Car driving oddly after thermostat housing change

[Rudedog]

On the 19th of September I had my thermostat housing changed under warranty after I took my car in following a slow coolant loss.

From the next day I noticed a few odd things in the way it now drives.

1 - It sounds really fruity... some would be happy with this but I noticed it straight away that the loudness was all the time I drove it, almost like a continuous cold start volume.

2 - It's now jerky at low speeds, the throttle seems really sensitive, if I just touch it the car lurches, before if I lifted off in traffic the car smoothly rolled forward but now it feels like it's wanting to go on it's own.

3 - Worst of all the MPG has dropped noticeably, I waited two weeks since it was done to confirm, I could feel the car was using fuel but thanks to the We Connect app I now have my driving data for 'before and 'after'.  Before without any effort I was easily getting 33 to 35 mpg on the morning run and around 27mpg on the drive home, now on the same run I can 'just' about manage 29-30mpg in the morning and a terrible 19-21mpg in the evening!

This can't just be coincidence.... I know my car and could feel something not right but how would a change of thermostat cause this?

I thought the thermostat was just like an old type thermostat but now I read it's all controlled electronically.

With the bad MPG I'm thinking that it's permanently running rich and using extra fuel all the time??

I'm going to book it in as it's due it's year three service and first MOT in March so they can do it all at the same time.

Hope they find something as this is spoiling an otherwise great daily.

Any ideas? Am I missing something?

[madstaff]

Sounds like they could have possibly disturbed something on the inlet manifold, eg, poorly seated gasket, forgot to put a vac pipe back on, etc, and is now introducing excess air into the intake now.

[fredgroves]

Judging by you saying it's making more noise and bad fuel economy, it sounds like something intake wise that's not been put back properly...

If you read this guide on how to do the replacement, you'll see there is quite a bit of scope for getting it back together incorrectly...

https://blog.fcpeuro.com/how-to-replace-water-pump-thermostat-housing-mk7-vw-gti-beetle-jetta-macan

[Rudedog]

Looking at that guide as you said plenty of scope to make a small mistake putting back together.

There's a small part of me who wishes I'd left it alone but I guess it was only a matter of time before it got worse so had it done while I was still in warranty.

Does anyone know if tech still do test drives because I'm sure they would have noticed something not quite right with the drive before signing it off.

[ajmoir36]

Check all connectors for all the sensors have been put back too, there are a lot to remove.

[Rudedog]

Got it checked in for third year service so they'll do it then, just got to pay for the extra fuel it's drinking in the mean time.

[NAS23]

I have just started having these exact symptoms on my 2019 GTi but out of nowhere without any recent works having been done.

Started with what I can only describe as a sudden slight misfire or splutter at moderate load around 2500rpm. I've had the car 12 months and routinely achieve 34-37mpg in daily use. This has now reduced to 26-28mpg at best and I can see the fuel gauge needle slowly dropping between every journey, meaning I'm on course to get 250 miles from a full tank instead of my usual 350.

The engine also feel like it's running slightly rough or rich, with periodic jerky acceleration from low revs. There's no EML light and I have a basic code reader that has revealed no stored or pending error codes at all. Interestingly I thought I had pressed the driving mode button into Sport by accident because of the droning exhaust but it's still on individual with comfort exhaust setting.

Car is due a service in the next 2 weeks along with the pre-warranty expiry check so hopefully it will get resolved and I'll compare notes afterwards.

[mcmaddy]

I have just started having these exact symptoms on my 2019 GTi but out of nowhere without any recent works having been done.

Started with what I can only describe as a sudden slight misfire or splutter at moderate load around 2500rpm. I've had the car 12 months and routinely achieve 34-37mpg in daily use. This has now reduced to 26-28mpg at best and I can see the fuel gauge needle slowly dropping between every journey, meaning I'm on course to get 250 miles from a full tank instead of my usual 350.

The engine also feel like it's running slightly rough or rich, with periodic jerky acceleration from low revs. There's no EML light and I have a basic code reader that has revealed no stored or pending error codes at all. Interestingly I thought I had pressed the driving mode button into Sport by accident because of the droning exhaust but it's still on individual with comfort exhaust setting.

Car is due a service in the next 2 weeks along with the pre-warranty expiry check so hopefully it will get resolved and I'll compare notes afterwards.

what petrol are you using?

[SRGTD]

I have just started having these exact symptoms on my 2019 GTi but out of nowhere without any recent works having been done.

Started with what I can only describe as a sudden slight misfire or splutter at moderate load around 2500rpm. I've had the car 12 months and routinely achieve 34-37mpg in daily use. This has now reduced to 26-28mpg at best and I can see the fuel gauge needle slowly dropping between every journey, meaning I'm on course to get 250 miles from a full tank instead of my usual 350.

The engine also feel like it's running slightly rough or rich, with periodic jerky acceleration from low revs. There's no EML light and I have a basic code reader that has revealed no stored or pending error codes at all. Interestingly I thought I had pressed the driving mode button into Sport by accident because of the droning exhaust but it's still on individual with comfort exhaust setting.

Car is due a service in the next 2 weeks along with the pre-warranty expiry check so hopefully it will get resolved and I'll compare notes afterwards.

Sounds like the symptoms that current model Polo GTI owners have experienced when the OPF / GPF on their cars is going through the regeneration process. Symptoms have included (some but not necessarily all of) the following while the regen it happening;

- Significant increase in fuel consumption
- Increased resonance in the exhaust. It becomes very 'boomy'
- Sluggish/hesitant performance, with the car being quite jerky in slow moving stop/start traffic
- Smell from outside the car - almost like a hot smell, if it’s actually possible to smell heat

I’ve experienced this twice in my car (a Polo GTI+) in around 6,000 miles. On one occasion, I was almost home so took the longer scenic route to give the regen process time to finish.

[Ex]

Hi I’ve got 2019 GTi performance my car has started with exact same symptoms loss of 10 mpg on all cycles gone from 40 + on mway to 28 & from 32 urban to 20 mpg steady driving.also exhaust tone louder and jerky in stop/start traffic.just wondered if anyone has solved issue as my local dealer is useless they say there’s nothing wrong as no fault codes or warning lights on
Many thanks