MPG has suddenly gone from kind-of-okay to worst

Sylvester

Sylvester

Member
I used to get around 29mpg with my 1.4 petrol, 8L=60 miles, I mostly do urban driving and ~15/20min drives, so meanwhile wasn't great mpg, it was okay.
Last month it has suddenly gone to 23mpg, 10L=60 miles, which is bad, especially it's summer time. I mostly drive the same distances too.
I'd changed the fuel filter, still the same.

I didn't notice any change with the car, the oil consumption is very low like before, reaching coolant temp like normally. I'd used valve/piston cleaners from time to time for years and cataclean around 4 month ago.
Any idea what to check? The spark plugs are 4 years old, I'd checked them last year and looked fine, battery is 6 months old.
Thanks
 
Sylvester said:
I used to get around 29mpg with my 1.4 petrol, 8L=60 miles, I mostly do urban driving and ~15/20min drives, so meanwhile wasn't great mpg, it was okay.
Last month it has suddenly gone to 23mpg, 10L=60 miles, which is bad, especially it's summer time. I mostly drive the same distances too.
I'd changed the fuel filter, still the same.

I didn't notice any change with the car, the oil consumption is very low like before, reaching coolant temp like normally. I'd used valve/piston cleaners from time to time for years and cataclean around 4 month ago.
Any idea what to check? The spark plugs are 4 years old, I'd checked them last year and looked fine, battery is 6 months old.
Thanks
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I like to change spark plugs every service, or at least every other service if your not doing high mileage, but 4 years, they need a change as only needs one to start playing up

 
Crazylegs said:
I like to change spark plugs every service, or at least every other service if your not doing high mileage, but 4 years, they need a change as only needs one to start playing up

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Since mpg has never been good, it needs some basic checks I reckon. Is it getting to temperature? And a scan to kick off.
Mac.
 
Crazylegs said:
I like to change spark plugs every service, or at least every other service if your not doing high mileage
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Cheers, I'm gonna have a look at the spark plugs, the gap supposed to be 1mm?
PlasticMac said:
Since mpg has never been good, it needs some basic checks I reckon. Is it getting to temperature?
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Howey said:
How's coolant temp with diag equipment?
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95% of the time I drive urban, so almost as soon as it reaches 90C, like ~10 minutes, few minutes later I turn the engine off. During winter sometimes doesn't even reach it before I get to work. It's getting up to temp really good, always did and that didn't change, then goes between 85-90C. Temp sensor been changed, and I know the thermostat might be stuck open a bit, but I've spent a lot on other things over the years, so that hasn't been priority, as it's working good for years and also I mostly do short drives. Cheers
Joga said:
Check for dragging brakes.
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Drum brakes were changed last year, they had a drag for a while, but it's been sorted, the rear wheels spin nicely now. Probably good idea checking the front ones too, just in case. Cheers
 
Presuming this uses the dual green coolant temperature sensor it's possible for one side to fail so your ecu could get duff information but dash correct hence why I said what is it showing with diag equipment
 
Howey said:
Presuming this uses the dual green coolant temperature sensor it's possible for one side to fail so your ecu could get duff information but dash correct hence why I said what is it showing with diag equipment
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The G62 sensor was changed less than 6 months ago, OEM from Audi's website, cost £60.
I'm not sure what diag equipment means, I assume it's different than full system scan? The fault codes are related to locking issues, I know the causes, and intermittent yaw sensor issue, which I've been looking it changing it, but nothing else.
Can bad compression cause these symptoms? Cheers
 
Dirty or clogged throttle body. Dirty or clogged EGR valve. Has the battery recently gone flat? If so the throttle body will need to be adapted in VCDS. Throttle body also needs to be adapted after cleaning.
 
Diag is simply diagnostic equipment, any software capable of reading the ecu live data doesn't have to vcds.
The lambda also adjust fueling so might be worth making sure that's also reading correctly
 
Sylvester said:
Cheers, I'm gonna have a look at the spark plugs, the gap supposed to be 1mm?


95% of the time I drive urban, so almost as soon as it reaches 90C, like ~10 minutes, few minutes later I turn the engine off. During winter sometimes doesn't even reach it before I get to work. It's getting up to temp really good, always did and that didn't change, then goes between 85-90C. Temp sensor been changed, and I know the thermostat might be stuck open a bit, but I've spent a lot on other things over the years, so that hasn't been priority, as it's working good for years and also I mostly do short drives. Cheers

Drum brakes were changed last year, they had a drag for a while, but it's been sorted, the rear wheels spin nicely now. Probably good idea checking the front ones too, just in case. Cheers
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So I think you are acknowledging above that "the thermostat might be stuck open a bit" but perhaps discounting that as the reason for poor economy as it has been like that for a long time and "you mostly do short drives". I think it's quite likely that your thermostat might be sticking open even worse than before (or opening even earlier).

It is on short drives that the thermostat is most important. you should think of the thermostat less as a high temperature regulator but more as an over cooling regulator; it prevents the engine from being cooled during the warm up phase. If the thermostat is stuck open or opens early your engine is being constantly over cooled. The more it is over cooled on short journeys the worse will be your economy.

Alternatively, have you suddenly started using AC in the hot weather?

Trevor
 
audifan said:
Dirty or clogged throttle body. Dirty or clogged EGR valve. Has the battery recently gone flat? If so the throttle body will need to be adapted in VCDS. Throttle body also needs to be adapted after cleaning.
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It had a new low milage TB around 2 years ago, cleaned thoroughly with new seals and adaptation was completed. The idle and acceleration is solid, the idle has always been a bit low at 640, but never been an issue.
The battery was replaced last year, voltage ~12.3v.
EGR was replaced a year ago, adaptation was successful.

I've spent a fair amount of money on my car, especially how much I drive, I also take care of it, like I always wait a bit before driving off, so the oil can circulate properly, hence this sudden mpg change is even more bothering. Thanks
 
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TAABVW said:
I think it's quite likely that your thermostat might be sticking open even worse than before (or opening even earlier).

Alternatively, have you suddenly started using AC in the hot weather?
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I've gone out for a longer drive to see check the coolant temp, but it's the same as before, reaches 90 in <10min, stays there for a while, then down to 85, then back to 90. It stays in middle much longer as well, but it's might be due the recent hot weather, so I assume thermostat didn't get worse, as stuck open valve should have the opposite effect.

I literally never use the AC, I just roll the windows down.

I'm gonna check the spark plugs and other smaller bits recommended above, then if still no change to mpg, I'm gonna have to look into changing the thermostat, which would be rather annoying as I'm still dealing with other bits like the alarm and central lock issues.
Thanks
 
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Sylvester said:
I've gone out for a longer drive to see check the coolant temp, but it's the same as before, reaches 90 in <10min, stays there for a while, then down to 85, then back to 90. It stays in middle much longer as well, but it's might be due the recent hot weather, so I assume thermostat didn't get worse, as stuck open valve should have the opposite effect.

I literally never use the AC, I just roll the windows down.

I'm gonna check the spark plugs and other smaller bits recommended above, then if still no change to mpg, I'm gonna have to look into changing the thermostat, which would be rather annoying as I'm still dealing with other bits like the alarm and central lock issues.
Thanks
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Once the temperature indicator has moved to 90C, it shouldn't move. As yours drops back, I'd think that's an indication of the stat opening early. You can check what's going on by opening a log of engine temperature on vcds, before your normal daily drive. You'll know the engine temperature exactly.
Logging is easy just select a couple of relavent Groups, open a log, stick the laptop under the seat and go. Oh, set the power mode on the pc so that it doesn’t switch off, or standby when you close the lid.
Mac.
 
Howey said:
Might be worth having the emissions checked to see what values it's achieving
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MOT was done 3 months ago, passed emissions first time, of that's a good indication. Cheers
PlasticMac said:
Once the temperature indicator has moved to 90C, it shouldn't move. As yours drops back, I'd think that's an indication of the stat opening early. You can check what's going on by opening a log of engine temperature
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I've done that before and the temps shown were rather accurate, like when it showed ~85C in car, it'd shown ~83C on the connected device, but I'll try to check it again.

Meanwhile the slightly wandering temp isn't great, the issue always been the same, if the open thermostat had got worse in last few weeks, tempwould've ve gone lower more, but it does the opposite lately and stays on 90 much steadier. Thanks
 
I've checked the plugs, on the pics kind of seems worse than in person, especially the top left.
Either way, just to be sure I'd ordered new 3 electrodes NGKs, will be interesting to see if they make any difference to the Bosch 4 electrode ones.
20230615_112400.jpg
 
PlasticMac said:
The temperature gauge is "weighted" to display 90C, when the temperature is between about 82C & 97C, so once the engine is up to temperature, it should not move. Hope this helps.
Mac.
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The "temperature gauge" is more of a comfort gauge than anything else. Just compare the gauge reading with the actual temperature shown by VCDS.

RAB
 
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