25 replies to this topic

[jakejakejake1]

Hi everyone,

I'm sure there is a problem with my car, but unsure what it is.
The car is a 1991, 998, with freeflow manifold, alloy inlet manifold and 12g295 head. Standard cam.
My MPG have been decreasing quite rapidly the last few times I have filled up.
I have made a spreadsheet and calculated my MPG by always filling the tank and noting down the mileage on the car, so no estimating from the gauge.

Now when i tried this 6 months ago i was getting between 35-50 mpg depending on where i drove etc etc.

But the last four mpg calculations have been 39, 30, 26, 23.
Now clearly something has happened and even when I used to floor it everywhere i couldn't get it below about 30mpg, now its dropped to 23 and I'm driving very carefully. And the 39 and 30 values were obtained over long distance at about 50-60 mph.
Admittedly the journeys haven't been the longest (5-8miles on average), but then again, they never have been.

What could be causing this huge fuel consumption/ loss of fuel?

Thanks, Jake

[BobJob]

Have you put on lots of weight recently?

[Ryang556]

Checked the fuel hose going into the carb etc?

[nafnaforiginal12]

Time for a service?

Just a thought

[jaydee]

Clogged air filter or poor fuel quality?
Traffic? Carrying extra weight?

[Wil_h]

First thing I'd do is clean the carb and check the dashpot oil. then check that there are no air leaks on the carb (after the throttle plate). Check that the vac advance is working, this is key to good mpg.

Other things that will affect mpg, binding brakes (the rears are great for getting stuck). flat tyres, although when you do the maths, they have to be really flat to make any differance.

Check also that your exhaust is in good order, a collapsed scilencer will be a problem. And finally renew your airfilter if it's dirty.

As suggested above a service of plugs, leads, dizzy bits etc. will be worth while.

[Ethel]

Look at the plug colour to see if it's a fuel issue.

[mk3 Cooper S]

Roofrack?

Binding brake?

[brad-the-bear]

is the head leaded or unleaded?

[Tommyboy12]

Good shout on the leaded head. 12G295 will be leaded as standard. That lead build up may be wearing away.

[mk3 Cooper S]

Easily checked with a compression test

[brad-the-bear]

[leroy26]

hgf?

[maggies_minder]

fat passengers

[Cooperman]

The way to improve the mpg, assuming the engine is set-up properly, is to do the following:

Fit narrow wheels and 145-section tyres, remove the wheel arch extensions, remove the big wing mirrors and replace with the small rectangular mirror only on the driver's side A-post near the top, drive with the windows closed, drive with the tank no more than 1/2 full, don't have a roof rack, drive in high gears without using full throttle, don't carry unnecessary weight around and don't have fat passengers.