[Closed] Why does my engine run cool?

Running cool could be the thermostat stuck open. Not sure it would explain the other stuff.

^^ defo this^^

^^^ yes my first thought, valves are designed to fail into the open position (from what I recall)

Temp sensor failure, seen this a lot in golfs, the engine thinks its running cool and overfuels. No DTCs stored in the PCU, as because its getting duff info from the sender, it thinks all is hunky dory.
Just because the sender has been replaced, it dont mean that the new un is any good.
There's a lot of Bosch fakes or even substandard Bosch from places like Euro car parts.
Go known second hand or go main dealer

Thanks for all that guys. Is there a way to test the thermostat?

ulysse, do you mean the coolant temp sensor? The clip-in green & black thing with brassy node on it? Is it really so likely that a new me would be defective? I got a generic one (£16, HAAS?) as it was the only one the motor factor had in stock and I was in a rush. I'll get a price from VW...

Testing thermostat.

You'll need to remove it.

If it's jammed open it will be obvious. As it'll be err jammed open.

If not. To test throw in a pan with a thermometer and water and put it on the hob

Should open at the number printed on . Usually 82 or 88c

I had this happen earlier in year was a bollocks cool as hell on m way and near over heating in traffic as was only jammed slightly open.

Yeah if i recall that engine, (its been a while) there is a hose stub / stat housing at the front of the head over the gearbox side , with either 1 or 2 temp senders clipped in.
ECP are getting quite a name for dodgy engine sensors, so i cant see why high street motor factors wouldnt source their parts from a similar places.

Also bear in mind the part could be fine, but the wrong electrical resistance value for your particular model- It does happen!

TPS are a genuine VW supplier, dunno if they'll do direct to customer, but worth a go
https://tps.trade/

HAAS sounds suspiciously like a Eurocarparts part, and yep, also do as Trail Rat suggests, boiling water in a pan, on the hob, out of the kettle isnt good enough as it looses heat by the second once away from the heating element

I should add that I've done at least a couple of thousand miles with the gauge showing cool. I figure if I'm going to take the thermostat out to test it, I might as well just replace it. They're only a few quid, I see. Is it a relatively quick/cheap job for a garage? I don't have time/tools until week after next, with a big trip in between.

Thank you all again, much appreciated.

ECP are getting quite a name for dodgy

Fake shit and substitution of branded parts with their own branded shit.

A wise man once said to me many years ago

Don't learn to be a mechanic. Learn how to use a multimeter.

Very little that cant be pin pointed with a multi Meter these days. Everything has a ****in sensor.

Says the guy who's car developed an annoying habit of main beam sticking on tonight ......yay for britpart headlight switches.

You might notice that the car heater isn't getting as hot if the thermostat has failed open. Turn the heater up to max and try that first

^^ good tip

The Audi I had showed a temp sensor fault code and engine light on when the thermostat failed. As you have said the thermostat is a cheap part and some time to replace.

I wouldn't take the stat out to test it straight away.

I would drive until the the gauge is starting to approach 1/4 (due to being ECU controlled they often do this quite suddenly, rather than a gradual increase). 1/4 as this seems to be normal operating temp if your sensor is dud or wrong type.

Then jump out and feel the radiator. If its stone cold, the stat is still closed and the sensor is probably at fault. If its lukewarm, the stat is probably stuck open as coolant has been flowing through as the car has come up to normal temperature.

Thermostats are cheap and sometimes a bit of a faff to replace depending upon where they are placed in the engine. Just replace it then you can tick that off your list. If it fixes the symptoms then that's your problem.

A similar issue I had with my old focus was actually the thermostat housing. Because for cost saving reasons, Ford uses a plastic housing rather than a more traditional metal one (i'm sure other manufacturers have probably followed suite) and the plastic ones were prone to cracking and leaking. Mine went, had issues with the car not warming up properly (more of an issue of being slow to warm up rather than not warming up at all), investigated the housing after a bit of googling, found evidence of a leak. The issue is not only the leak but the coolant bypassing the thermostat altogether. I seem to remember the housing cost a princely sum - something like £60 or something, and the thermostat itself was cheap - I just replaced the lot.

I bought new thermostat sensor and 'box' of plastic housing with actuated valve in for the Galaxy. Both from online Ford parts supplier, both branded as the original make supplied.
The sealing ring around the sensor did not seal with the plastic housing, ergo water loss as the engine heated up.
I spent a week chasing down where the hell it was leaking - I had assumed one of the half dozen or so coolant pipes had split or not sealed up when I refitted.
Took it to garage, who just immediately said 'was the sensor a bright green colour? If it was, they have dodgy seals and you need to buy a different make...' 10 min job to sort with new part...

The whole OEM/manufacturer/buy from known place seems a lottery.