The emission warning light came on on my 1991 945 with the 2 litre non-turbo engine. I was a couple of hundred miles from home so I drove back with a slightly lumpy running engine.
The exhaust and plugs are very sooty black so I am certain the engine was running very rich after the warning light came on (running perfectly beforehand).

So my question is: Will all of that carbon in the combustion space and exhaust give me any problems if I just replace the lambda sonde and the catalyst? (I am assuming that the cat will be damaged by all that sooty carbon so intend to replace it).

My concern is that the sooty carbon in the combustion space and the exhaust upstream of the cat will simply ruin any new lambda sonde and cat that I fit. Do I need to do any cleaning up (if that's possible?) to remove the carbon?

Thanks for any informed advice!

Best regards

OwenF

Dear OwenF,

Hope you're well. The first step is to see what codes are found in the On-Board Diagnostic (OBD) system. See this site's FAQs for instructions as to how to obtain any OBD fault codes.

Some fault codes are very specific, others not. For those, that are specific - e.g., Oxygen sensor's signal missing" - check the wiring the the sensor itself. If the sensor is factory-original, then replace it. In a car that is 27 years old, the factory-installed sensor doesn't owe You anything.

The soot coating on plugs suggests - as you note - a "rich condition". That could result from a failed engine temperature sensor. If the engine control unit thinks that the engine is "cold", it will keep the mixture rich for far longer than is needed (or helpful). Again, if that sensor is the factory-original item, it is long past the end of its service life.

The spark plugs can be easily replaced and likely should be, if they're coated with baked-on carbon deposits.

Correction of the underlying problem(s) could result in the catalytic converter clearing any soot by "burn-off". Thus, I'd not rush to replace the catalytic converter. The factory-installed units are robust and replacements are not cheap.

Hope this helps.

Yours faithfully,

Spook

Thanks Spook, good to hear from you. The OBD read-out indicated 2-2-1, this translates as "Fuel supply too weak in part load stage". I took this to suggest that the lambda probe was not monitoring the mixture correctly?

I haven't yet examined the OBD after my 200 mile drive back home, but the car ran OK at speed and the engine temperature didn't deviate from normal.

I did replace the cat with an aftermarket item in 2005 after failing an emission test, but I didn't have to replace the lambda sensor at that time.

I think you are suggesting just replace the lambda sensor then, and see if that cures things? Do you think I need to clean the downpipe as best I can to remove the soot before fitting a new sensor?

Many thanks again

OwenF

Actually you could try to rejuvenate the old O2 sensor if you want to add a bit more miles to it. Get the old sensor out, wipe out any carbon deposits on its tip and heat the tip (until red-hot) at a gas cooker/burner. Hold the sensor body using some long-handle pliers as it could get really hot with your hands near the flame.

If you could connect a digital voltmeter (set to very low voltage reading) to the sensor while doing the above you could see the voltage fluctuating up and down (0.100 to 0.800 mV). The sensor may respond sluggishly at first. Keep heating it for about 5 minutes or longer before it starts to respond. Consistent low voltage reading (less than 0.100mV) means the sensor is already out/kaput.

And clear the OBD codes after installing the rejuvenated sensor. Either disconnect the negative battery cable (but this may need you to enter the radio codes again) or press the ODB button longer than 8 seconds, the LED stays lights up, then press for longer than 8 seconds again until the LED turn off. Check back immediately if the codes are cleared by briefly pressing the button and you get a 1-1-1 code.

Bosch three-wire O2 sensor recommended replacement interval is between 60k to 100k miles. For Regina (NTK sensor from NGK) about 150k miles.

Done this many times.

May this helps,
Amarin.

Thanks everyone for these helpful comments. Got back home fine (200 mile trip) but by this time the 940 was running a little rough.

Checked the OBD1 codes again and everything pointed to the Lambda sonde being at fault (original 1991 sonde, circa 300k miles).

So new Bosch OE spec sonde purchased from Germany, fitted (easily) and everything cured - car runs fine, no lambda warning light on dash, no fault codes on OBD1 (after clearing old codes).

Didn't replace the cat, but will wait and see what the emissions are like at the annual MoT test in March next year.

Thanks again

OwenF

Dear OwenF,

Hope you're well. The engine coolant temperature sensor supplies a signal to the engine control unit (ECU).

A separate sensor sends an engine temperature signal to the instrument cluster fuel gauge.

Thus, the ECU can "think" the engine is still cold, when the engine's temperature - as shown on the instrument cluster temp gauge - is normal.

If the ECU "thinks" the engine is cold, the air/fuel mixture will be "rich", i.e., extra, unneeded fuel will be supplied.

Thus, I'd check the engine coolant temperature sensor. If memory serves, that sensor is mounted under the third intake manifold runner, i.e., towards the back of the engine (closest to the firewall).

I'd not try to clean the inside of the down-pipe, the pipe from the exhaust manifold to the catalytic converter. Any loose soot will be blown towards the cat and there burned-off. Soot that stays in the down-pipe doesn't hurt anything.

Hope this helps.

Yours faithfully,

Spook

That's very helpful Spook, I'll check that out. Have run a number of checks today and interrogated the on-board OBD several times - each readout indicates that it is a lack of control over the air/fuel mixture causing the problems. Haven't yet checked the ECU temperature sender yet of course, but I guess that would show up as a readout on the OBD - code 1-2-3?
all the best, and thanks again

owenF

If you have a cooking thermometer, an ohm meter, and a way to heat up some water, you can verify if the coolant temperature sensor is good or bad. According the 700/900 FAQ temp vs resistance values are:

32F (0C)-- about 6000 ohms within a range of +/- 10%
68F(20C) -- about 2300 ohms
104F(40C) -- about 1300 ohms
140F(60C) -- about 600 ohms
176F(80C) -- about 300 ohms
212F -- about 190 ohms

I can’t remember if the Bosch and Regina systems use different sensors nor does the info in the FAQ differentiate. I’m guessing you have a Regina car if it’s a non-turbo. But that’s a guess.
--
Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

The Bosch temp sensor in Bosch/Regina car is the same with part #0280130032. This from the blue Volvo Pocketbook 1991-97. However a recent check at Bosch website shows the part number was discontinued. It is being replaced with part #0280130069.

Regards,
Amarin