Poor fuel economy TDI90

Un4tural

Member
Hi All,

Figure this should be the next thing to tackle as it is becoming rather grim, got 280miles out of 30liters last fill, on sport rims but still.

I've picked up filters for intake and oil etc. and will give it a quick oil change and service however i doubt that'll sort it.

No lights nor did i have any codes - will get vcds set up at some point to have a play.

I strongly suspect a leaking intercooler - will probably take hoses off plug one end and try to use a ball inflator or something to test it to avoid taking the bumper off as ideally i'd like to leave it for when i have a replacement bumper to replace it with. It smoke a bit if i put foot down and it need to build up boost from nothing, 2nd for example, sometimes takes up to 3k rpm for it to start pulling properly with it noticeably smoking a fair bit out the back end until it gets there starting at about 2k rpm.

DIS is fairly accurate i noticed over the time I've driven it being within a few mpg - reporting 36 this fill up. i do leave it for 5-10min to warm up now as it is fairly chilly overnight but wouldn't expect it to drop average too badly.

Tried injector cleaner which apparently made it worse (that or the cold weather)? Tyre pressure are fine and brakes aren't sticking (at least not on stilts), bearings do not seem to be complaining either, putting back together the gearbox there didn't seem to be what felt like excessive resistance for wheels turning/gearbox etc. nothing seems to be seizing up or putting up resistance mechanically.

Gearbox oil is fresh and with new dmf/clutch there aren't any odd sounds.

I'll record the car running, there is a bit of an odd noise, kind of like those shakers with rice in them from engine bay and when it is raining a lot it sounds like a slight belt slip but other than that it I've not caught any funny sounds. It does still vibrate a bit too much when pulling away in 1st without giving it some go pedal but i reckon it'll just be engine mounts as they do look like 150k miles later.

What would be the easiest things to start with checking, doubt doing the service of oil/filters will sort it as it has been not great on economy since i got it but it does appear to be becoming worse - wouldn't expect fuel additive to decrease mpg to any significant amount.
 
EGR needs a clean ?
Air intake sensor clean (maf?)
Wheel alignment maybe to a lesser extent esp with wide tyres.
 
my tdi90 mpg seems to have improved quite a bit since i removed roofbars, renewed gearbox oil and did a service inc cleaning the maf. roofbars make more difference than i expected. i can now easily get 60mpg on a motorway run. skinny lightweight wheels.

i think ive read about temperature senders causing fuel economy issues??
 
ive got a similar issue on a tdi75 - I get 280miles to a tank as well - I have changed the sensor and its still the same
 
I am dreading looking at EGR as that is leaking oil :/ but i guess i'll have to if that would ruin economy noticeably.

MAF seems to function ok looking at values for air flow.

I do need to do alignment on fronts as Halfords messed it up bad, but it doubt it would cost me 20ish mpg.

I'll do some poking around this weekend if it's not too wet, but not 100% on where to look other than doing my service.
 
A good place to start would be a VCDS scan. That should point you in the right direction.

RAB
 
You mention that your car smokes a bit when accelerating, and boost is noticeable at 3k.It could point to a leaky charge air problem.It could be intercooler or one of the rubber hoses.The turbo is compensating for the issue, so your boost is arriving late.I have a 90 and boost is very early on acceleration and smooth up to peak boost.
So if you do have a combination of charge air leaking and temp sensor/thermostat not working this will definitely impact on economy.
 
did a service (still need to do fuel filter) and it does sound happier - did some zip-tie-fu to tidy up loose plastics under the car which got rid of the rice shaker sound from engine bay.

I've had a look over the intercooler, though with limited access and it does not appear to be damaged. I'll get some rubber bands or something and give it a test over when weather is clearer and i have some daylight by removing the hoses taping over one end and see if it leaks air when i blow some air in it.

Next will be EGR i suppose - I've seen some removal guides, hopefully weather is nice to tackle this. Would it be worth ordering a replacement and doing a swap, presumably it wont be pretty if it is leaking oil as it seems to be doing and with 150k miles on the clock. Or would it be better to do an EGR delete and straight pipe the valve like Timmus project car, to keep MoT man happy?

Noticed DIS showing good MPG (60ish) when at lower rpms but when i go over about 1.8-1.9k indicator drops to about 30-40mpg when on even road and using CC which again makes me think it is air/boost related, however it does it even when load is low, when turbo would likely not be spooling for boost.

EDIT: So on a few hours on motorway I've noticed an interesting behavior and seeing as DIS is pretty accurate with my fuel consumption as well as fuel gauge going down slower... seems i get MPG as I'd expect between idle-1.8k then 3.1k+ rpm range, between 1.8k-3.1k i get 30-35mpg, going 75mph in 5th displays about 40-45mpg, going same speed in 4th i get 60+(falls below when there's an incline), way back in 4th average MPG on DIS went up by a good few MPG.
 
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did a service (still need to do fuel filter) and it does sound happier - did some zip-tie-fu to tidy up loose plastics under the car which got rid of the rice shaker sound from engine bay.

I've had a look over the intercooler, though with limited access and it does not appear to be damaged. I'll get some rubber bands or something and give it a test over when weather is clearer and i have some daylight by removing the hoses taping over one end and see if it leaks air when i blow some air in it.

Next will be EGR i suppose - I've seen some removal guides, hopefully weather is nice to tackle this. Would it be worth ordering a replacement and doing a swap, presumably it wont be pretty if it is leaking oil as it seems to be doing and with 150k miles on the clock. Or would it be better to do an EGR delete and straight pipe the valve like Timmus project car, to keep MoT man happy?

Noticed DIS showing good MPG (60ish) when at lower rpms but when i go over about 1.8-1.9k indicator drops to about 30-40mpg when on even road and using CC which again makes me think it is air/boost related, however it does it even when load is low, when turbo would likely not be spooling for boost.

EDIT: So on a few hours on motorway I've noticed an interesting behavior and seeing as DIS is pretty accurate with my fuel consumption as well as fuel gauge going down slower... seems i get MPG as I'd expect between idle-1.8k then 3.1k+ rpm range, between 1.8k-3.1k i get 30-35mpg, going 75mph in 5th displays about 40-45mpg, going same speed in 4th i get 60+(falls below when there's an incline), way back in 4th average MPG on DIS went up by a good few MPG.
Hi Just wondering if you got to the cause of your poor fuel economy, my tdi90 hasn't been used much for the last year due to not going anywhere much. Now I've started using it again i notice the fuel economy is terrible, like average of 30. Even in hilly Cornwall that's poor. I'll do the egr, maf, filters etc at the weekend, just interested if there are any other prime suspects to tackle. cheers nod
 
@nod I think it was combination of tired turbo, oil making it's way into intake and generally poor maintenance of motor historically.

It has blown due to timing tensioner bolt snapping, chewed itself up so I bought a replacement which needs valve work, which i am in process of.

Hopefully that resolves any issues but I haven't yet stripped her down as been busy with work... poor Tuna been sat for nearly half a year now :(

What I did notice is fuel additives had no effect/change on my ATL A2 mpg or noise, but A4 with BPW engine seems to react well to premium fuel and additives, making engine quieter and run noticeable smoother.

Will definitely do a good digging when I have it back up and running again and see if MPG improved and how she goes, though do not look forward to draining coolant again... wish it had a nice lil tap or something to drain it easily.
 
@nod I think it was combination of tired turbo, oil making it's way into intake and generally poor maintenance of motor historically.

It has blown due to timing tensioner bolt snapping, chewed itself up so I bought a replacement which needs valve work, which i am in process of.

Hopefully that resolves any issues but I haven't yet stripped her down as been busy with work... poor Tuna been sat for nearly half a year now :(

What I did notice is fuel additives had no effect/change on my ATL A2 mpg or noise, but A4 with BPW engine seems to react well to premium fuel and additives, making engine quieter and run noticeable smoother.

Will definitely do a good digging when I have it back up and running again and see if MPG improved and how she goes, though do not look forward to draining coolant again... wish it had a nice lil tap or something to drain it easily.
Oh dear, shame the engine let go and your replacement is being difficult. Mines on 140K so the turbo is probably pretty tired and I have noticed oil in the pipes leading to egr. Will take a look at the turbo if I get time. Unlikely... When I striped down and cleaned the turbo on my previous tdi90 it made alot of difference to the performance. Good luck with sorting yours. Cheers nod
 
Your turbo could still have some life left on it @nod - mine is also on 140k and when inspected had only a smidge of play that my garage judged would be taken up by the oil when in service. I will have it inspected again and make a call on it at 175k miles and then if all good, replace at 200k miles as a precaution.

In terms of fuel economy woes, once all service items have been taken care of and the thermostat / both temp / MAF sensors are replaced or proven good, I would try a diesel purge and then check the torsion values on the injectors.

Even without doing the latter (it’s next on my list), on a long run my TDI90 is now capable or returning mid-60s mpg at a 65-70mph cruise driving economically (allowing speed to drop / build up on hills. I have a six speed, so take maybe 5-8% off that and you should be able to crack 60mpg no problem when doing the same.

Interestingly my DIS grossly over-estimates my consumption by about 12%; it said 57mpg average on my last pump-to-pump calculation when the correct figure was just over 64mpg.
 
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DIS does seem to be quite a ballpark figure, mostly frequent my A4 whilst I get A2 sorted and it seems very close fill to fill, but depending on driving and letting it coast/engine freeroll more DIS seems to really like this and bump up the figure.

What I noticed with A2 it seemed to be fairly accurate most the time but then I'd get abnormally high or low drives too, not sure why as overall consumption was same measuring fill to fill.

My original motor had a turbo rebuild in 2011 if memory serves, but I suspect engine wasn't very healthy due to quite a few hints of it running low on oil in service papers... which can do a turbo in fast.

I could not see any issues with the turbo eyeballing it but if it was starved of oil it makes sense for seals to go and excessive amount of oil going into the intake, plus extra blow by through breather.

Could be a good idea to check compression is good, kicking myself on not spending the 50 odd quid for one to test engine before dragging it all the way home, should give a decent idea of health, at least compression wise, as my old one had quite a bit of blow by as well.

another thing definitely worth checking is the intake and egr gubbins, mine weren't cakes up too bad (thanks oil leaks washing it away!) but donor motor had quite a bit of caking to remove and air flap valve thingy was quite sticky, not sure how free it needs to be but it'd pop up a fault code now and then using the sticky one.
 
Interestingly my DIS grossly over-estimates my consumption by about 12%; it said 57mpg average on my last pump-to-pump calculation when the correct figure was just over 64mpg.
A sure sign that your coolant temperature sensor has failed. It is in fact two sensors, one used by the ECU and one by the instruments for calculation. More likely, the ECU sensor is under-reading, meaning that your fuel consumption is sub-optimal.

RAB
 
A sure sign that your coolant temperature sensor has failed. It is in fact two sensors, one used by the ECU and one by the instruments for calculation. More likely, the ECU sensor is under-reading, meaning that your fuel consumption is sub-optimal.

RAB
I hope not RAB as the car is on it’s third in 9 months, all genuine / OE!!

But as you’ll no doubt say, it’s certainly not impossible and I do need to do a VCDS scan and check the temp readings when I get my VCDS back from Mark @Mka306

As a side-note, if the temp sensor is indeed duff that would both explain why my oil was so filthy after relatively few miles. Also if so then I should be able to get within a whisker of 70mpg on a run!
 
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Check the coolant temperature with VCDS if you can, in the engine section and the instrument section. They should be the same. If not, then you may need a fourth! Check the connector for corrosion.

RAB
 
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