sigma supersport engine running problem

[fondelli]

Just back from a few days fun in north wales. However my sigma supersport has developed a running issue. Here is what is happening:

 

1.Start from cold normal - cold start etc fine

 

2.Engine performs normally for a period of time

 

3. Engine then starts to misfire and get fluffy at very light throttle openings

 

4. When this happens the idle speed hunts between 500-2000 rpm and then stalls

 

5. If I stop and restart the cycle repeats.

 

I have a funny feeling it might be a dodgy lambda probe? Hopefully someone has had something similar. I know that a lot of aftermarket mgt systems use the lambda probe at idle and then switch to the tps. Over to the experts

[Stationary M25 Traveller]

When my lambda sensor failed recently, I got excellent advice on BC to simply disconnect the connector in the cable to it. Works fine without it. If this cures it, remove it, take the ref number from it and get onto eBay.

[Boss Hogg]

I had exactly the same problem with my supersport, changed the lambda sensor, problem solved. Mine also over-fuelled like crazy so was fine on full throttle but at around town on light throttle, misfires and a serious stink of petrol! I understand Caterham lambda sensors are not great so got the replacement from another source (bosch), no problems since .

[fondelli]

Thanks for this. Does anyone have any documentation on how the ignition/injection system works? I would like to understand more if i can

[Boss Hogg]

can't help with documents, but my basic understanding is that the lambda detects oxygen levels in the exhaust gas, if it senses too much O2, more fuel is added by the ECU (less fuel if too little O2). The optimal O2 level is based on what the CAT converter requires. The CAT only works when hot, so i suspect lambda readings are not considered by the ECU until everything hot enough (hence car OK when cold). With a broken/failed lambda sensor, wrong/no signal is sent to the ECU and, from your symptoms, the ECU over-fuels (fluffy/misfire at low throttle). Also accounts for rev hunting as the ECU is looking for a signal that's not there. At high/full throttle ECU is delivering max power/fuel so the car runs and responds as normal.

[oldbuffer]

I had the same problem fixed as advised above but CC also increase idle speed to 1000rpm.

[fondelli]

Spoke to the factory- they confirmed my (and yours) suspicions. Got the exact same (genuine ford) part on ebay, so hopefully problem solved. Tip of old sensor was really sooty and black.

[Aitch42]

Hi there

I had a similar problem which turned out to be the MAP sensor which measures the atmospheric pressure and then alters the fueling to suit. Initially Caterham suggested it was the Lambda sensor and then changed their mind. Try and unplug the MAP sensor - black box by the battery and see if it solves things. Unfortunately this has been a repeating problem and I am now on my fourth sensor within 3 years. I ended up buying a job lot of sensors from China which seem to be a lot more reliable than the Caterham ones. If you need a MAP sensor let me know as I have a few left.

Howard

[fondelli]

Howard- thanks for the offer. I changed the lambda. The engine seems to be idling ok. Next test is a good drive to see how things are.

I did notice that the old lambda was very black- which suggests mixture richness.