Normal ALH Coolant Temp - Replace Thermostat ?

bogcaster

Active member
Joined
Nov 9, 2008
Location
Poole, UK
TDI
Mk IV Golf TDI SE
Not sure if I need to replace my Coolant Thermostat on my 2001 ALH Golf (un modified) .....

It all started when I was fettling the injection timing following Timing belt replacement and I thought the car was struggling to get over 80 deg C.

VCDS Block 7 logging reports warmed up temperature (G62) between 76.5 and 84 deg C. With most of the time around 80 deg C. Logging started at 15 mins in after starting my journey. This feels a bit low to me. New thermostats (044 121 113) say they open at 87 deg C. Does this mean they start to open or are fully open at 87 ??

External temperture according to Dashboard was just below freezing between -2 to 0 deg C through the journey.

G81 Fuel intake temp was between 20 to 28 deg C in the same period and similarly G72 Air Intake was between 3.6 and 8 deg whilst moving. The Air Intake temp rose as I came to a standstill to 12 deg as expected.

This was recorded over a 55 mile (just over an hour) journey all on motorway (highway) at 60 mph, approx 2000 rpm 5th gear. The car returned 68mpg (UK gallons) and has always historically returned this sort of good mileage from new 11 years ago. Its now done 200k miles.

Question : Do I need to replace the thermostat, is it really worth disturbing it ? On one hand, my engineering experience says, if it works well - why change it ? Equally - like many of you, I like to optimise for fuel efficiency in this age of rapidly rising diesel prices ? What realy mpg difference will 5 to 10 deg make ?


Many thanks!
 

Yblocker

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Sep 1, 2010
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Oakland, CA
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1997 Passat
My 1Z was behaving much like yours- operating "a bit low", so I was holding off until I had an excuse to change it. As it happened, I began seeing coolant under the car on cool mornings so I knew I had a leak. The leak was from the connection between the thermostat housing and lower hose elbow...anyway yours is different, but if the coolant has never been changed in 200k miles, maybe that's reason enough.

Automotive thermostats begin opening at their rated temperature, and are fully open about 5 degree C above that.
 

bogcaster

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Nov 9, 2008
Location
Poole, UK
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Mk IV Golf TDI SE
Thanks for reply. I have been thinking of an excuse too - perhaps Coolant change is the one. Its had many top ups with G12 mix after water pump change and thermo sensor change but never in its entireity.

I guess my thermostat is starting to open way before 87 as it struggles to go above 85.

Did the thermostat change make any difference ?
 
Last edited:

Yblocker

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Sep 1, 2010
Location
Oakland, CA
TDI
1997 Passat
Mine was never reading over 80 C according to vagcom. I have not checked it with vagcom since I swapped, but it does read higher on the gauge (195 F indicated) for what that's worth.
 

Fauvay

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Joined
Oct 12, 2011
Location
Minneapolis
TDI
2000 Golf
I swapped out my OEM thermostat that was not allowing the car to get up to temp at 150k miles. A little spilled coolant later, I was back to straight up at 190F.
 

Strykeviper

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Mar 17, 2010
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Orlando, FL
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2000 Jetta GLS 5 speed swap, 2002 Jetta Wagon, 2015 Passat SEL, 2013 Jetta Sportwagen
My 2001 was reading around 80 on vcds. I changed it and it now reads around 90-95. The gauge will read 190 no matter what if it is close to the correct temp. Some people call it a "dummy" gauge because of that.
 

boertje

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May 24, 2002
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Coeur d'Alene, ID
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'01, '01, '03, ‘06 NB - TDIs all.
To the OP, I replace them even if they are 5 degrees down. Better efficiency and better economy is my experience.
 

Yblocker

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Location
Oakland, CA
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1997 Passat
So you got me all curios, so when I drove to the Bee club meeting tonight, (15 miles on the freeway each way), I had the VCDS hooked up as I drove to check the temp spread on my 87 degree C thermostat. Level driving at 65 to 70 mph at 55 degrees ambient temp, it stabilized at 91/92 C. Pulling a grade, it would go to 93/94 C. That's at least 10 degrees hotter using the new 87 C thermostat compared to my original(?) one that had trouble getting the coolant over 80 C.
 

JB05

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Oct 20, 2005
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Il.USA
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Golf,2005,anthracite blue
So you got me all curios, so when I drove to the Bee club meeting tonight, (15 miles on the freeway each way), I had the VCDS hooked up as I drove to check the temp spread on my 87 degree C thermostat. Level driving at 65 to 70 mph at 55 degrees ambient temp, it stabilized at 91/92 C. Pulling a grade, it would go to 93/94 C. That's at least 10 degrees hotter using the new 87 C thermostat compared to my original(?) one that had trouble getting the coolant over 80 C.
I'm seeing the same exact temperatures since changing my t-stat recently, and I bought it from one of our on-line vendors.
 

bogcaster

Active member
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Nov 9, 2008
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Poole, UK
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Mk IV Golf TDI SE
That's it then. Getting new thermostat tommorrow. I'm going to check the coolant sensor too with a thermocouple to make sure that's not reading low as well.... Will post up results soon. Thanks all.
 

JB05

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Just so you know, my warm-up times unfortunately, did not change with the new t-stat; but again, the engine does run a bit hotter. I have my front radiator blocked off with some foam padding and I have yet to see this new thermostat open where the temperature will drop to 87C.
 

boertje

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May 24, 2002
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Coeur d'Alene, ID
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'01, '01, '03, ‘06 NB - TDIs all.
Just changed my bug Tstat today along with the alternator (oh what fun on a bug). Now the temp is 93 instead of 86. I used an OEM tstat that boraparts sells. Perfect.

Also replaced the tstat in my 06 bug BEW 2 days ago. Warmup times were cut by 2/3 and now my wife is happy because the heater throws warm air again.
 

DSL HED

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Aug 14, 1999
Location
S. Portland, Maine
TDI
2012 Jetta wagon DSG
tupdegrove - You'll lose some coolant unless you're extremely quick at swapping it. I replaced my thermostat the same day I installed my FrostHeater so I ended up adding about 1/2 gallon of fresh coolant.

The thermostat change how-to says to cover the alternator with something because coolant is going to come out of the block when you remove the thermostat/housing assembly. So be prepared! Read the how-to to make sure you're all set to do it. It's a pretty easy job if you buy all OEM parts. I bought a new thermostat, housing and O-ring and it cost me about $40 online.

Before changing it, my thermostat never got above 163 F according to my Scangauge. Now it will eventually get up to about 191 and that's with my front end covered up with pipe insulation.
 

bogcaster

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Nov 9, 2008
Location
Poole, UK
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Mk IV Golf TDI SE
Ordered an OEM thermostat today.

My coolant sensor is bang on - that does not need changing. I'm happy that its accurately and not reporting low against true temperature. I checked the coolant temp using a thermocouple on the return feed to the explansion tank across several temperatures around 80 mark, but 1st calibrating it against melting ice in water and steam.
 

tupdegrove

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Jan 13, 2012
Location
Allentown, PA
TDI
2005 Jetta TDI PD Auto
Why a new housing?

I bought a new thermostat, housing and O-ring and it cost me about $40 online.
So you covered the radiator hoses with pipe insulation? Year round? Radiator not covered?

Before changing it, my thermostat never got above 163 F according to my Scangauge. Now it will eventually get up to about 191 and that's with my front end covered up with pipe insulation.
 

bogcaster

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Poole, UK
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Mk IV Golf TDI SE
Thermostat changed. 1hr job. After 200k miles, the coolant was immaculate. Changed it anyway. Now running 10 deg hotter at 91 now. Will see what it does to economy this week. I have to say the radiator drain plug is really nifty
 

IXLR8

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Cushing, ME
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12 Passat Platinum Gray, 02 Golf Black, 01 Jetta Black
I just changed the T-stat in my 02 Golf, with 251K miles on it. The scan gauge reading of my coolant temp almost never got above 172dF on the highway and about 168dF around town. On a cold, 0dF, day last winter driving around town it wouldn't get above 158dF??!! With the new, OEM, T-stat my readings around town varied between 192-195dF. There is one hill I like to do my Italian tune-ups on, not very long but enough to get things cooking. With a rolling start at the bottom in 3rd gear at about 25mph, with WOT, by the top I am typically doing about 55mph. Yesterday, after changing T-stat and the engine up to temp, by the time I got to the top, I was doing 65mph and when I backed off I found the car had gone into limp mode and no boost. :eek: Quick off/on of key and all was back to normal... that never happened before. Time will tell if there is any mileage improvement, but there is definitely a power improvement. :) I also hope to stay warmer in my travels for the rest of the winter.
 

Yblocker

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Location
Oakland, CA
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1997 Passat
Nice. Glad to hear it worked out. Keep an eye out for leaks though. On the 1Z, the thermostat housing connects to a plastic elbow, and they are sealed with another o-ring. I had a persistent leak at this connection overnight which was only solved by a new housing and elbow. So I guess once the system is taken apart, even just to R&R the thermostat, the two piece, over engineered business (that could have just as easily be made one piece) should be replaced. I used dealer parts because I grew a bit weary of dropping that power steering pump to access the thermostat bolts!
 

bogcaster

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Nov 9, 2008
Location
Poole, UK
TDI
Mk IV Golf TDI SE
Although a bit subjective, my view is a definite improvement in Smoothness & Responsiveness. Economy has improved too by a few miles per gallon.

Definitely worth doing. Thanks for everyones contribution to this!
 

fruitcakesa

Top Post Dawg
Joined
Feb 18, 2007
Location
Vermont
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04 jetta 5 spd wagon
I waited 2 years before finally getting around to changing out the tstat and of course wish now I had done it sooner. We finally had cabin heat and 2-4 mpg economy increase! Oem parts only!
 

Henrick

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Aug 24, 2010
Location
Ireland
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Golf VI TDI, 77 kW (CAYC)
VCDS reports the coolant temperature is ~79 C after 20-30 minutes of driving. Does this sound normal or should I replace my thermostat?
 

Henrick

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Aug 24, 2010
Location
Ireland
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Golf VI TDI, 77 kW (CAYC)
Replace it. mine did exactly that until I changed it.
I did a quick check this morning. I started the car and drove some time until coolant temperature reached around 60 C (VCDS). Then I shout it off, popped the hood and grabbed the hose which goes from the T-stat housing to the radiator. It was chill. I touched the engine block and it was hot. Doesn't this quick check tell the T-stat isn't stuck open/is okay?


bogcaster, how much of coolant did you need to add after the change? How much did it spill out?
 

JB05

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Il.USA
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When I changed my t-stat I did not drain the radiator like other's have mentioned. I think as long as the t-stat is closed the coolant in the radiator will not pour out unless I am missing something. Unless opening the drain cock on the radiator will siphon all the coolant out through the top hose. IIRC, I lost about half a gallon when I pulled off the lower hose. I also removed the oil cooler hose where it attaches to the upper radiator hose, but very little came out. I left this hose disconnected when I went to refill the system in the hopes that this would purge out any or all air in the system.
 

bogcaster

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Nov 9, 2008
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Poole, UK
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Mk IV Golf TDI SE
I did a quick check this morning. I started the car and drove some time until coolant temperature reached around 60 C (VCDS). Then I shout it off, popped the hood and grabbed the hose which goes from the T-stat housing to the radiator. It was chill. I touched the engine block and it was hot. Doesn't this quick check tell the T-stat isn't stuck open/is okay?


bogcaster, how much of coolant did you need to add after the change? How much did it spill out?
I ran a vcds log before the thermo change and it clearly showed the thermostat starting to open at 75 deg instead of the 87ish. A couple of observations against your test... You are correct that the thermo is not stuck open but you also need to check at what temp the thermo opens, mine was too low. I had suspected mine was opening too early at 75 ish and to convince myself, I had put the old and new into a pot on the stove with thermometer and easily spotted the old one opening 10 deg earlier than the new.

The water from tstat housing to the rad is the cold return feed after cooling going to the engine. It should be cold. I was in an ambient 0 deg C when working on mine and it was always cold. The upper feed hose to the radiator was hot though when my tstat opened proving it was circulating thru the rad. I also think your tstat has not opened yet at 60deg. It needs to be hotter, try and get it to a stable temp which should be around 90.

After draining rad, I lost about 500ml when I pulled the thermostat out. As the coolant was cold, the thermostat was closed, and the coolant came out of the engine block. There's no gravity path to the drain plug when it's closed. To get round this, you could drain it whilst hot but I think that's a bit risky to both my hands and engine. You really want it to cool down properly avoiding hot spots.
 

Henrick

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Joined
Aug 24, 2010
Location
Ireland
TDI
Golf VI TDI, 77 kW (CAYC)
Okay, thanks. It seems that thermostats do degrade with time. If they're not stuck open/closed, then their opening temperature lowers. Now the only thing I doubt is which brand of thermostat to choose: OEM one or Wahler.

Also, I lost my sleep last night so I had time to think about car maintenance. In fact I won't replace the t-stat only, I will do the whole timing belt (since water pump was skipped at 80k miles, etc etc). I'll need 6-8 new OEM bolts, TB kit (TB, roller, tensioner, nuts, water pump), the Thermostat and probably 3 liters of G12++ to be dilluted to 6 liters of mixture. I plan to change as much coolant in the system as possible. $$$$$$....

I have already decided on particular brand for each part except the thermostat. OEM one is BEHR, it fits the housing nice but doesn't come with a seal. Wahler is aftermarket, comes with seal but doesn't fix in the housing... :confused: :confused: :confused:
 

bogcaster

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Nov 9, 2008
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Poole, UK
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Mk IV Golf TDI SE
I got a genuine vw thermo in the end. I found someone with excess stock on flea bay. 14 pounds and a pound for a seal. Good luck with the overhaul.
 
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