My monti Carlo 1.2rsi is normally great on petrol doing between 46and 54 mpg  and almost done 100k miles but today it used 1/4 of a tank todo 18 miles every once in while it dose on a short 20 mile journey do a low mpg any one have any ideas what could the problem be 

thanks in advance 

soamer 

Please note - I am not a mechanic or expert in anything.

 

Hi welcome, a quarter of a tank to do 18 miles doesn't sound right, if you take the tank as 10 gallons (45 litres) then it's 18 miles uses 2.5 gallons which is about 7 mpg,

 

If you are just taking the figures from what the car's computers/gauges tell you without checking them this could be the problem.  You need to check by filling the fuel tank to first click of the petrol pump nozzle, zero your trip then when you next refill fill the fuel tank to first click of the petrol pump nozzle, divide the fuel quantity into the mileage covered on the trip (and reset it).  Divide the litres by 4.546 to give you the number of gallons.  This will give you an average mpg on this refill, it may or may not be very close to what the car tells you.

 

Shorter journeys will generally give lower mpg but at 7 mpg I'd think you'd either be towing a house or have a noticeable fuel leak.

 

HTH, do an mpg check as described above on a 20 mile journey if you really want to check just a short journey.

Edited 29 October, 20223 yr by nta16
ETA: Please note - I am not a mechanic or expert in anything.

Sorry should of said full tank last night 

Many thanks first quarter is normally about 130 miles dependent on sped and road type it is odd on how it did this but tonight been on a 36 mile trip fuel gauge hadn’t moved I’m starting to think the sender unit 

Welcome.  Have you refilled to see what you used?   So have you got a locking fuel cap?    It would need to be a very choked air filter to have that bad economy and that does not happen overnight.     Winter spec petrol was staring to be delivered in Scotland and the north of England this week. That would make no difference.   

Edited 29 October, 20223 yr by roottoot

Yes could be the sender unit or the connections to the unit, my MK2 had this very issue and the only way was to fill tank wat untill it dropped the quarter or so then fil up again turned out to be the unit devoloped a fault and had to replace it, mine was under the rear seat and I purchased a new unit fot £46 , after fitting everything seemed normal again

Please note - I am not a mechanic or expert in anything.

 

It sounds like you're just reading off partial fuel gauge readings to see the mpg, don't do that.  My wife's previous car, a Vauxhall (GM) product, from a full tank would go over 100 miles before showing about a gallon use, and this was from brand new, then it'd suddenly drop to somewhere near more accurate.

 

Use the fuel gauge as just a 'gauge' and not an 'accurate' and don't let you fuel get too low and I used to when younger and had to push the car or use the spare can in some awkward places.  Also don't fully trust what the computers might tell you with regards to mpg again use the info as a guide only.

 

Some of the older folk here will probably remember driving old cars where the fuel gauges were broken and unreliable and using the tripometer to gauge when to fill up and using the rev-counter (on cars that had one) for road speed when the speedos were unreliable or inaccurate - now it would be using your brain to check the cars brains in some areas.

 

Edited 30 October, 20223 yr by nta16
spelling

Please note - I am not a mechanic or expert in anything.

 

Meant to put - does the sender appear to stick after the car has been parked up for a few days or it sticks intermittently with regular frequent use, if not parked up then it does sound like perhaps sticking sender but if intermittent perhaps it could be after sender, a connection, wire, or gauge end.  Plugging in a scan tool might show something up perhaps with live data.

 

Edited 30 October, 20223 yr by nta16

24 minutes ago, nta16 said:

 

It sounds like you're just reading off partial fuel gauge... check the cars brains in some areas.

 

I still do this with the trip meter, with a trip computer in the car. To put how inaccurate this can be over a single journey, I did half a tank (roughly) commuting, then did a trip twice the distance to pick up a visitor from a local ferry port (so same driver, same car and mostly the same roads) and came back with a longer predicted range than I left with.

I’ve had the car 4years and the last twelve months once filled up which normally will last around 430miles but once and a while you can go a short distance and use 1/4 of a tank in 18miles which normally would be around 110 miles engine runs fine just odd that once in 10 weeks it dose this 

Odd.  I take it you use E10 petrol.   Maybe stick in a tank of E5 99 Ron for the sake of under £3 extra it costs since you are up on how much range you get.  Even if you get no more miles you are getting some detergent as well as the higher Octane.       Are the spark plugs all good and replaced sometime in the past 4 years?  Is the air filter clean and dry,?    Brakes serviced, tyre pressures set for winter and just keep paying attention on brim to brimming. 

Edited 30 October, 20223 yr by roottoot

Please note - I am not a mechanic or expert in anything.

 

3 hours ago, Soamer said:

once and a while you can go a short distance and use 1/4 of a tank in 18miles

But how are you measuring this fuel consumption?  If we take the 18 miles as being pretty accurate or near enough how is the 1/4 tank use measured?

 

At about 7/gallon you'd probably be able to smell the petrol leaking or coming out of the exhaust (at standstill with engine running and you at end of exhaust).

 

Personally I take no notice of the what the computer tells me is the predicted range as it only relates to whatever measurements were made in the last time frame and set of circumstances which may or may not be replicated and I don't know the level of its accuracy anyway.  It's like the times when someone tells you there's plenty of fuel in the car when there isn't.

 

Edited 30 October, 20223 yr by nta16