K-series lambda sensor

[56]

I have a 2001 1.6 k-series (EU3) Caterham and I'm having some issues with the engine misfiring due to the lambda sensor.

It seems to be a problem with the heating of the sensor. The misfire is only present at low load, at higher loads it runs fine presumably because the engine is producing enough heat to get a proper reading from the sensor. With the sensor disconnected the car also runs fine which is how I've been using it for the past year or so but I thought it was time to get to the bottom of the issue.

I have tried replacing the sensor but still have the same issue which leads me to believe it's an issue with the wiring. I then thought it could be the MFRU causing the problem but I was recently reading the article on the starter motor 'k-click' problem (another thing I need to fix) which states that the oxygen sensor relay in the MFRU is not used on a Caterham. Looking at Revilla's excellent wiring diagram and seeing as everything else is working properly then it looks like there are only two wires which could cause it. And if it's not those then it points towards an internal ECU problem?

 

Has anyone else had this issue? Any other ideas?

Thanks!

[garybee]

The heater is there for start up to get the sensor working quickly.  Once it's up to temp the sensor stays hot regardless of engine load.  If the sensor heater was at fault you would only have problems for the 1st couple of minutes after starting from cold.

As yours is EU3 have you plugged in to an OBD diagnostics to see if the ECU has recorded a fault?  That may give you some guidance.  Don't just look for fault codes though, you need to look at the output from the sensor once it's up to temp.  Can advise once you have a fault code reader.

What spec is your car?

[revilla]

Hi 56,

I've seen two EU3 ECUs where the lambda sensor heater drivers had failed (it normally drives the heater with a PWM signal, pulsing fully on and off - we connected a bulb in place of the heater load and instead of flashing on and off as it normally would it just glowed dimly).

However, I tend to agree with Gary's post above, your symptoms don't sound like a heater failure. The reason that it doesn't misbehave at higher loads is not that the engine is producing enough hear to keep it hot but that the ECU ignores the lambda sensor under higher loads. Standard narrow band lambda sensors only give sensible readings at AFRs close to 14.7:1 and at higher loads the ECU needs to run the engine richer than that, so the lambda sensor is just ignored.

Get an OBDII scanner plugged into it, preferably one that shows live sensor data not just codes. Cheap on eBay, just look for "live data" in the description.

There are other reasons why the lambda sensor could be giving false readings to the ECU. Reasons that come to mind immediately include:

1) An exhaust leak ahead of the sensor. These tend to suck in air (NOT "blow" as you might expect) meaning that the sensor sees oxygen in the exhaust and thinks it's running lean when it isn't.

2) Is the sensor in one exhaust primary or downstream of where they all join up? If in one, and you have a problem with that one cylinder (e.g. a clogged, sticking or leaking injector) then the lambda sensor will only see the cylinder that is running wrongly and make all the others run incorrectly in an attempt to correct it.

3) You haven't used any non-sensor-safe silicone in sealing any of the exhaust joints have you?

Andrew

[56]

Hi Both. Thanks for the replies. When I first had the issue I did check the error codes and I had:

- Oxygen sensor heater circuit malfunction (P0135)

- Misfire detected with low fuel (P0313)

If I manage to get a live scanner then what should I be looking for with the lambda sensor reading? Am I right in thinking the reading should sort of oscillate in normal conditions?

I'll check for exhaust leaks as well but I definitely can't hear anything unusual.

The car is a standard 1600 k series and it's got the short 4-1 manifold with the sensor just after the collector so it's reading all 4 cylinders.

Andrew, do you have more info on the light bulb test? Does it need to be a certain type for the ECU to send the signal and should it start pulsing straight away when the engine is started?

Thanks again!

[garybee]

Yes, once up to temp the reading should oscillate between lean and rich with the [narrow band] sensor in your engine.

I would also check Air Inlet Temp figures (do this before starting your engine) are realistic and roughly reflect the actual air temp.  Have a look at your long term and short term fuel trims at idle and at 3500(ish) RPM.  This will give you evidence of intake leaks.

Don't get hung up on the heater code, all that will be causing you is a slight delay in going between open and closed loop running.

I can highly recomment the 'Torque' app (android phones) and a compatible bluetooth OBD2 adaptor.  

[revilla]

Pretty sure I just wired a 21W indicator bulb across the two heater wires. I can't remember now whether it started flashing as soon as the ignition was turned in or when the engine was startes.

Although you clearly do have some sort of fault showing there, as Gary says it is unlikely to be the cause of your rough running once the engine has warmed up a bit.

Ideally the lambda signal should swing between around 0.1V and 0.9V once a second or more when running closed loop, but you will never see that on OBDII live data as the refresh rate is too slow, so what you will see is apparently random samples of that waveform at different points. So long as you are seeing some values down around 0.2V or less and some up around 0.8V or more and it appears to be jumping around rapidly between readings it's probably working fine.