why would this happen

[carrots 1]

hi my 1.8 16v was reading 39mpg on a run after i done the head gasket.2 years on and i am lucky to get 23 on a run.i no the mfa a not 100% true reading but it made me feel beter when it showed 39mpg

[Roger Blassberg 0]

Firstly, what is your real mpg based on fill-up to fill-up? Has it really dropped or not?

 

On your car I think that the MFA takes its mpg reading from a vacuum sensor connected to the inlet manifold via a small bore hose. If either of those is not airtight there will be an error; if it sees less of a pressure drop it will assume that the throttle is open and will give a correspondingly lower mpg reading.

 

Best wishes

 

RB

[carrots 1]

if i am lucky i can get 150 miles to £30 i will check the hose cheers

[Roger Blassberg 0]

That's 30mpg as near as makes no difference which isn't a bad average. If your MFA says 23mpg, that is outside of the usual error range. On the other hand, the earlier 39mpg was too!! Check also that your brakes aren't binding; does it roll easily if you coast up to a set of traffic lights in neutral for example or are on a very gentle slope? Sticking calipers, especially on the rear, are an occupational hazard.

 

RB

[carrots 1]

i tried the roll test its fine.suppose 30mpg not to bad for an old car

[Roger Blassberg 0]

To get the performance out of a 16V engine, it really needs to be revved quite high and the gear ratios are designed accordingly with a reatively short final drive. And the fuel injection metering isn't very sophisticated to be honest. Particularly if your driving style is a bit "press on", you can't expect more than 30mpg average.

 

But certainly, to answer your original question, the MFA should be within 2-3mpg of the actual figure unless the hose is leaking or the diaphragn sensor is defective.

 

Best wishes

 

RB