JTD suspect thermostat problem...but only when weather is cold - Alfa Romeo Forum
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JTD suspect thermostat problem...but only when weather is cold

Hi
Hope someone can shed some light on a suspected thermostat problem with my GT JTD
I replaced the thermostat after i bought the car in june (every car i buy seems to need that little job doing) and temp gauge has shown no issues on a journey since, goes up to 90 and stays there.
That is until the start of the cold mornings. When the outside temp shows to be less than 10 degrees on my way to work it takes ages to get to 90 and then sometimes slowly drops down to 80 the final couple of miles along 30mph zone.
Driving home with outside temp back above 10 degrees no problems.
This morning coldest so far (3 degrees) and it didn't get close to 90 the whole 11 mile trip. Got up to 80 then dropped to 70 the slow end of the journey. Drove home lunchtime and it was back to normal, no problems.
Hope all that makes sense.
Could a thermostat be affected by outside temperature?
Not had a chance to investigate under the bonnet in the mornings. When it happens again i'll feel the top hose for heat.
If anyone else has had this strange issue please let me know.
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Sounds like stat related to me,when weather is warmer to air flowing over rad is not as cold so if stat is only slightly opening too early the effect will not be as bad as if air is colder having a greater cooling effect.

What make stat did you fit?

In past on other makes of car I have used many different makes of stat and only ever found the OE ones work the best.
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It will take longer to reach thermostat opening temperature in cold weather. Engine and fluids are colder and you will be using the heater more. I suppose if speed is fairly low (generally not above 50mph) and heater temp is set to high and fan heater fan is running at high speed, it may be possible that the thermostat doesn't open.

Generally my JTS thermostat takes around 5 miles to open but half of that is 70mph on the dual carriageway.
On an A road when driving perhaps 50mph when the outside temperature is freezing, I've seen it take 10 miles.

A few years ago I did a 30 mile journey when the temperature was -20 degrees and I had to drive faster than was prudent to try to get the gauge to almost 70 degrees. Needless to say, the thermostat never opened and the radiator was cold enough to stick skin to at the end of the journey.

The heater matrix is a small radiator. The more air passing over it to try to heat the cabin, the more it will cool the coolant.

From my point of view congestion and SMART roads may only serve to slow traffic so much that engines stay cold and stay on cold start enrichment for the whole journey which equals greater emissions.
Not very smart.

Next time it seems to be staying cold, after about 5 miles, pull over, open the bonnet and feel the hoses. One eater hose should be hot, the other fairly hot but the radiator top hose should be almost stone cold if the thermostat hasn't opened.

Radiator top hose same temperature as heater hoses = defective thermostat.
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Thanks for your comments, much appreciated. I'll check the hoses next time it happens.
It was a Vernet one from Eurocarparts I fitted.
So prob a new thermostat then. Oh joy.
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Just drove home 6 miles through mostly urban conditions. Engine had not been run for over 4 hours. Outside temperature was 7 degrees. Heater temperature set to 22 degrees. Fan went to a middling speed. Thermostat didn't quite get to opening temperature.

The JTS seems to take longer to warm up than the TS but both thermostats are about 18 months old. The JTS has a stronger anti freeze mix which I think makes a difference.

My TS uses a Vernet thermostat which seems to work well.
The JTS has a Valeo thermostat.
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Originally Posted by Fruity View Post
Just drove home 6 miles through mostly urban conditions. Engine had not been run for over 4 hours. Outside temperature was 7 degrees. Heater temperature set to 22 degrees. Fan went to a middling speed. Thermostat didn't quite get to opening temperature.

The JTS seems to take longer to warm up than the TS but both thermostats are about 18 months old. The JTS has a stronger anti freeze mix which I think makes a difference.

My TS uses a Vernet thermostat which seems to work well.
The JTS has a Valeo thermostat.
Cheers for your update. Have now seen plenty of threads regarding diesels of various makes being slow to get up to temp in winter. This is the first diesel i've owned. I'll keep an eye on things but may be that's the norm for my JTD in winter.
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A diesel that is not working hard can easily cooled by the cabin heater. If mine is sitting in traffic and I whack the heater on full, the temperature will drop. On the motorway it has more than enough waste heat to run the heater.
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Also, diesels have an EGR valve to put exhaust gases back through the engine. The idea is the spent gases cannot support combustion so this keeps cylinder temperatures down. It seems anti-logical but that is the idea to lower NOX emissions.

I don't know what temperature the EGR valve function is enabled but it could be a reason diesels seem to run cooler.
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It may take longer, but IME a diesel with a thermostat working properly will eventually get to 90 and stay there, even at subzero temps.

In 20C weather my car hits 90C after 2mls of 30mph urban road - at a particular junction, it's that repeatable. In 0C it's another 2-3mls before it gets to 90C. Whatever cabin temp is set doesn't seem to make any difference.

I think any other behaviour suggests the thermostat is sticking partly open.

There are a lot of crap aftermarket thermostats about, to judge from this forum.
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Quote:
Originally Posted by halftone View Post
It may take longer, but IME a diesel with a thermostat working properly will eventually get to 90 and stay there, even at subzero temps.

In 20C weather my car hits 90C after 2mls of 30mph urban road - at a particular junction, it's that repeatable. In 0C it's another 2-3mls before it gets to 90C. Whatever cabin temp is set doesn't seem to make any difference.

I think any other behaviour suggests the thermostat is sticking partly open.

There are a lot of crap aftermarket thermostats about, to judge from this forum.
Disagree. My 156 JTD 1.9 16v has never 'stayed at 90C'. It'll approach the 'twelve o'clock' position in warm weather and traffic, but under normal driving conditons the needle will settle between 10 and 11 o'clock. The specialist tested this with the needle in this position by plugging in the laptop and all was perfect. This is with a brand new dealer supplied stat with the new tempo sensor already fitted. You know when the stat is struggling when in cold weather the needle struggles to get off the ground. These comments are based on experience of 130,000 miles with this Alfa from new.

The OP has nothing to worry about from the symptoms he has described.
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Quote:
Originally Posted by halftone View Post
It may take longer, but IME a diesel with a thermostat working properly will eventually get to 90 and stay there, even at subzero temps.

In 20C weather my car hits 90C after 2mls of 30mph urban road - at a particular junction, it's that repeatable. In 0C it's another 2-3mls before it gets to 90C. Whatever cabin temp is set doesn't seem to make any difference.

I think any other behaviour suggests the thermostat is sticking partly open.

There are a lot of crap aftermarket thermostats about, to judge from this forum.
That is my thinking aswell, my 1.9 always hits the 90 mark and stays there no matter what outside temp is or what setting heater is on. Only thing I notice is it can take a little longer in cold conditions to reach 90,it never reaches 90 and drops down if sat at lights or driving slower etc.

I suspect the thermostat is sticking partially open or is opening prematurely. Had an old VAG PD that used to run cool and temp would drop if engine rpm dropped. Thermostat tested in boiling water and found it to work ok,however it was weak so the water pressure behind it allowed stat to open too soon rather than at correct temp.
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Quote:
Originally Posted by Spider95 View Post
Disagree. My 156 JTD 1.9 16v has never 'stayed at 90C'. It'll approach the 'twelve o'clock' position in warm weather and traffic, but under normal driving conditons the needle will settle between 10 and 11 o'clock. The specialist tested this with the needle in this position by plugging in the laptop and all was perfect. This is with a brand new dealer supplied stat with the new tempo sensor already fitted. You know when the stat is struggling when in cold weather the needle struggles to get off the ground. These comments are based on experience of 130,000 miles with this Alfa from new.

The OP has nothing to worry about from the symptoms he has described.
Yeah but.. the 156 doesn't mask +/- 10 degree fluctuations, 147, GT and 159 do. The ECU pretends the temp is constant 90C within that range, and the needle only moves if outside that 'normal' range. On those cars, you can only see the true temp via OBD read.
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Last edited by halftone; 04-11-18 at 08:09.
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On a 156 it matters little where the needle settles, as long as it is consistent.

The symptom of a thermostat problem is that the temperature drops when you are going fast, especially on a cold day. Mine sits at about 80 on the gauge, MES thinks it is 90, but once warm it only moves from that position in two cases:

- really belting it on a hot day - the sort of speeds that get your licence removed in the U.K., the temperature will climb a bit

- sitting in traffic on a cold day with the heater going full chat - it will drop a bit. This is simply because the diesel engine is not making the 3kW or whatever of heat that the heater can demand.

Simple test for a busted thermostat - get the car warm, and then drive along an A road or motorway at a decent speed. If the needle drops, then the thermostat needs changing.
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That makes sense. The point of a thermostat isn't just quick warm-up but constant adjustment of flow to maintain an even operating temp.
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Also consider that a lot of the after market thermostats seem to leak a little even when new. The Alfa ones seem to be leak free. The small leakage is enough to cause problems on cold days.

Is the EGR functioning normally ?. If it's been blanked you lose the heat input to the coolant from the EGR cooler which will cause problems on cold days. The diesels just don't produce much heat at low engine load. Go down a long hill on a frosty day with the heater on and the EGR blanked and the engine will struggle to maintain 80 C which is the minimum required to drive the gauge to the mid 90 C position.
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Originally Posted by Alexie View Post
Also consider that a lot of the after market thermostats seem to leak a little even when new. The Alfa ones seem to be leak free. The small leakage is enough to cause problems on cold days.

Is the EGR functioning normally ?. If it's been blanked you lose the heat input to the coolant from the EGR cooler which will cause problems on cold days. The diesels just don't produce much heat at low engine load. Go down a long hill on a frosty day with the heater on and the EGR blanked and the engine will struggle to maintain 80 C which is the minimum required to drive the gauge to the mid 90 C position.
The EGR is blanked on my GT. Didn't know it could effect the coolant temp. Cheers for the info.
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I can’t say I have noticed any difference in warm up times since blanking my EGR. It seems to warm up to 90 in same time producing hot air in cabin and not dropping even on coasts downhill etc.
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