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.
 

oko

A2OC Donor
EGR needs a clean ?
Air intake sensor clean (maf?)
Wheel alignment maybe to a lesser extent esp with wide tyres.
 

nod

Member
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??
 

Kaz

Member
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
 

Un4tural

Member
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.
 

RAB

Technical Specialist 1.2 TDI
A good place to start would be a VCDS scan. That should point you in the right direction.

RAB
 

O07JESTER

Member
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.
 

Un4tural

Member
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.
 
Last edited:
Top