E30 Zone

Hi All,

Am I right in thinking that because my 318is came without a cat that it also doesn't have an oxygen sensor? I had a look on the exhaust manifold today and can't seem to see one.

Leads me to another question, how does the ECU measure the mixture if there isn't an oxygen sensor?

Thanks
David

If you don't have a cat then you won't have a lambda sensor but if you did it would be more or less in the middle of the down pipe. Can't help with the second part!

LGK6D wrote: Leads me to another question, how does the ECU measure the mixture if there isn't an oxygen sensor?

Answer to that is simple - it doesn't, and doesn't need to.
The sole purpose of a lambda sensor is to prevent damage to the cat. if for any reason the mixture becomes too rich.
The theoretical times of the injector pulses for an optimum mixture is written into the software in the ECU's 'chip' for all rev. and engine load values. As long as the engine is in good condition, especially the injectors, this will be good enough - almost infinitely better than can be obtained with a carburetor.
To enable the CO level to be set at idle to an acceptable value, on the M40/42 engines, a potentiometer is mounted in the AFM housing, which allows an adjustable voltage to be applied to the lambda sensor input of the ECU. The voltage set by the pot. is fed to the ECU by a white/black wire, and under the bulkhead wiring cover there is a plug and socket in this wire, which is left disconnected when a lambda sensor is fitted, so allowing the sensor to take control instead.
AFMs produced after the date that cats., and hence lambda sensors, were compulsory, do not have the pot. fitted, so this needs to be considered when buying a replacement AFM for any car without a cat./ lambda sensor.

Thanks both, that's exactly the info I was after.

So, I was speaking to MarkD about having one of his chips (suppose to be really good on this engine). The American/Canadian cars all have cats and oxygen sensors. Mark has said that the car won't start if the chip goes in and there's no Oxygen Sensor. I'm guessing he's using the sensor as a closed loop fueling monitor to make things more accurate?

Does anyone make a 'Theoretical values' chip which doesn't need the lambda sensor?

LGK6D wrote:. Mark has said that the car won't start if the chip goes in and there's no Oxygen Sensor

Highly unlikely! A lambda sensor needs to be hot to work, and it won't reach the necessary temperature to operate until several tens of seconds AFTER the engine has started, at best.
It's quite possible that all US 318is do not have the CO setting pot. in the AFM, but it's not rocket science to add a pot somewhere to give an adjustable voltage to simulate a missing lambda sensor.

Note that a lambda sensor controlled engine will run on a fuel mixture strength that produces minimum emissions, rather than maximum power. Max. power requires a significantly richer mixture.

Brian, you are an asset to this forum.

So, if Mark has his new set of parameters on the chip, the potentiometer in the AFM will provide the adequate information the ECU is looking for without the lambda sensor and all should be well in the world?

I naively thought that the lambda sensors job was to measure the exhaust gas and report back to the ECU where the duty cycle of the injectors/advance and what not adjusted accordingly to make sure the mixture was always spot on. Everyday's a school day.

A lambda sensor is an oxygen fuelled battery, which produces a voltage when hot and when supplied with oxygen.
If the mixture fed into the engine is lean, there will be insufficient fuel present to combine with all the oxygen in the mixture, so oxygen will be present in the exhaust gasses when it reaches the lambda sensor, which will cause the sensor to start producing electricity.
This voltage is fed to the ECU, and instructs it to make the mixture progressively richer. Eventually the mixture will be rich enough to consume all the oxygen present, so none will reach the sensor, and it's voltage output will fall to zero.
With the voltage gone, the ECU now makes the mixture progressively leaner, until oxygen once again reaches the lambda sensor, it start generating again, and the ECU once again starts to make the mixture richer and the whole cycle repeat itself over and over again, about twice a second if all is working as it should.
This keeps the average mixture spot on, but spot on for low emissions, rather than power.

My 1992 318i (M40) starts perfectly, then after a minute or so bogs down and sputters. I have to rev it to keep it running, and a burst of smoke follows and strong petrol odour. After revving to 2000 for a few seconds it clears and runs perfectly. Oxygen sensor? Any thoughts?