Poor fuel economy - potential reasons and solutions

Linas.P · February 9, 2019

So recently my IS250 became much thirstier then before. There were few changes so it was hard to attribute it to the engine, obviously colder weather, but as well after fitting Michelin PS4s I noticed the tyres to be the reason to at least some extent event though they were rated same as the ones I have replaced (Dunlop Sportmaxx RT2s - C for fuel). However, after running the car for some time now it is clear there must be something more... my Tank MPG used to be in 28-29MPG range. End of summer after tyre change it dropped to 26, when colder weather hit it was 24 and now I am approaching 20s. Engine feels fine, starts fine, idles smooth, no misfires, last MOT all pollution figures were 0.00 (I was even wondering if have forgotten to put probe in), but the fuel consumtion indicates not all is rosy.. .

There is one obvious job which needs doing - the spark plugs overdue, not 80k miles whereas it should have been replaced after 60k, so I will address it during next service. Secondly, I feel that 183k miles motor now has quite a bit of blow by both petrol leaking into oil and vice versa. Even few years ago after ~150k miles I could smell petrol in the oil and I do 5-6k miles oil changes + I have noticed car likes to burn little bit of 5w30 (or blow it in the cylinders). Now both of those things are normal for DI high pressure engines so I am not overly worried, but with the miles I am starting to suspect it now reached level where blow by recirculated via PCV and now starts to gunk-up intake valves.

Anyhow, going to take off intake manifold of and try to see how intake valves are looking, replacing spark plugs is planned anyway. Other thing I found - engine likes 0w40 oil (or should I say dislikes... doesn't even sip) - probably makes sense thicker base oil, less blow by and at the same time less carbon build-up from PCV. Now I really would like to have an option of 350....

Finally, just accidentally found this video by savagegeese specifically for this topic - preventative maintenance for DI (wondering if there is good reason to consider oil catch can). Have you considered any preventative maintenance considering these DI engines will most likely surfer from similar problems?

Farqui · February 10, 2019

That's a good video that, as are many of his.

I fitted an air/oil separator last May when I replaced the pcv. I've been monitoring the results and emptying the can every now and then.

My vehicle hasn't covered your mileage and I couldn't find any useful info on what is an acceptable amount of blow by.

Rather than a catch can, the air/oil separator feeds the clean vapours back to the inlet. To do this you'll need strong walled hoses that can cope with a vacuum.

What catch can are you looking at? Mishimoto do a nice one but it's expensive. I opted for a similar design but a much cheaper version/knock off. I'm quite surprised how good it is actually.

Finding a suitable place to mount the can wasn't easy in my engine bay and I currently have a long run of hose. Which flows downwards to help drain the oil and pull the vapours back up.

I'll get some details online later.

On a related note, were you aware that F1 cars were/are deliberately feeding oil into the combustion cycle, to boost performance.

is200 Newbie · February 10, 2019

Quote

Good video - interesting about the oil

When I had my last IS I compared two oils, Castrol Edge and Magnatec and the Edge didn't perform as well as Magnatec. The car would, I could only presume, use the oil (burn it) as the level did not stay constant as I needed to top it up through use of the car, not regularly but it was noticeable. The Magnatec did stay stable - have stuck with the Magnatec since and its always been stable.

I do the oil changes on a 6 monthly basis also. I guess using a catch can would prevent the discolouration of the oil a bit also.

Farqui · February 10, 2019

@Linas.P, have you ever checked/cleaned your MAF?

Linas.P · February 10, 2019

55 minutes ago, Farqui said:

@Linas.P, have you ever checked/cleaned your MAF?

The engine was hydrogen cleaned 2 years ago, apart of that I didn't have any problems so wasn't looking for any. Will check when I do intake valves check.

You doing your preventative maintenance well, despite having port+DI - well done you! Can you add some info of oil catch can you have?

As for F1 cars that is different story (I know you didn't intend to compare like for like), the issue with burning oil is not one of engine not having right seals or compression, or that of pollution, but more about intake valves and what PCV does - it sprays air with oil onto intake valves. On 2GR-FSE it is less of a problem, because you can do "Italian tune-up" form time to time with added fuels system cleaners and your port injectors will clean valves. Not the case for me.

As for oil, my car uses ~0.5-0.8L of whatever Castrol Lexus uses (it is 5w30, but not Magnatec) and probably about 1L of Magnatec. That is per 6000miles. I have used Mobil1 0w40 and it stayed at the same level for over 4000miles, I haven't checked it for the last bit but it was far cleaner after 4000 miles, less petrol smell and no burn at all. Not saying Mobil superior to Castrol, just something I have experimented with and seen difference + whereas 0w40 Castrol Edge is 2x the price of Magnatec, I was able to get Mobil1 for even slightly cheaper.

ColinBarber · February 10, 2019

Plugs won't cause that amount of a drop in economy without you also noticing poor running or a misfire error code occurring.

Clean the MAF

Check your brakes aren't binding

Possibly poorly performing O2 sensors, not enough to trigger a warning but will cause over fuelling. Need an OBDII reader to check your short and longterm fuel trims - other sensors should also be checked, e.g. coolant temp sensor.

NemesisUK · February 10, 2019

Tyre pressures ok? The colder weather will cause a drop in pressure

Linas.P · February 10, 2019

ODBII gives no errors and all the sensors reports ok, however I never looked into fuel trims as honestly I don't know what to look for there.

Brakes are fine, had all 4 calipers refurbished and all new pads less then a year ago. Spark plugs are just the maintenance thing, I didn't meant to say they are bad just that replacement is behind the schedule.

I keep the tyre pressure on the higher side most of the time + this is long term thing, started well before weather got cold.The only puzzling thing - the doesn't feel sluggish, power seems to be ok with stuffed-up intake valves I would expect impact to performance, not only economy.

ColinBarber · February 10, 2019

26 minutes ago, Linas.P said:

ODBII gives no errors and all the sensors reports ok,

Sensors may not work correctly without triggering an error. A decent mechanic should be able to check this along with the fuel trims.

Linas.P · February 10, 2019

5 minutes ago, ColinBarber said:

Sensors may not work correctly without triggering an error. A decent mechanic should be able to check this along with the fuel trims.

Agree... in general, but it is easier said then done.

However last time decent mechanic was fixing my A/C, he turned the air from outside in winter when temp was 7C and noted that A/C performance is excellent - "blows 7C air". As a matter of fact it was not working at all.

In UK we have very few real mechanics left, who can actually take a car and find an issue. People do not keep cars past 10 years old and nobody really fixes older cars, parts are replaced as units and most of "mechanics" only knows how to replace oil and most basic things, like brake pads, coolant, Battery... even then they screw-up.

Fixing such "nebulous" problem like low MPG in older car is just not worth it (they still could not find why A/C does not work) - even if I find decent mechanic it will cost me more than a car is worth. So I rather try few things myself.

ColinBarber · February 10, 2019

If you have some ODBII software you should be able to get sensor readings. Make sure temperature sensors are displaying something that looks correct - e.g. if it is 7 degrees outside then if your sensor is stating -20 then there is an issue - likewise for the coolant temp which you should see rise as the engine warms up.

If you can see the long term fuel trims they should ideally be in the middle around 0% (goes from -100% to +100%), and more importantly the left and right banks should be similar.

Linas.P · February 10, 2019

Ok I will see if I can capture fuel trims. How log is long term? 17 miles to and from work would be long term I guess?

One other thing - I was doing some math on the range and it seems my car is reporting lower MPG then it actually is e.g. previously I could do 340-360 miles per tank which on average fill of 58-60L ~ say around 27MPG, which was approximately what was indicated. The long term tended to be higher as I would do weekend trip to Scotland or something on 37MPG tanks etc. Now I do ~290-310 Miles to the tank which is ~23MPH. My last Tank AVG indicated 21.2MPG after 308 miles and 58.92L brimmed, which makes it closer to 24MPG. It seems car thinks it sends more fuel then it actually does?!

Anyhow, even 21.2MPG is not as low as it could get... I now tend to drive really conservatively compared to before. If I drive same as when I was getting 27, 28, 29 MPG... I would be sub-20. Before it was like, I drive harsher or get into massive traffic and it drops to 24MPG, but then just few miles of smooth motorway and it picks-up straight away back to 27, 28. Now if it drops to say 22... it stays there. Equally before if I get half tank at 36, 37 and spend rest of the tank in the traffic the overall would still be 33, 34 etc. Now I just get into traffic can it drops right don't. It is not like I cannot achieve high MPGs e.g. last Thursday I refuelled on my way home quite late in the evening and the road was very quiet so I could cruise at dead 50 MPH for 10 miles and I got 44.1MPG... so it not like I cannot achieve it, but I need to try much harder and drive extra smoothly to get there. Clearly might highs were much higher and my lows were not as low. Previously, as long as engine is warm after refuelling I can drive home without showing off and even with traffic lights etc would would get easy 38-42MPG.

Anyway... enough story telling - in short something is a miss. If I press accelerator fuel goes down like into the drain. All considered MAF sounds quite logical conclusion...

Farqui · February 11, 2019

On 2/10/2019 at 10:51 AM, Linas.P said:

...

You doing your preventative maintenance well, despite having port+DI - well done you! Can you add some info of oil catch can you have?

...

Thanks! Refreshing the PCV valve was a simple maintenance task and LED to me fitting an air/oil separator so I can get some insight into the condition of the engine.

Oh yes I do enjoy a good DI flush of the valves

The can I have is available a little cheaper now; https://www.ebay.co.uk/itm/Universal-Car-Racing-Baffled-3-Port-Oil-Catch-Can-Tank-Air-Oil-Separator-Black/382718012049?hash=item591bc56a91

I blanked one of the ports and used heavy duty hoses than do not collapse under a vacuum (beware of some of the softer silicone hoses as they can collapse).

The can isn't very big and over 2-3k miles has only filled up with 20mm of oil/condensate over the colder months. For comparison, in the summer over 5k miles I only accumulated 8mm of mostly oil. I don't use my car daily tho so if used regularly then you may need to empty the can more frequently or acquire a bigger unit.

Farqui · February 11, 2019

21 hours ago, ColinBarber said:

Plugs won't cause that amount of a drop in economy without you also noticing poor running or a misfire error code occurring.

Clean the MAF

Check your brakes aren't binding

Possibly poorly performing O2 sensors, not enough to trigger a warning but will cause over fuelling. Need an OBDII reader to check your short and longterm fuel trims - other sensors should also be checked, e.g. coolant temp sensor.

Thanks for the guidance, I haven't tracked fuel trims before but have set-up torque to log these and temps/loads so will be interested to see the results.

ColinBarber · February 11, 2019

On 2/10/2019 at 9:30 PM, Linas.P said:

Ok I will see if I can capture fuel trims. How log is long term? 17 miles to and from work would be long term I guess?

It doesn't matter. The long term trims are stored in the ECU even when the ignition is turned off. This is one reason why an ECU reset / disconnect Battery is a good idea if you switch to a different fuel grade, so the long term learning is reset.

Linas.P · May 15, 2019

So... just a bit of update...

As mentioned above I started looking into fuel trims instead of trying to do anything physically with the engine. The saved trims were following:

Bank 1 Long ~ +10
Bank 2 Long ~ +18
Bank 1 Short ~ -4
Bank 2 Long ~ -3

Then I had few longer trips and fuel economy overall increased, but fuel trims remained roughly the same, so I reset ECU to see whenever it could be as simple as resetting trims.

After reset (~400 miles):

Bank 1 Long ~ +0.9
Bank 2 Long ~ +8.3
Bank 1 Short ~ -1.2
Bank 2 Long ~ -2.1

As far as the trim themselves I understand anything between -20/+20 is "within the spec.", however what worries me is the difference between banks - it seems sticking out point is bank 2 running +8 rich in long term.

Secondly, I have recently discovered that there is 7th injector on 4GR-FSE, which "aids" for cold starts. I have found one case where it was reported leaking causing one bank to run rich... wondering whenever that could be the case.

Finally, what is considered normal MAF readings? mine is reading 52g/s @2500RPM ... if that makes sense at all.

ColinBarber · May 16, 2019

I don't believe the cold start injector is fitted to European series II IS250s, although it is to the series III IS250 and GS250. It is located on the common intake and therefore if it was leaking it would affect both banks.

The difference in the long term trims could be one or more injectors playing up, or even the O2 sensor itself becoming lazy.

Marleez · December 12, 2020

My car drinks more fuel since I changed my tyres. If you have wider tyres at the back than the front. Your car will use more fuel than having 4 tyres at the same width e.g. 235/x/x on all 4 wheels.