Since new my trusty 2001 Octavia 2.0 Estate has reported a running temperature of exactly 90 degrees in all weathers - sizzling drought ridden summers with air-con on, freezing winters with heaters on - after 5 mins or so the engine temp dial has always, and only, pointed directly up (to 90 degrees).

Until recently that is.

Over the last few months I've noticed the reported engine temperature hanging between 70 and 80 degrees. When cruising at 70mph on the M1, or drifting through town unimpeded by traffic at 30mph, the dial reports 70 degrees. When pulling uphill it'll get to 80 degrees. It only gets to 90 degrees when the car is going nowhere (e.g. in slow traffic, or parked) having been out on a run for 15mins or more.

Should I be concerned? I guess the lower temp will affect fuel consumption, but will it cause engine damage over time?

Two garages have failed to find any obvious problem (coolant level is fine, no radiator leaks, dial reports engine temp accurately, car has full service history).

What could be going wrong? What should I make sure the next garage looks at?

try replacing the temperature sender

can easily be done yourself too

about £30 or so

Does it drop when driving at speed for example then warm up when stationary? If yes then i would be looking at the thermostat.

Could be a faulty coolant temp switch if its just the gauge messing around.

could be the thermostat sticking, allowing the coolent to be constantly cycling, which would stop the engine getting to operating temp.

oh and the engine running cold can potentially be just as bad as running hot, as the oil wont be hot enough to lubricate components as well as it was designed to do.

thermostat would be what i'd check out first

the oil wont be hot enough to lubricate components as well as it was designed to do

I dont agree with that as thats why we have multigrade oils.

I suspect however it doesnt actually have a problem, its just the gauge telling porky pies.

IME, the gauge reading low was a failed thermostat, and the gauge intermittently not moving at all was a failed sensor. But in both cases this was for the Fabia, which could be different?

I've just developed (or noticed, given the weather) this problem on my Mk 1 Octavia too. I logged mine during a journey and high speeds lowered temperature, so I'm pretty sure I have a faulty 'stat.

Is it easy to change? (Or is it easy to swap, say, whilst changing a cam belt? )

The stat is a pain in the **** to change on the 1.8T but you might as well do it when changing the water pump if you think you have an issue.

Just had a look and if you suspect the sender -

Last year, I bought 059919501A (4 PIN TEMP SENSOR), plus clip and washer for less than £22 delivered (VAGPARTS).

Took 5 minutes to change.

when I had a fault with my sender it throgh the engine management light on... but I think that was because mine was jumping high then low...

If I had to place my bet it would be on a thermostat... sounds more realistic...

Put me down for the thermostat.

Available for £10 on ebay if it's the same 059919501A as on the tdi (green 4-pin)

If that does not resolve, check water pump operation & change thermostat (genuine). If still issues, caustic & reverse flush system, if still issues new radiator (ensure fan operation).

Greg.

try replacing the temperature sender

can easily be done yourself too

about £30 or so

I dont agree with that as thats why we have multigrade oils.

yes but oil does have an operating temp that it 'works' at, hence why you should warm up an engine before you rag it.

oil will change its viscosity level when hot, i.e get thinner, which will lubricate better and protect the engine better, running a cold engine can damage it. just not as quickly as running too hot will. multigrade oils just help.

If the warmup is slow, I'd say thermostat; if the warmup is ok, but the readings are low on the gauge, I'd say sender!

There are two styles of 4 pin sender units........depending on the year of car you have. remove the connector off the sender unit and check if it is Square or D-Shaped.....so you can buy the right one. Saves a trip to GSF / EUro Car parts.

get a vagcom cable and see what temp the motor is reading - is the fan is coming on? then the water must be hot :-)

if its just the sensor for the display reading low then either change it - or don't worry about it until it dies properly.

oil will change its viscosity level when hot, i.e get thinner, which will lubricate better and protect the engine better.

not quite true - that one - thin oils can wreck engines too, otherwise we'd all be using sewing machine oils & WD40 :P

- thinner oils give better mileage on better made engines with close tolerances.

but you are right - too thick/cold oil isn't good either...

Replace the thermostat. Your thermostat is sticking, allowing the coolant to be constantly cycling.

not quite true - that one - thin oils can wreck engines too, otherwise we'd all be using sewing machine oils & WD40 :P

- thinner oils give better mileage on better made engines with close tolerances.

but you are right - too thick/cold oil isn't good either...

i know mate, i was juat saying as the oil gets warmer it gets thiinner, not saying the thinner the oil the better, just that when at its thickest(cold) it wont work as well as when its warm(thinner)

Does it drop when driving at speed for example then warm up when stationary? If yes then i would be looking at the thermostat.

Could be a faulty coolant temp switch if its just the gauge messing around.

Thank you Lummox, those are indeed the symptoms - in stop/start traffic the temp' is (eventually) fine, but on the clear runs the temp stays low.

could be the thermostat sticking, allowing the coolent to be constantly cycling, which would stop the engine getting to operating temp.

oh and the engine running cold can potentially be just as bad as running hot, as the oil wont be hot enough to lubricate components as well as it was designed to do.

thermostat would be what i'd check out first

Thank you for the explanation Alan_VRS; that helps my understanding of why a failing thermostat would keep the engine cool rather than cause it to overheat. If the thermostat controls the cycling, presumably another separate system "knows" how to keep the coolant cool - and if that failed the engine would get too hot?

Providing additional cooling when the coolant temperature gets significantly above "normal" is the function of the electric fan system. If you've ever been really bogged down in traffic, seen the gauge rising, and then heard a fan cut in and seen the gauge starting to fall, that's the electric fan being operated by its thermoswitch.

when I had a fault with my sender it throgh the engine management light on... but I think that was because mine was jumping high then low...

If I had to place my bet it would be on a thermostat... sounds more realistic...

Cheers Kylethompson30; based on the other comments on this thread, and the behaviour I see while behind the wheel, I am inclined to agree - the temperature as reported in the instrument panel does move smoothly and predictably.

Thank you for the explanation Alan_VRS; that helps my understanding of why a failing thermostat would keep the engine cool rather than cause it to overheat. If the thermostat controls the cycling, presumably another separate system "knows" how to keep the coolant cool - and if that failed the engine would get too hot?

The thermostat is failsafe, in that it closes against a spring so that if the needle or mountings break, the plunger is forced open and coolant flows through the radiator (this is also what happens when the thermostat is working fine and the coolant reaches ~90degC). When it is closed, the coolant loop is short-circuited and coolant only flows around the engine block. Therefore the only failure mode that has a risk of overheating is if the thermostat needle sticks closed, however the likelihood of this happening is much less than the failure modes described previously.

HTH

My 1998 Octavia 1.8 20v has developed the same problem as discussed on this thread. When the engine is under low stress and the traffic is free flowing the temperature goes right down.

when I stopped today I left the engine running and the fan kicked in before the temp gauge got to the vertical central position.

so is it a faulty thermostat, Temperature sensor, etc.

I'd like to replace the offending part myself to save a few quid, where are they located in the engine bay. Cheers