Interpreting OBD2 readings

[markh1]

Hi all,

I recently got an OBD reader off the back of another forum members recommendation as I felt the car wasnt quite running right.

The bits that look a bit off are the fuel trims between the different banks and also the oxygen sensor readings maybe.

Any help on interpreting these readings would be much appreciated!

 

[Alex]

Have you checked for any fault codes and also, what are the symptoms of it not running right?

 

[markh1]

I dont have any fault codes but from a bit of background reading it looks like a fault code wont be triggered until the fuel trim exceeds 10%

The symptoms are slightly rough idle and excessive fuel consumption.

Further reading suggests it could be a vacuum leak?

 

[Alex]

Could be a numerous amount of things, not just vacuum leak. If it hasn't tripped a CEL then everything is within limits so should be ok.

What's excessive fuel consumption? 5? 10? 20?

 

[markh1]

on the 45 miles or so it averaged 23.69mpg and half of that was sat at 70mph on the motorway

 

[Alex]

Call Hartech, as it's had one of their capacity increases and they may be able to help you.

 

[Shalmaneser]

What would you describe as your main complaints?

High fuel consumption?

How is the idle? How does it sound? How is the power delivery?

Negative fuel trim indicates the ECU is reducing fuel, as it is detecting rich combustion. Seems odd with a motor that has been enlarged.

How new are the O2 sensors? Were they replaced as part of the rebuild? Were the injectors cleaned and tested?

 

[markh1]

So the main complaint is below par performance as per surrey rolling road test for a 3.9.

The idle does fluctuate slightly and the power delivery can sometimes be a tiny bit jolty.

I also notice that one bank smokes more than the other on startup.

I dont have any records of the injectors or o2 sensors being replaced

 

[Alex]

If it's a 3.9 oversize Hartech engine it's only been done in the last couple of years - call them. Where's the rolling road place getting their comparison from, another Hartech 3.9 engine or a 997 3.9?

 

[Robbidoo]

Charlie at Surrey rolling road has a pretty good gut feel for when something isn't making power, in my admittedly limited experience.