When you did the top engine clean through the intake, did you stall the engine while getting the last bit of fluid out and then let it set for 1/2 hour or more? That is the most effective way of using the top end cleaner. When I did that on my 01 Yukon at 90,000 mile, I had to replace the spark plugs before I noticed better results.

Yes, I stalled the engine and let it sit for a while.
Actually it was a little difficult to turn the engine off: it acted like a diesel and did continue to run even without sparks
After replacing the spark plugs it seems that the mileage is going better, but I honestly don't feel any particular change.
I still have to check the O2's and the cats, however: cats are teh last thing I want to mess with. F-bodies have the cats welded to a Y-pipe and to add pipes as suggested by JR will be pretty difficult...

I know.
I once wanted to see what are the fuel trims in SD (that was about 2% less than with the MAF).
I now wanted to check the function of the MAP forcing the PCM to run in SD, just to see if after the cleaning I was able to see similar fuel trims as before.
The fuel trims are the same and that should mean that the MAP is OK (I wasn't sure if the cleaning liquid was safe for the membrane inside the MAP).

Use a thermal probe or a infared thermal gun and measure

1. temps at each exhaust primary pipe
2. incoming to both CATs
3. Right after the CATs
4. in the muffler tips

See if all points left/right are about the same temp at each test point

I suspect at each primary about 300-500 degrees depending on exhaust manifold or header used

About 800 plus going into CATs
About 170 degrees in the muffler tips

I doubt MAP itself is an issue UNLESS oil got sucked into it
At idle should be about 35 KPA and of course if at sealevel with boost should hit 104 KPA at WOT.

MAP is about 30 kPa at idle and otherwise it shows the same values as an external pressure sensor that I use to inject water/alcohol. Baro is around 95 kPa and rises to 104 after going WOT under boost.

This weekend I checked the injectors tips and they are all nice and clean. I was able to see the intake valves of a couple of cylindes: all nice and clean too.

I checked the Oxygen sensors: no residuals, look almost new (they are 40'000 miles old).

I checked the catalysts and they look clean.

Other things I checked: compression, timing, fuel trims, spark pugs. And the car is still making 16 - 16.5 mi/ga

I remember I had the same situation once, in the same period of the year. We have now 37F in the morning and 60F during the day with a pressure of 950 hPa (= 950 mbar = 1984 psf = 13.78 psi) ...I don't know what unit you use over there.

Is it possible that certain wheater condition cause an eccessive fuel consumption? In winter or in summer I never noticed that, but in similar wheater conditions like now... honestly yes.
I know it doesen't make sense, I'm just looking for an explaination

Bosch says O2 life for best performance is 30,000 miles max.
Beyond that the O2 degrades and functions slower.

You cannot see inside the O2 tip as that is where the sensor is so the shell color could look good ( esp when w/meth useage reduces carbon buildup) but still be dirty internally.

Your country might as they do here in the USA in spring time switches fual makeup.
Here they add more ethanol as much as 10 to 15% which reduces fuel mileage as it does not produce the same power as straight gas.

Should I replace all 4 O2-sensors?

Or just replace those in front of the cats? In this case can I put those that are now in front to the rear of the cats? They are "only" 40'000 miles old. If they are slow they are bad for fuel metering but maybe not to check the cataysts function (I hope so).

I researched about ethanol in fuel in Europe but I haven't find any information. Maybe they do it...
Do you see a difference in fuel trims when the fuel contains ehtanol?
In the USA they put ethanol in every kind of fuel? Every grade? Or just in the grades with a low octane rating?

The rear O2s are NOT just for monitoring the CATs.
Do a search on this forum as I have posted GM info as to what rear O2s do

They do play a part in what the PCM decides is the average part throttle injector pulse width for a drive cycle

The rears are more trusted on start up since the fronts have more of a temp change and since AIR is injecting air the rear O2s have a more stable exhaust makeup and temp range.

Rear O2s due to their wire length require more current load thus the rears are not the exact design as fronts thus should not be swapped front/rear

Rears should I think function longer as temps and exhaust is better but I would not want old fronts in the rears

At the least replace the fronts, review this forum as I list the new GM designed O2s and their part number

E10 or E85 due to the ethanol will change the fuel trims as it is supposed to be richer then regular gas but it takes more volume of it to produce the same HP as normal gas

Yes as of 2 years ago EPA was requiring using this junk in all grades and of this year most gas station are even using higher percentage of ethanol and less performance and MPG is seen.

Also our cars were never designed for it and can effect O2s and even cause internal rust in fuel system to break off and clog pumps, filters and injectors.

Reports are ethanol, esp E85 drops MPG from 10 to 20 %

OK, I'll change them all.
Maybe the decarb procedure left some residual on the inner part of the sensors.
The front O2's are 40'000 miles old, the rear sensors 85'000 miles.
In the front I have installed original sensors but not exactly for that position. I think I choosed those with the longest wire because at that time I had a set of LT headers and I didn't like the idea of installing extensions.
Maybe not the best idea.
I'll post the results as soon as I'll change them.

Stefano

Being there is a airchannel path between the wire and insulation since O2s need outside reference air make sure there is no kinks or crimping which would shut down that air path.

I find when people using the wrong length wiring they bundle up the excess wire and like tiewrap it thus again crimping off that internal airpath.

Exts are a joke since the coldest airpath is now gone as the wires used are common wires and not the special wiring O2s require.

The older the O2s are the slower they they function and response is slower.

A decent OBD scanner like I use comforms to EPA and does O2 tests and tells you if O2s are functioning properly and within spec window.

Biggest failure I see people do is having the wiring connected to PCM wiring harness and then when turning the O2s to get them on or off cause the wires to twist around themselves and crimp off that internal reference air path so always have O2 wiring harness free and let them spin as O2 is turning.