90 Spec GTI 8v Over Fuelling - 42mpg to 9mpg - please help!

[90-Spec]

Hi all, any advice with this gratefully received, I've had an 8v GTI for a number of years and it's suddenly started over fuelling big time. The symptoms are very rich smell, constant fuel pump whine with engine running and MFA indicating 6-10mpg.

The car has just been put back on the road after 12 months laid up. Yesterday was indicating 42mpg, after a good drive I pulled up at some traffic lights and the idle started hunting between 400-1500rpm. Today the idle is suspiciously smooth and it's running rich as hell.

So far:
1. New blue coolant temp sensor (then disconnected, revved 3 times to 3000rpm to clear ECU memory and reconnected)
2. Checked earth straps on gearbox, side of rocker cover, coil and starter. All are attached and look as clean as anything else in a 23 year old engine bay. Have I missed any earth straps out?
3. MFA was indicating 42mpg on a gentle run yesterday, today reset and 6-10mpg
4. It's a non-cat model

Other than this it runs perfectly, in fact better than it did before this fault cropped up.

Would appreciate your thoughts!

[90-Spec]

Should also say it's just had the timing and CO content done, and passed an MOT emissions test 2 days ago blowing 0.3% CO at idle.

[apollo996]

You need to try another ECU.

[javalin]

wiring fault - check the wiring condition to the injectors..... Common problem that it will crack/split and its possibly earthing and completing the circuit.

[90-Spec]

Thanks for the comments, just to close this one off for now, I have determined the MFA is not providing an accurate reading. Went on a 50 mile run in the car and calculated fuel consumption the old fasioned way as 34mpg. Still smells rich when warm but at least that keeps people from tailgating 

Will replace a couple of tatty looking rubber L-pieces on the vacuum lines to see if that nails the mis-reading. Noticed this evening it also thinks it's -14 degrees outside, so perhaps still can't completely eliminate ECU on the way out. If I get to the bottom of it I'll post again.

[Mr Blue]

MFA is a big lie. vacume lines may be the problem yes.

Get the timing and mixture adjusted. Also check the fuel lines as they can weep (old cracked rubber)

[90-Spec]

Finally got to the bottom of this after many hours swearing/dismantling things. The vacuum lines to the MFA, air box and inlet manifold were all badly deteriorated, particularly the rubber L-pieces.

There was also a split in the warm air pipe located at the bottom of the air box that runs to the back of the engine, which explains why the car was rich when warm. I guess it's an important bit of plumbing in relation to fuelling when warm as these old cars don't have heated o2 sensors.

Replaced the lot, re-did the timing, adjusted the idle and we're back to 42mpgs trundling down the A44 again. Winning!