Engine cooling system

79TA7.6

Veteran Member
Joined
May 8, 2006
Location
Live: Wilbur/Creston; Work: Moses Lake Washington
TDI
2003 Jetta GLS 5 Speed
So I was driving to work this morning when my low coolant light came on. I have had issues in the past with a slow leak at the t-stat housing, which I thought I fixed by adding a bead of silicone to the flange, so I was not too worried. The temp gauge did not move, and yes I know this is just a dummy temp gauge as it does not read true. I did not want to pull over and add and coolant anyway as it was below zero Fahrenheit this morning driving in and the engine would have been too warm to add that cold of coolant. After work I popped the hood and the coolant was not just low, but the entire resivour was empty. I started looking around the engine bay to see if I could find the culprit when I finally realized my thermostat housing was loose. Mind you I have had nearly 50k miles of driving with my “fixed” t-stat housing repair. The bottom bolt was almost completely out. The only thing I can atribibute this to would be all the harsh roads I have been driving this winter. I have been bouncing over some pretty narly drifts getting to work. Especially this last week. The snow has been drifting real bad this year. Is it possible that all the bouncing could have loosened the bolts? I have been driving this car for 250k miles and never had the bolts come loose.

The other question I have is it seems that I only lost about 2 liters of fluid, the best I can figure anyway. I tried to burp the system and get nothing. I drove it until the temp gauge read normal and a little long to make sure the engine was up to temp. Checked the resivour and it had not gone down any. Decided to check the radiator hoses and the hose attached to the t-stat housing is stone cold. Drove it another 10 miles and rechecked. Same thing. I drove all the way home, 65 miles in total, and I have zero heat in the t-stat hose. I hooked up my VCDS to check actual coolant temp and it is sitting at 90c. Is it possible that my car is not heating up enough to open the t-stat? Coming home it was about 20-25f. I have the radiator 2/3 blocked with coroplast if that makes any difference.
 

Nero Morg

Top Post Dawg
Joined
Oct 19, 2017
Location
OR
TDI
2003 Jetta wagon, 2001 Jetta TDI, 2014 Passat TDI
I'm just a pinch south of you and we're getting hammered with some bad wind chill. Still fully warms up, but takes an extra mile or two.

I've learned that these cars, some burp air, some actually push all the air out when filling up. Are you getting coolant flow out of the small line to the surge tank? If no, then your water pump is bad. I'd also suggest changing the thermostats housing out. Over time the plastic gets brittle. Just as a start.
 

79TA7.6

Veteran Member
Joined
May 8, 2006
Location
Live: Wilbur/Creston; Work: Moses Lake Washington
TDI
2003 Jetta GLS 5 Speed
Thanks Nero. I checked this morning and my overflow bottle is being filled with hot coolant from the small line going into it. Water pump is fine. I have changed the coolant many times on this car, the last being 15k miles ago when I did the timing belt. Water pump was replaced new at that time with whatever the upgrade from idparts is. I know these can fail, but pretty unlikely at only 15k miles, and I obviously have coolant flow if my overflow is being circulated. In all the times I have changed coolant I have never had an issue getting the car to accept coolant. Also, the t-stat housing was changed 50k ago. That is not why it came loose. The bolt quite literally came out of the engine block. I got it screwed back in last night at work and double checked it when I got home. I am not super worried about why the housing came loose as it was likely operator error when tightening it. It is likely compromised now so when it warms up a little I will probably change it again. My biggest concern is why my t-stat does not seem to be opening. Again, according to VCDS, I am not running hot, but maybe just a hair under where it should open.
 

jettawreck

Top Post Dawg
Joined
Aug 2, 2004
Location
Northern Minnesota-55744
TDI
2001 Jetta and 2003 Jetta
If you are running the cabin blower fan much speed over "2" you won't probably build enough extra heat to cause the stat to open-unless you are working the engine pretty good.
Perhaps the edge of the stat has been caught under the flange o-ring (cause the prior leak and eventually lead to the bolts coming loose??) and enough cold coolant is entering past the stat and keeping temps slightly cooler than it would be if everything was correct.
My understanding is the coolant flows from the radiator to the stat, so unless the stat is open the radiator/stat housing hose would be cold.
Couple wild guesses...
 
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79TA7.6

Veteran Member
Joined
May 8, 2006
Location
Live: Wilbur/Creston; Work: Moses Lake Washington
TDI
2003 Jetta GLS 5 Speed
I had thought about the heater core cooling the coolant too much so I drove home last night with the fan off once the cabin got warm enough.

I did not think about the o-ring maybe causing a non perfect seal, therefore allowing the housing to not sit flush. This makes perfect sense.
 

Nero Morg

Top Post Dawg
Joined
Oct 19, 2017
Location
OR
TDI
2003 Jetta wagon, 2001 Jetta TDI, 2014 Passat TDI
I'm going to second the thermostat. Unless one of your cooling fans is stuck on, but I doubt that. Last night on my way home from work it was 28°f and it warmed up enough for the upper hose to get hot. Good thing these thermostats are easy to change.

As for the bolt coming loose, it may have been operator error.
 

BobnOH

not-a-mechanic
Joined
May 29, 2004
Location
central Ohio
TDI
New Beetle 2003 manual
Coolant systems need a certain pressure, so even a small leak will cause them to misbehave. Recommend r+r the thermostat. Use just the rubber seal with clean surfaces.
 
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