Sensors

Is the lambda too high or too low?

I’m pretty sure it was miles too high but can’t remember now everything else was in perfect though after a dose of lambda cleaner

Trying not to lose myself in getting too technical here!

Lambda, as measured by an emissions tester is a measurement of the amount of oxygen in the exhaust gas.

If all is ok and the fuel mixture is 14.7 to 1. Its the ideal mixture, known as the stoichiometric point (might want to check the spelling there, LOL!). Its what the ECU is constantly trying to achieve based on input from its sensors. When you see the O2 sensor flick from rich to lean to rich etc. that is it feeding back the actual, instant amount of oxygen in the exhaust gas trying to keep the engine running as close to that point as it can.
The point that the engine achieves this is called Lambda. When the stoichiometric point is acheived Lambda should be 1.

If your engine is running lean the air/fuel ratio will be higher, more bits of air per bit of fuel being burned. As a result the emissions tester would show the Lambda to be too high. Your O2 sensor shows rich, meaning it wants the ECU to enrichen the air/fuel ratio.

Lean mixture could be caused by a dodgy sensor or air/exhaust leak. If it improved after cleaning it with lambda cleaner it could be a sensor, then again, you may have just cleaned the carbon off it that has built up with the ECU trying to compensate for an air leak.

When I got the car the centre pipe of the exhaust was knackered so changed that and now the back box is blowing slightly but I guess that wouldn’t cause the rich smell would it? Would a good service help? Don’t know when it had new plugs or even an oil change but would that actually make much of any difference? Could it be one of the coils not working properly? It’s not midfiring (or doesn’t seem like it). The car does feel underpopwered compared to the m13 engine but again I don’t know if that’s normal or not.

An exhaust leak will cause a high lambda, because oxygen gets into the exhaust gases.
The rich mixture will cause high HC- but that is an over simplification, as a lean mixture can cause high HC too due to lean missfires.

High HC plus high lambda (with no exhaust leak) indicates an over lean condition.

High HC plus low lambda indicates over rich.

A missfire due to dirty plugs would cause high HC and the lambda would be 1.000 I reckon.

The usual failure mode of the pre-cat sensor is to switching at an over-rich mixture, which won't put the EML on until it runs out of rich fuel trim and still can't close the loop.

I would suspect you need a new pre-cat sensor to improve the mpg, and to fix the exhaust leak if you want to pass the MOT. (I don't know why lambda is in the MOT, but it is)

Right so a service wouldn’t make a great deal of difference here then. Should I just throw a ew pre cat sensor in and see if it makes any difference? Is that what you’d do? I know there’s a few different sensors available for the Jimny engines ranging from £17 to £79 ( trade prices) how do I find which one is the right one? Should the like is euro know which one it is? Or should I get a generic one with the same number of wires? Don’t want to be wasting any money atm as I’m starting a new job soon so this payday will be a very small one but will need the car to be running better to get me there more economically....well cheaper really lol