Vanishing MPG

Right then.

When I first got my car it turned over a nice stable 24ish mpg on my daily grind run to and from work, plus a little bit of pleasure use on the side.

I have had the car for three years. Over the last year or so I have noticed a gradual drop off in MPG (I'm one of those anal people who keep records of that kind of thing). Anyway, since, say, the early part of 2003 I've seen a steady decrease in MPG from 24 down to about 20 (a drop of 17%).

If I was really careful I used to be able to squeak 500km from a tankful. These days I barely scrape 420 without the orange light coming on (a drop of 16%).

OK, by now you might be thinking "Digsy knows about these engine things. I'm sure he knows what is going on." Problem is, I don't.

My driving style has not changed. My routes have not changed. I am using the car a lot more, but that is just miles under the tyres. I'm not going drag racing every other weekend. Most of my use is to and from Norwich, down the same roads at the same speeds, time and time again.

My car does not seem to be running rich. Last MOT (a month ago) the lambda reading was slap bang in the middle of the acceptable scale. The exhaust is not sooty or smoky. I would assume that this means my O2, MAP and MAT sensors are all OK and the fuelling is fine.

Performance seems about the same as ever.

I have no leaks. I cannot smell petrol outsideor inside the car. There are no telltale stains on my driveway.

So, where the hell have those 4 mpg gone? Any ideas?

My first port of call will be to dig out the basic check list for the NA and measure a few voltages (O2, MAP, MAT) and see is everything is truly OK. If those check out then I'll be scratching my head.

I'm trying to cut some running costs out of my car at the moment, so a plummetting MPG and climbing petrol prices are exactly what I don't need. Thoughts?

new wheels and tyres?

I have changed wheels but only very recently (within the last month). I have changed tyres several times (rears twice and fronts twice) during ownership, but the MPG reduction has been gradual over the whole time.

are you up to 63K miles yet.

spark plugs?

I've had two sets of plugs installed. One set at 63000 and another literally within a few weeks that were put in by accident at 81000 by my local Toyota dealer at my last service. I got those for free

I'm going to plot out all my MPG readings tonight and see it if is indeed a gradual drop or whether there is an "event" somewhere along the line that I can tie to a service or something.

Put on weight?

Parked in my garage?

Closed loop lambda sensors readings/tests wouldn't reveal a slight overfuelling situation if it wasn't the O2 sensor at fault - the closed loop system would be working and would compensate. So if you were overfuelling all the time, but not by much, you may well see a drop in overall mpg caused by non-closed loop driving. Acceleration and liftoff I believe, and as your daily grind is a lot of back lanes, I guess it isn't going to see closed loop that often, compared to a motorway slog for example.

So, check pump voltage, fuel pressure, O2 readings on acceleration, maybe even the injectors...

-Ian

ignition systems tend to lose integrity/insulation resistance over a time most people dont change them as the car still starts and runs fine but it will cause increased mpg-could be that

Cheers for the replies guys. I sense a weekend at Ian's with the multimeter coming up...

Jagman, I'd g with something breaking down too. ANy ideas on what tests I could run to diagnose the ignition system? The NA has a dizzy and a coil which won't get mentioned in the routine servicing schedule (although I do ask the garage to check them periodically). MAybe I'd better have that apart and see if anything is amiss.

MInd you, I just plotted the MPG figures out and maybe I'm making a fuss over nothing. It just feels like I'm not going as far on a tankfull though...

See for yourself:

Hmm, I dunno, I see a shift downwards of about 22mpg at around 22/06/2004 onwards.

Which is, I shit ye not, roughly when you had your car in my garage.

-Ian

I see what you mean. There's such a scatter though that it's difficult to tell.

Another odd thing: This is on my mind at the moment so I'm tending to watch the fuel gauge a lot. Bear in mind that every time I fill up I really brim the tank. Last tankful by the time I'd done 100km the needle was on the first 1/4 mark. I distinctly remember thinking "that ties in with doing 400km on a tankful now."

I filled up again on Sunday (brimmed again) and this time I did a full 100km before the needle got to the very first small mark on the gauge. Seemed to drop faster after 100km.

I know you can't really use the gauge for accurate measurements but that kind of difference surely can't be right?

I see 25 miles (40.32258064516129032258064516129km for those Europeans amongst us) for each notch on the gauge for the first quarter of a tank but after that it goes a bit mad. And the last quarter disappears alarmingly fast. It's usually repeatable though - dunno what's going on there, you need more data points

-Ian

Originally posted by Digsy

I see what you mean. There's such a scatter though that it's difficult to tell.

Looking at that graph, I think Ian's right - there does seem to be a distinct decline over the last few months. You've got a cluster of lower points that looks (to me) almost distinct.

Are you basing the conclusion that you're getting a consistent and steady fall in mpg on this graph? I'm wondering if that cluster of lower mpgs at the end was enough to make the plotting program give the line of best fit a smooth decline overall? Because other than that, the scatter of points looks overall to be pretty level to my eyes.

I don't know how those programs work, so it's just a thought.

Cliff

You haven't started using different petrol or something? From, say Optimax to normal SUL or normal UL even?

ref ignition checking- if you run the car in the dark(ie a garage at night time with the lights) you can sometimes see a glow from the leads which is the leads breaking down- usually they go at the joints in the leads -or sharp bends-wherever there is a joint it will have less insulation resistance this is usually where trouble starts due moisture from condensation which gradually degrades insulation further, quality leads have fully bonded ends-(no joint)

and are 100% silicon ,much like synthetic oil spec manufacturers can get away with only 10% silicon content and still market them as "silicon leads" once again you pay for what you get.

the ignition on the tt although using coil packs is not everlasting and will break down insulation eventually,but I see few people on here considering replacing them ,despite upping the boost,single conversions,etc , the coil packs work in hot surroundings and are now 8,9,10 years old. I think its worth checking your ignition system or simply replace components for peace of mind

I assume you are plotting these graphs based precisely upon the amount of fuel put in the car at each fill up versus the exact mileage you did to use that fuel. And that you fill the car up to the same point each and every time? Also that you don't have a variable load ie you now give someone a lift or a teenager that is rapidly weighing more and more.

You could never gauge your exact MPG by saying that you have a 60 litre tank and fill it up when the light comes on and assume that you have put 60 litres in each time. Fuel gauges have to be the most unreliable piece of kit I have ever come across.

There are also seasonal differences especially in lower distance travelling cars as the weather gets colder due to the choke being proportionally on for longer than in the summer.

There will also be an age factor involved in rotating components efficiency. Also to consider are braking components. Any mechanical system over time loses efficiency and you will take power loss's over time as things wear out slowly. Braking components also get a bit sticky and even a slight rubbing will manifest itself in lost MPG.

Just for clarification, what I do is this:

1) Fill up the car until the tank is brimmed. And I do mean brimmed. Until I can't get a drop more in. I've squirted a oad down the side of the car more than once, while making sure the tank was full.

2) Reset trip A. Drive about. Tum ti tum.

3) Fill up again, as per step one. Record amount of fuel in litres, record trip A in km. Bang figures into spreadsheet.

4) Repeat until fade.

I don't fill up at the same pump every time so I know I can't be sure that the pump is "clicking off" at precisely the same point, but volumetrically speaking if I am filling up to the top of the neck each time then the figures must be roughly right.

Right then - check this out.

In oder to make a bit more sense of the data and to try to account for inaccuracies in my measurements I have plotted out the average MPG over the preceeding 20 tankfuls against time.

This shows the picture a LOT more clearly.

So what the hell happened in June 2004? I had my best ever MPG and then it all went downhill.

*edit* I had a new set of tyres fitted all round three days after the all time high reading, but that's it. Surely that can't be the cause?

I fitted glass front headlights too. I know that there is an air intake under the driver's side one, but again...surely not?

Yeah thats exactly how I figure my MPG out as well. No other more accurate way really. Only 1 thought occurs here on an indication side. Is the odo getting a true signal? One way to check is with a GPS system. Do a set length of drive on GPS and see if the figures tally up with the odo, but I can't really see the odo reading degrading over time. (its the last variable in that equation though).

As far as the tyres are concerned you would notice if there was perhaps dirt trapped between the brake disc and the wheel causing it to run a little untrue. You may also notice if a different profile tyre had been fitted. Some tyre companies do make claims that their fuel saver tyres can improve the cars mpg by 10% but I am inclined to think of that as utter BS myself.

With your figures you can't accept it as a true down turn until the 22/07/2004 as its not until then that it seems to dip below your other low points.

Perhaps the brakes aren't that crazy a place to look though. They would have albeit marginally been disturbed when you put the new tyres on. They may just naturally be starting to catch and not release fully due to ageing. Intermittent brake rubbing would account for a variable loss of MPG IMO.

Mm, there are genuine econotyres out there DB. But the pricce you pay is less sticky rubber (therefore less rolling resistance therefore less fuel used to cruise). My housemate's Golf company car has got some on and they are embarassingly squealy at the most sedate of cornering speeds.

Now, if you changed brands of tyre and got a more grippy set on, you may well notice a drop in fuel economy...

You also had that sticking rear caliper a while back - it suddenly stuck and overheated, and you smelled it and freed it up, but if it's suddenly sticking again enough to drag but not enough to overheat...

-Ian

Originally posted by Ian C

Mm, there are genuine econotyres out there DB. But the pricce you pay is less sticky rubber (therefore less rolling resistance therefore less fuel used to cruise). My housemate's Golf company car has got some on and they are embarassingly squealy at the most sedate of cornering speeds.

 

Now, if you changed brands of tyre and got a more grippy set on, you may well notice a drop in fuel economy...

 

You also had that sticking rear caliper a while back - it suddenly stuck and overheated, and you smelled it and freed it up, but if it's suddenly sticking again enough to drag but not enough to overheat...

 

-Ian

I used to do 70 miles to work and back for over 9 years and used different brands of tyres from grippy continental super sport contacts to michelin fuel savers etc. There was no marked difference in fuel economy hence my sceptiscism.

STicky caliper huh well theres a a coincidence

I'm pretty sure its not the return of the sticky caliper. That happened in December 2003. I freed it off by some gentle excercise with a G clamp and it hasn't returned since. Since that day I regularly check my calipers after a drive by going round and touching them to check how hot they are. These days they all stay the same normal "warm" after a blast.

I'd also be surprised if it was the tyres, but it does seem like a coincidence, and probably the only thing that directlty ties up.

There is, however [Columbo] just one more thing, Sir. [/Columbo]

And I'm kicking myself for not thinking of it sooner.

This summer was the first period that I left the aircon on pretty much all the time. DOH! I hear you all cry. What a numpty for not realising that. However, switching the aircon on should be a step change in fuel consumption which would knock a set amount off. If this was the case, I would expect to see a stabilisation in the averages after 20 data points. Since June I have had 30 tankfuls and apart from a slight glitch in September, the average MPG still appears to be falling.

I have now switched the AC off again as Autumn has well and truly set in. Let's see whether the figures recover.

Somethine else interesting to note is the gradual decline from the start down to November 2003 and then the rapid rise again. In november 2003 I had my 63000 mile service which included new spark plugs.

Just in case there are any single girls reading this, I am a prefectly normal guy with a healthy social life, lots of friends and a good sense of humour. I don't spend my whole life logging MPG figures, OK?

Originally posted by Digsy

Just in case there are any single girls reading this, I am a prefectly normal guy with a healthy social life, lots of friends and a good sense of humour. I don't spend my whole life logging MPG figures, OK?

Yeah whatever you say mate...

Glad the penny has dropped.

The amount of power the aircon needs depends on the external temperature, so the hotter it got, the more it would affect your fuel consumption, would that explain your decline in economy?

-Ian

Possibly. I still would have thought the readings would have stabilised by now though. Its not like we had a blisteringly hot summer...

Time will tell I guess. This tankful is looking like a good one, by coincidence. 250km and still not at the halfway mark.