Audi A4 1.9 B6 Low MPG and Coolant won't reach 90 since service...

[Sprinter2012]

As in the title really, when I bought the car about a month ago the MPG was fantastic.

The trip computer said I averaged 56.9 MPG on the 90% motorway trip on the way home.

I took it for a service at Audi 2 weeks ago, just oil and filter. And now the MPG has dropped from an average of about 45-50 to struggling to stay above 40MPG!

The coolant temp also now never gets to 90, it's sits a few mm below the middle mark.

Could anything they did during the service have caused these issues, or is it just coincidence because the cold weather arrived during that time?

Any advice much appreciated guys.

 

[Jimmy TDI]

Sounds like your thermostat has failed and is now sticking open.
No way Audi could have done it as it's obviously sealed in the block, so will just be a coincidence.

 

[VagCabby]

Thermostat, or coolant temp sensor

 

[Sprinter2012]

Today when I went out in the car I tested the car by driving with the heaters off, and it reached 90 fine. Then when I put the heaters on it dropped to about 80 then came back up to 85ish.

Could this be linked to the MPG issue or is that just down to cold weather and different oil?

 

[john777w]

Just a thought, did the garage or you add any antifreeze??

 

[Sprinter2012]

Just a thought, did the garage or you add any antifreeze??

Shouldn't have done, I just asked them to change the oil and filter, nothing else.

I might just get the temp sensor and thermostat changed plus new coolant, as I'm not sure when it was last changed.

 

[Martyn_V6]

I was having the same problem with mine the other week. Got it serviced then noticed it wouldn't go past 70 on the gauge. Fitted a new coolant temp sensor as the thermostat was done at the start of the year. Made it worse, would get off 55. Was told I could have a air lock. Ran it for a while with the cap off etc to try get rid of it with no luck.

Turned out to be a tiny bit of plastic was sitting on the sealing face of the thermostat keeping it jammed open! Not sure where it came from but might be worth checking yours.

 

[Sprinter2012]

Definitely think it would be safest to just change both just in case.

Does anyone know what I'd be looking at for parts and labour to change the sensor and thermostat plus new coolant?

 

[Martyn_V6]

If i remember correct my thermostat was under £40 from Audi same with the sensor. I just done the sensor myself, litre and a half of G13 coolant from Audi was just under £13. You will need more than that though if your changing the coolant out

 

[john777w]

I was having the same problem with mine the other week. Got it serviced then noticed it wouldn't go past 70 on the gauge. Fitted a new coolant temp sensor as the thermostat was done at the start of the year. Made it worse, would get off 55. Was told I could have a air lock. Ran it for a while with the cap off etc to try get rid of it with no luck.

Turned out to be a tiny bit of plastic was sitting on the sealing face of the thermostat keeping it jammed open! Not sure where it came from but might be worth checking yours.

That's why I asked about the antifreeze, it's very corrosive & I thought it might have broken up some crud which could have jammed the thermostat!

 

[Sprinter2012]

I was having the same problem with mine the other week. Got it serviced then noticed it wouldn't go past 70 on the gauge. Fitted a new coolant temp sensor as the thermostat was done at the start of the year. Made it worse, would get off 55. Was told I could have a air lock. Ran it for a while with the cap off etc to try get rid of it with no luck.

Turned out to be a tiny bit of plastic was sitting on the sealing face of the thermostat keeping it jammed open! Not sure where it came from but might be worth checking yours.

Just had a thought, the earlier spec water pumps had plastic impellers which could break off and find their way round the cooling system...

That's why the later spec ones had metal impellers. Might explain where your mystery plastic came from!

 

[Jimmy TDI]

I've also seen the little plastic clips/fingers that hold the stat into the plastic housing snapped off before and getting caught in the stat. They usually get snapped off dying removal of the old stat.
This is usually down to inexperienced people doing the job who don't know that the housing is designed to hold the stat for fitment into the block.