the engine was completely cooled down. So current prognosis is a head gasket leak that is allowing combustion gases into the coolant. Apparently, the leak down test may not be able to show the impact of a small head gasket leak.

Hydrocarbon test will be done next week and we'll take it from there...
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Old 03-02-2015, 08:06 PM   #21
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Alright so here are the latest (repeatable) observations:

If the car is driven right after the air in the system is burped, no overheating occurs. However, the coolant overflow tank will be bubbling after the drive, and the coolant level will rise AFTER the engine is shut off overnight If the air is not relieved when I next drive it, it will overheat very quickly upon driving (dash gauge rises close to 2/3 within 5 minutes).

Hydrocarbon test turned up negative. But if not a head gasket leak, I'm not sure where else the air is coming from. It seems to be a significant amount of air as the coolant level in the overflow will rise by about 2 to 3 inches. Is it possible that the hydrocarbon test is unable to pick up a small head gasket leak? This would however contradict the seemongly large amount of air in the system though.
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Old 03-03-2015, 01:09 AM   #22
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Quote:
Originally Posted by Gerald81 View Post
Alright so here are the latest (repeatable) observations:

If the car is driven right after the air in the system is burped, no overheating occurs. However, the coolant overflow tank will be bubbling after the drive, and the coolant level will rise AFTER the engine is shut off overnight If the air is not relieved when I next drive it, it will overheat very quickly upon driving (dash gauge rises close to 2/3 within 5 minutes).

Hydrocarbon test turned up negative. But if not a head gasket leak, I'm not sure where else the air is coming from. It seems to be a significant amount of air as the coolant level in the overflow will rise by about 2 to 3 inches. Is it possible that the hydrocarbon test is unable to pick up a small head gasket leak? This would however contradict the seemongly large amount of air in the system though.

no...don't think so

you MAY have a hose that isn't quite clamped tight
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Old 03-03-2015, 11:43 AM   #23
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Yeah I'm thinking there has to be a leak somewhere that is allowing air in, be it via the head gasket or a hose.

Is the air in the engine cylinder under pressure after the engine is shut off? I'm wondering if it is a head gasket leak, perhaps once the cooling system cools off and de-pressurises itself, the pressurised air in the cylinder(s) where the head gasket is leaking is able to seep into the cooling system overnight, hence the symptoms observed where the coolant level is elevated overnight.

Whatever the case may be, the guys at the shop have now changed the thermostat (swapped to a new stock piece), burped the system thoroughly, and as observed before, the car drove perfectly fine with no overheating (right after burping), and the Defi water temperature sensor was generally consistent with the ECU reading. The only problem is that the coolant overflow tank is still bubbling after the test drive. Basically, similar to what was observed before.

The car was left to sit overnight, and it was observed in the morning that the coolant level was now very slightly lower than the "warm" level noted the day before. Pictures below:

Warm level (at 92C):


Cool level (at 30C):


It is really marginal to me and from what I used to recall, the difference in coolant level between 90C and 30C was definitely more noticeable, at least 1/2 to 3/4 of an inch. So I'm not 100% convinced the car is back to normal. The good news was that, at least the coolant level did not rise as before, and when taken out for another test drive (without attempting to relieve any air out after sitting overnight), it did not overheat as it used to before, and there was no bubbling in the overflow tank now. However, the Defi water temp sensor was now lagging behind the ECU reading (again). For example, when the ECU read 88C, the Defi gauge would show only 70C+.

Would the fact that the Defi sensor is located on the upper radiator hose cause such a discrepancy, especially if the stock t-stat has yet to fully open?

Could a faulty t-stat really have caused bubbles in coolant and pressurised air in the cooling system overnight?
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Old 03-04-2015, 03:13 AM   #24
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T-stat. Atleast yours is stuck open not closed lol

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Old 03-04-2015, 11:13 AM   #25
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Looks like I'm not out of the woods yet. Today, the new stock t-stat failed to open and the OEM gauge started to rise, again. Coolant temp was logged this time at about 104C which registered itself in the gauge as slightly above the halfway mark.

So looks like some air is still making its way into the system and is interfering with the t-stat recognising the proper temps needed to start opening. Hydrocarbon test came back negative again but I'm really not sure about anything at this time...
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