The average mpg on my girlfriends 100hp has always been around 35mpg for short journeys and about 45+ on a run.
Worse in the colder weather and better when warmer as expected.

Now for some reason, it is similar to before on short journeys (maybe a little lower) and I am struggling to achieve much above 40 on a good run!? Even sticking to 60mph barely makes a difference.

I have checked for ecu codes, made sure tire pressures are ok and checked brakes weren't binding or anything.

Any ideas?

You've checked for codes. Does this mean you have MES? If so, connect it and watch the readings for coolant temperature and fuelling, especially the two oxygen sensor readings. Airflow and intake air temp if available might be useful.

If it is pushing more fuel through, it is doing so because it thinks it needs it. If air or coolant temps are reading lower than actual, it will overfuel. A leaking injector will screw things a bit, and the oxy xensor readings may give a clue to that.

A common thing is a failing thermostat, running coll enough to increase fuelling, but warm enough to give an adequate gauge reading inside. From cold, start it up and monitor the top hose. It should stay 'cool' for some time, then suddenly get hot as the stat opens. There will be some heat transfer along the hose, so feel nearer the rad than the engine. If it warms gently, the stat is open.

are the 100hp that bad on long runs.

Driving gently the 1.2 averages 62-68 on long runs ? if you keep the top speed to around 55 mpg

even just short 3 mile runs its High 50s


I ran a faulty thermostat for 6 months and didn't notice any difference in MPG
mixed driving it was always above 62 mpg.


Wind does effect MPG. Same 200 mile journey tail wind 68 MPG headwind 58mpg.

As does the road surface.

I had a 1.2 dynamic don’t be fooled by the mpg readout my was 10mpg over most of the time
Ime a 56year old git and never got more than 48actual mpg out of mine

No I have a 100hp shows 44mpg actual 42 mpg so not that far off
But I have noticed the last 2 weeks my mpg has dropped by 5mpg and I think it’s because of the high winds we’ve been having ?????

100HP is not an economical car but the OP's does sound low. Thermostat is a likely culprit.

A slight missfire can also cause overfuelling.The O2 sensor reads the unused oxygen (but not the unused fuel caused by absent sparks) which the ECU takes as underfuelling so makes it richer overall.

Sticking brake caliper possibly? Not enough to notice but enough to drag the mpg down.

Kronkron said:

Sticking brake caliper possibly? Not enough to notice but enough to drag the mpg down.

Click to expand...

As mentioned in my original post. I have gone over the brakes.
I replaced rear discs and pads as well as changing one of the rear wheel bearings as it was picked up on the MOT as being a little noisy.
The rear calipers have also been treated to a rebuild.
Fronts are fine, just a quick clean and grease where needed.

I don't have MES, just a generic fault code reader.

It generally drives fine, I am sure there is a slight lumpy idle. It's not very noticeable, but it doesn't feel 100% to me.

The car has had 2 thermostats in the time she has owned it (just over 4 years) last one was fitted just over a year ago. I guess it could be that, but this wouldn't cause a lump idle.

Had it's MOT a few weeks back and nothing stood out on that, so it's not that bad.

May have to splash out on MES to allow me to fault find the issue. Not worth randomly replacing stuff.
I may bang a set of plugs in it first though as it's probably due those being changed anyway.

dac69er said:

As mentioned in my original post. I have gone over the brakes.

Click to expand...

Apologies, in my haste I missed the last sentence on your original post.

Throughout these forums there are tales of short-lived thermostats. General recommendation is to pay the extra for OE, or ensure a good brand name.

Stick coils don't last for ever. If one is going down, that will cause the missfire issue. MES might pick it up but sometimes coil faults have to be found from the other symptoms.

MES have a free version so you'd only need the connection cables.

portland_bill said:

Throughout these forums there are tales of short-lived thermostats. General recommendation is to pay the extra for OE, or ensure a good brand name.

Click to expand...

The first one I got was Quinton and hazel I think, was from local motor factors.

Next one was an original birth one. The temp sensor failed after 6 months, so had to replace that.

Tbh, the thermostat appears to be working ok. The temp rises as expected. Fan comes on when expected.
When the thermostat was gone before we never experienced mpg issues like this, it only made a slight difference.

DaveMcT said:

Stick coils don't last for ever. If one is going down, that will cause the missfire issue. MES might pick it up but sometimes coil faults have to be found from the other symptoms.

MES have a free version so you'd only need the connection cables.

Click to expand...

That is true. But they usually last if the plug changes are done when required so they aren't strained.
I may have to look into new coils though.

Have downloaded the free version of MES and already have a cable, so will give it a try and see if that shows anything obvious.

Will report back once I have given everything a look.

I had some serious fun and games with a BMW bike that use stick coils. Its the air/oil cooled boxer twin with two primary coils and two secondaries. The latter are to meet emissions rules as the engine will hard run on them alone.

I was having a intermittent vibration and high fuel consumption. O2 sensors where soot black. Swapping the coils left/right made no appreciable difference. The bike eventually went onto one cylinder making it obvious. I got a new coil (£80 HOW MUCH!!!) and the other coil failed on the way home so an about turn for another £80 bill. Fuel consumption went back to normal vibrations improved and thankfully the O2 sensors were not ruined.


My point - it can be very hard to diagnose failing coils, but if you pull any out that smell of hot insulation it's likely they are packing up. A fairly crude test but both of my old coils were smelly. 25,000 miles later, the new ones are completely unfragrant.

I repeat a thermostat that doesn't shut properly doesn't harm the MPG too much. There's hardly any if an improvement after being replaced.


a faulty coolant sensor will. But you should see this on the gauge and doesn't sound like the case here.

post a picture of the spark plugs. Let see how the fuelling is and if all the cylinders are the same

koalar said:

post a picture of the spark plugs. Let see how the fuelling is and if all the cylinders are the same

Click to expand...

i have ordered a new set of plugs anyway as they have done just over 20k miles.

will get some pics when i swap them.

i connected up with MES and nothing jumped out at me. i have taken some screen shots of the figures and will post later.
it seemed to be idling pretty smooth when i was going through it this morning.

only odd thing i noticed is that when i looked at the graph of the idle speed it was generally ok, but had the odd dip every now and then!?!? not sure if that is a bug with the software or something odd. i assume it is picking the idle speed up off the cam position sensor, or is there a crank position sensor also?

i am tempted to try the ecu reset option that is available on the MES software to see if resetting everything helps?

please see info i got from MES this morning. car had been driven about 5 miles or so to work from cold.

only thing that i noticed is the spark advance never changed, but i didn't actually go for a drive.

the graph is the idle rpm, as you can see, there are some strange random dips. not sure if this is a software/connection type issue or an actual issue. as you can see, its not dipping at a constant rate.

should get new plugs in the next couple of days, so will have a look at the coils, measure the resistance (this may not tell me much, but i can always have a go), and will take some pictures of the old plugs.

any other suggestions would be greatly appreciated. thanks

Well the free version of MES doesn't allow you to actually do the reset

Got the new plugs today so will try and change them tomorrow.

koalar said:

post a picture of the spark plugs. Let see how the fuelling is and if all the cylinders are the same

Click to expand...

Look ok to me.

Stuck the new plugs in anyway, will see if that improves the mpg.