I've a 58-plate (2008 Mk2.5) 1.8 petrol Focus that broke down last week due to a crap battery. The recovery guy said the alternator wasn't doing its job. He fiddled with a "small connector" and the alternator kicked back in again, but only for a few mins. He commented that the alternator looked new but the wiring could be dodgy.

Managed to get home and take it to an independent. He changed the battery (was only pulling 5V) to a Bosch S4 one. Tested the Alternator wiring as far as he could up to the main loom and there are no breaks. The alternator's 3-pin plug is getting a signal to and from the ECU too apparently. It's one of those SmartCharge things Ford is adamant is a good thing.

He reconnected the alternator and the car's idle went mental, rising to 2.5K and more without touching the accelerator pedal. We disconnected the 3-pin plug so it's now working as a standard alternator. Everything was fine for a few miles but now the following happens:

• Changing gear from 1st to 2nd leaves the revs high and they don't come down until 2nd is engaged. If I change gear at 2-2.5K they remain at 2-2.5K until 2nd is engaged.
• If I coast with the car in neutral / clutch down, the revs drop to around 1.2K but hunt between 1.2 and 2K as if someone is blipping the accelerator
• The car will sometimes automatically rev up to 3K when at a standstill and the only way to stop it is to engage a gear. I like like a right knobhead at traffic lights sometimes!
• I've noticed the current MPG counter shows 99.9mpg when I have it in say 4th or 5th but aren't giving it any gas. It registers an acutal figure when I do the same in 1st and 2nd, like gas is still being applied
• The current MPG counter also doesn't go to 99.9mpg if I have the car in neutral and am moving. It reverts to say 0.3l/100 or whatever when i come to a standstill
• There's no EML showing, although the battery light is on because of the 3-pin being disconnected
• There are no DTC codes when I do the dash diagnostic trick
• Everything else seems fine, although MPG is pretty low to what I was getting previously

My OBD2 or whatever code reader came and I had a look around the different sections / readings.

Everything else seems OK but there was a Oxygen Sensor Output Voltage test. BS-S1 was massively fluctuating from like 0.005 to 0.700V but BS-S2 remained constant at 0.665V

I then got it to do the test and under the Bank1-Sensor test, there was one called $80 that showed these results:

Test ID: $80

Test Value: 0.7448V

Minimum Limit: 0.5000V

Maximum Limit: 0.0000V

Status: Fail

Some of the other readings at idle once it'd been running 10 mins or so:

Absolute Throttle Position: 13.7% to 14.1%

Intake Manifold Abolsure Pressure: 7.5% to 7.8%

Short Term Fuel Trim: minus 3.9 to 0.8

Long Term Fuel Trim: 7.8%

Ignition Timing Advance For #1 Cylinder: 3.5 to 9.5

Is this an indication of a vacuum leak somewhere?