Hi everyone!

I hope that someone here can give me some ideas on what the problem could be here, since I have pretty much run out of ideas and things to try...

In a short introduction, I bought this car a couple of years ago for my wife to "start" driving regularly. This is one of the first Golf IV, from 1998 and has the simple 1.6 8-valve engine with A/C (no EGR) and coupled to a 5-speed manual. The car is just over 300.000km now.

Since I got it, it has always had a fairly poor mileage (around 8 to 9l/100km) when driving even softly and a certain inertia specially under 3000-3500rpm. The power is there and it will pull over 180km/h, but it is very "lazy" in the initial throttle response at low revs. Also, it suffered from poor driveability, specially at light load and when applying or removing the gas the engine would "kick" due to the throttle closing or opening sharply...

I have done some things so far, such as new batery, air mass meter, new spark-plugs, cleaned throttle body and greased control mechanism, replaced the servo vacuum pipe (as it was leaking) and general maintenance as well (oils, filters, etc). I have also searched for faults on the OBD and nothing!!

I noticed that whenever I disconnected the battery for long periods, the first startup and throttle response after reconnect would improve quite dramatically but would go back to its "lazy" response a couple of kilometers after. This was also the case after a reset of the ECU (without disconnecting the battery).

I thought I had narrowed it down to a problem with the emissions regulation control, hence the next suspects would be the lambda-sensor and potentially the catalyst, which at 300.000km could have partially collapsed or be blocked (this car only has a lambda sensor before the catalyst, hence catalyst efficiency is not monitored). Before changing any of this, I replaced the back of the exhaust which was still the original , but it did not solve the problem. I did at this point make a "discovery". When removing and fitting the new exhaust behind the catalyst, the gasket between the exhaust manifold and the catalyst pipe broke and caused a leak. This was audible but I took the car out for a test-drive anyway. To my amazement, the response of the engine was absolutely perfect, the driveability issue disappeared and the fuel consumption reduced around 2l /100km when driving at stabilized speed in the motorway!! PERFECT!... I thought...
At that point I was convinced that the issue was the lambda sensor or the catalyst or a combination of both so the next step was changing them and fixing the leak. After all this was done, the car went back to its old habits...

At this point I have no idea what can be causing the problem as everything I know about mechanics and engines seems unable to explain this... strangely enough, the driveability issue seems to have cured itself from one day to the next, but the "laziness" at low revs and high fuel consumption are still there. I have tried to reset the ECU by disconnecting the battery when the lambda sensor and the catalyst were fitted and I felt the engine went through a "re-adapt" phase to map the fuelling correction parameters with the new parts.

The only thing I know at the moment is that the engine is running rich, but can't explain why when there was a small leak before the lambda sensor, the car ran perfectly... Any ideas?