Polo 1.4 massively negative LT fuel trims (-85)

[grahelcurt]

Hi
I am a home mechanic wanting to learn. Hopefully someone can help me.
I have a 2006 Polo 1.4 with massive negative fuel trims. At idle it runs slightly lumpy and when stopping at junctions feels like its going to cut out. When driving, it drives very smooth with no hint of a misfire.
History so far
When i bought it there was a leaking injector. This was replaced and all were checked... all now working ok
Map sensor changed with new to remove code
After driving around for a couple of months the water pump went so replaced water pump and timing belt.
Oil then was leaking into expansion bottle. So head gasket was replaced (skimmed and checked for cracks, also valves and guides checked)
PCV Valve was then changed because it was causing the power steering fuse to blow (both on the same fuse)
The radiator replaced as a leak was spotted.
A/F sensor (1) replaced as code came up.

This is when it started struggling at idle.

Checked the
Coil packs .....all 4 ok
Spark plugs all new
Compression check... within limits in haynes manual
Fuel pressure...... correct according to haynes
Cleaned throttle body (butterfly valve not moved)
O2 sensor 2 is showing around .8v so obviously over fueling.
I need a bit of direction now as my knowledge has ran out. Hopefully someone can help me with what to check next.
Or if i have missed something

Gray

[E-bmw]

Two things jump out at me straight awa in your post.

grahelcurt said:

PCV Valve was then changed because it was causing the power steering fuse to blow (both on the same fuse)

How was it explained to you that a PCV could cause a fuse on a different circuit to blow?

Each PCV that I have come across has been a purely mechanical thing, could be wrong of course.

grahelcurt said:

A/F sensor (1) replaced as code came up.

This is when it started struggling at idle.

When you write it like that don't you think those 2 events could be connected?

2 possibilities for me:

1 You have either not connected the air hosing around the MAF correctly, or have damaged it.

2. The sensor fitted is not correct, or not working. Some manufacturers (BMW being one) are extremely sensitive to cheap/pattern engine sensors.

[sam hill]

What was the code that lead you to believe the oxygen sensor was faulty? Was the new oxygen sensor a cheap aftermarket part? These are notoriously crap, rarely do they work correctly and if they do it's not for long.

[grahelcurt]

hi
thanks for your reply

the power steering went and a code came up (cant remeber what), so whilst doing a google search i came across someone who had the same happen. he was saying that he checked the wiring diagram, he unplugged a couple of sensors that were connected and the power steering started working.
I just followed his route of thinking and the fuse had blown. replaced the fuse and it immediately blown. pulled the connectors off 1 by 1 and thats it found the problem. Replaced the pcv valve and deleted code and power steering sorted.

The a/f sensor had a code pop up. the heater had gone so replaced it. Actually the misfire was showing itself before that was done.
O2 sensor 2 is showing .8 so i think that is my next checks to do. It does respond to added fuel upwards and vacuum leak to lower it. I think i need to get the exhaust emissions checked just to make sure it is showing the correct reading.


regards
Gray