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Old 17-03-2019   #1
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100hp mpg drop?

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?
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Old 17-03-2019   #2
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Re: 100hp mpg drop?

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.
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Old 17-03-2019   #3
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Re: 100hp mpg drop?

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.
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Old 17-03-2019   #4
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Re: 100hp mpg drop?

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 ?????
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Old 17-03-2019   #5
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Re: 100hp mpg drop?

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.
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Old 17-03-2019   #6
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Re: 100hp mpg drop?

Sticking brake caliper possibly? Not enough to notice but enough to drag the mpg down.
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Old 17-03-2019   #7
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Re: 100hp mpg drop?

Quote Originally Posted by Kronkron View Post
Sticking brake caliper possibly? Not enough to notice but enough to drag the mpg down.
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.
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Old 17-03-2019   #8
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Re: 100hp mpg drop?

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.
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Old 17-03-2019   #9
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Quote Originally Posted by dac69er View Post
As mentioned in my original post. I have gone over the brakes.
Apologies, in my haste I missed the last sentence on your original post.
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Old 17-03-2019   #10
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Re: 100hp mpg drop?

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.
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Old 17-03-2019   #11
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Re: 100hp mpg drop?

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.
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Old 17-03-2019   #12
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Re: 100hp mpg drop?

Quote Originally Posted by portland_bill View 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.
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.
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Old 17-03-2019   #13
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Re: 100hp mpg drop?

Quote Originally Posted by DaveMcT View Post
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.
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.
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Old 17-03-2019   #14
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Re: 100hp mpg drop?

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.
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Old 18-03-2019   #15
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Re: 100hp mpg drop?

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.
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