Low fuel mileage (C20NE)

[vectra1903]

I have a Vectra A with the early Motronic 4.1 system ECU and C20NE engine. My catalytic converter was cut out ages ago. In the summer of 2021 the car seemed like it didn't burn any fuel at all but now it does 11L/100km which seems a bit high to me. This figure doesn't change even on a longer trip or when eco driving. My grandpa always said that this car was around 7/100. I don't have any engine lights on. I also have a problem with an unstable idle, could these problems be associated to each other? Is it some sensor wearing out?

[Robsey]

Usually when a sensor is faulty, the engine malfunction light should illuminate. (EML / MIL) - unless the sensor(s) is running 'just' within limits.

If there is no engine light, then it will probably be something more 'basic' -

Now - I apologise, but I will go back to basics to ensure we cover all the potential causes.

As you have a fluttery engine, you could check -
General condition of the ignition components - plugs, leads, rotor arm, distributor cap.

Looking at the plugs should indicate if the car is running okay, rich or lean.
Creamy coloured deposits - okay
White deposits - too lean.
Dry black soot - too rich
Wet black oily deposits - possible seal issue.

Remember that it has been very cold recently, so the fuel enrichment will be on for longer until the car reaches it's normal running temperature.
Also - slippy / icy roads usually mean you have been driving slower, in a lower gear.

Other causes for l/100km are poor tyre pressures, or bad wheel alignment / tracking.

Do all the wheels spin freely?
Thinking here of brake drag or wheel bearing wear.
Brake drag - possibly dirty or sticking callipers or mal-adjusted rear brake shoes. (hand brake).

Bearing wear - no explanation needed.

Then there is driving style and working environment - hills, lots of slow traffic and so on.

Last on the list - fuel injectors. Are they spraying in a good pattern?
Any leaks or dribbles would also cause a rich mix.

I am sure you have probably covered most of these, but worth a prompt all the same.

[vectra1903]

Plugs, rotor arm, cap and leads are new. The coil was checked with a multimeter as instructed in the Haynes manual.

I only have one of the old plugs which probably doesn't help much, but that's how the plug looks:

Looks fine to me, I'm no mechanic though.

We've had pretty good weather here for the last month or so, so good that I'm running summer tires now. All of the wheel stuff is checked out, pressures are set, rims are straight, tires are brand new.

The brakes shouldn't be dragging as the wheels spun freely when I had the tire replcement done. My handbrake is pretty weak, doesn't hold the car if it isn't engaged to the last possible spot on the ratcheting mechanism. One click down and it rolls freely. But that's not the problem here, just saying that it surely doesn't drag.

The bearings I haven't checked, I don't have any whining or anything like this though. Only whine I have is from the gearbox, in 1st gear and then rapidly slowing down from a high speed.

My regular commute and the trips I have taken were usually on flat roads with moderate traffic. I hardly ever get in traffic jams, I either try to avoid them or get out of them quickly.

The injectors are also a mystery. I haven't had them checked. My mechanic told me that this isn't needed but I don't know really. Should I get them checked out?

What about the oxygen sensor on the exhaust?