I was looking for the reason of an eccesive fuel consumption: do you think the compression test I did isn't enough to know if the cylinders are OK?

The tips of the spark plugs looks fine (to me) and all the PCM values look OK too. The O2's values (before and after the cats) and fluctuation aren't different than before and the wideband shows the same AFR than before (14.7 - 14.9).

I scanned the timing values to see if the PCM was using less timing than the high octane table, but it looks fine.

Is it possible that carbon deposits went from the combustion chamber to the catalysts that are now blocked?
I think they get really hot inside (glowing) and this should eliminate every deposit...

I can't imagine more than what I already checked Any other idea?

Usually who drive those cars isn't caring too much about fuel (me too), but over here we pay $6.90 per gallon right now and this is becoming an issue

Doing compression testing is not enough, leakdown would then tell if there are any leaks to valves, seals, etc

Cost of AFR needed for boost does have negative effect of CAT live so a good test if possible is to swap both cats with a pipe and see what PCM reports via OBD-II

Being the engine is old with high miles it is possible CATs are getting plugged up. If car is less then 8 years old, at least here in the USA GM has to warranty CATs as to EPA smog warranty is 8 years

You could measure the exhaust temps in muffler and see if both sides are equal and also measure temps at exhaust primaries
using a cheap thermal infared tester

Still after resetting PCM engine ran better but degrades after PCM relearn miles then I'd tweak the tune being off a bit

OK, let me put some mile on the car and see if it's still thirsty and what the PCM is trying to tell me

Last few days we had a sunny wheater so I reinstalled the summer tires... this weekend we'll have snow again so I'm not sure how soon I'll be able to post a result.

Stefano

What country do you live in ?

No better here, about 40 deg day, about 10 deg night with snow maybe the next few nights

Switzerland, 20 miles SE from Zurich (a small place called Wetzikon)
Look here for a map
http://maps.google.com/maps?f=q&hl=de&geocode=&q=wetzikon&sll=37.0625,-95.677068&sspn=52.152749,92.285156&ie=UTF8&z=13&iwloc=addr

Here a webcam of a place close to my house
http://www.opentopia.com/showcam.php?camid=3842

Ah, one of our teammates lives in your country and is a pilot of Swissair and does long distance overseas flights

cannot read that map as not in english.

I am at 3,500 elevation so understand the weather issues

Swissair no longer exist (has been grounded in 2001)
Now it's just Swiss...
can you disclose the username of this teammate? It would be interesting to have this contact.

Google automatically set the German page for the map... Try to put Wetzikon Switzerland in your map

Elevation is 1760 ft, still enough for some snow, hopefully the last one this spring!

I cannot recall Hans last name

I met him once as he had a flight to west coast and he came by my place to visit for a couple of hours. He has a ZR-1 Corvette

OK, hopefully Hans will read this thread

I'm still testing stuff...
Here what I imagine could increase fuel consumption:
- MAP sensor racting too slow or wrong
- Oxygen sensors racting too slow or wrong
- injectors tips dirt because of sludge washed out
from the intake mainifold causing bad spray pattern
- Clogged catalysts

So far tested:
MAP = OK (I forced the PCM to run in SD and fuel trims are the same as when I did it last time, AFR remains constant even with sudden load changes)

What else could cause an higher than normal fuel consumption?

Stefano

As I mentioned a test could be is to replace CATs with pipe and see what the AFR difference is or assure the O2 sensors are functioning correctly

Once you bring SD into this then your on your own, I am not on the SD cool-aid trip and in no way can SD adjust for changes of conditions.