Idle and over-revving issue since changing battery 58-plate 1.8 petrol

[YorkyPuds]

I've a 58-plate 1.8 petrol Focus that broke down last week due to a crap battery. The RAC 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.

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 trick
• Everything else seems fine, although MPG is pretty low to what I was getting previously

I'm not technically minded at all, but I've lifted the bonnet to see / feel / listen for any possible leaks and can't see anything. I've a generic code reader coming before 10pm today, will this possibly read some codes not displayed by the dash trickery? Any thoughts as to what it could be?

[YorkyPuds]

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

Any idea what this sensor is so I can get it sorted?

[YorkyPuds]

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 Timeing Advance For #1 Cylinder: 3.5 to 9.5

Is this indicative of a vacuum leak somewhere?

[YorkyPuds]

Bumping to see if anyone has experienced anything similar.

It's going into the garage Tuesday so will post with the outcome. Let's see if your guesses are correct!

[TomsFocus]

What do we win if we guess correctly?  

Generic OBD readers won't be of much help, you'd have been much better off buying a modified ELM for use with Forscan though I think postage takes a few days on those.

An alternator that looks new might just have been a cheap refurbishment job, lets hope not though.

The rev issue looks like either a vacuum leak or a throttle body fault....but could be a voltage issue, so that's really what needs sorting first.

[iantt]

If you have disconnected battery to replace it the throttle body may need re learning. 

[Botus]

I was thinking the same....

I believe the process is around ….

disconnect the battery completely (with battery nowhere near anything !!! ) .... touch the main battery leads to one another for a minute (plus and minus - clears any stored voltage / in capacitors / modules), reconnect and start the car.  Ensure you don't touch the throttle pedal at all (during cranking or after start) allow the car to reach operating temperature without any intervention

 

did I miss anything?

[YorkyPuds]

1 hour ago, Botus said:

I was thinking the same....

I believe the process is around ….

disconnect the battery completely (with battery nowhere near anything !!! ) .... touch the main battery leads to one another for a minute (plus and minus - clears any stored voltage / in capacitors / modules), reconnect and start the car.  Ensure you don't touch the throttle pedal at all (during cranking or after start) allow the car to reach operating temperature without any intervention

 

did I miss anything?

I'm wondering if the guy testing the alternator wiring reconnected the battery and drove it straight away so the ecu or whatever never had time to relearn.

Seems weird this rev problem was never and issue until the battery had been disconnected. It wasn't even doing it on way home from breaking down when the battery was almost flat.

 

[Botus]

if the battery has the right voltage and isn't disconnected for long the throttle body adaptions shouldn't get forgotten

[YorkyPuds]

5 hours ago, Botus said:

if the battery has the right voltage and isn't disconnected for long the throttle body adaptions shouldn't get forgotten

The battery was practically dead when he took it off, was only giving out 5V.

[Botus]

exactly, I watched a video on throttle body set up

 

as I put (although may be short out for 2 to 5 mins), ensure lights, heated windows, mirrors and AC is OFF, allow cluster to do self test before cranking.....   start and leave to idle to full operating temp.  after this go for a normal drive (not slow not flat out), the phase of idling through to normal operating temp sets up the base throttle position (it moves with engine temp) and then a normal drives ensure the rest gets set up...

explains what's going on and why its needed, https://www.youtube.com/watch?v=ZNxXM75zJOw

 

 

[Botus]

just writing that and thinking about the video sounds more like it clears the PCM's adaptions and they rebuild following that procedure !!!

[YorkyPuds]

Quick update, Garage that I bought the car off have replaced the alternator despite the old one looking pretty new. Doing so has thrown up an error code in the ecu that suggests a faulty EGR valve, although no warning light came on.
 

They've ordered a new one to be fitted tomorrow.

 

[YorkyPuds]

Just had a call from the dealer who is trying to sort my issues and it feels they're clutching at straws.

They've now checked the hoses / pipes and there are no splits or holes. Their mechanic says everything leads back to a pressure control valve or something so they're going to order one of those and see if that does the trick.

[YorkyPuds]

PCV valve didn't fix the problem so the mechanic kind of gave up.

Dealer decided to reset the ECU by disconnecting the battery, letting it idle etc and it's completely fixed! FFS!

[FatHead1979]

On 9/3/2020 at 11:34 AM, YorkyPuds said:

PCV valve didn't fix the problem so the mechanic kind of gave up.

Dealer decided to reset the ECU by disconnecting the battery, letting it idle etc and it's completely fixed! FFS!

At least you got a few free brand new bits!  (brand new alternators aren't cheap you know!) 😁