Fuel trim, short and long. wierd values (OBD)

[tomasw]

Hi from sweden
I'v got a 1999 tahoe that has run 300000km= 186411mi (?)

I did a test with a ELM327 wireless and OBDcardoctor, because I had fuel starvation due a clogged fuelfilter earlier.
I'v got no fuel starvation now, but something else that cant be ok..

Short term fuel trim % moves around 0% one or two up and down, nothing funny here.
Long term fuel trim % rocks at +20% on boths banks at 70mph, that cant be right, could it?

Do this indicate manifold gasket break down or some other air leak or what?

 

[1998Suburban]

I'd look for an air leak at the exhaust manifold or donut in front of the primary oxygen sensors. You can suck air in through an exhaust leak at higher engine speeds, especially on longer downhill grades. How do the spark plugs look? That's another good indicator of whether you're actually running rich or lean.

 

[tomasw]

hi
I'll check that out this weekend.
I dont think my car got a leak, but will check if there is any small one.
Sparks will also be checked
Thank you for the feedback

 

[tomasw]

checked one sparkplug on the drivers side of the engine, medium gray, seems ok
checked with scanXL and now i got P0430 (
Catalyst System Efficiency Below Threshold (Bank 2))
it seems that:
"
A code P0430 may show that one or more of the following has happened:
-The catalytic converter is no longer functioning properly
-An oxygen sensor is not reading (functioning) properly (acording to the software it does)
-There is an exhaust leak.... <--

How small leak should I be looking for?
I have not notice anything that could be a leek, but I may need too look closer ...

 

[1998Suburban]

Yep, I'd keep looking for the leak. There's a flange right behind the cats that's exceedingly difficult to seal. For some unknown reason GM put one of the secondary O2 sensors just behind it. You can have a very minor leak (barely feel air puffing out but can't hear it) and it will trip a catalyst efficiency code.

Keep in mind the secondary O2 sensor doesn't influence how the engine runs. With that in mind:
http://www.afterthoughtsauto.com/o2sim.html