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MG MGF Technical - Running rich ?

I have noticed that my exhausts on my 1998 1.8i are very black and sooty. I get 28-31 mpg ave with mainly motorway driving, although I often do 90+. The car has had a few mods, mildly ported engine and inlet manifold [the RS140 kit], K&N, 160 TB and de-cat. Recently I changed the exhaust manifold to the Janspeed 4-2-1. I am wondering if in doing so I have caused my Lambda sensor to give up the ghost.

I do not have multi meter so measuring the resistance (?) is not really an option for me. Before anybody asks I have not yet taken out the plugs and had a look [job for the weekend if its fine].

What should be the expected behaviour if I was to disconnect the sensor? Would I be able to notice the difference?

Concidering that most of the mods are induction I was expecting my worry to be under fueling not running rich!

Any ideas?

Steve Ratledge

> What should be the expected behaviour if I was to disconnect the sensor? Would I be able to notice the difference?

I dont think the MEMS will like it much and may go into some sort of safe/ get you home mode???
Bob Millar

induction mods might* cause a car to run lean, but not rich.

>What should be the expected behaviour if I was to disconnect the sensor?

The car will move to a rich map (also the case for a broken sensor), this is because a over rich map is safer than a over lean map.

>Would I be able to notice the difference?

Probebly not, but you MOT guy would be able to tell you very quickly (and the most he can charge you is the cost of an MOT, it's part of the test!)


* in the absense of engine management and it's sensors, which automaticly counteract this!
Will Munns

There may not be a problem here Steve: certainly the fuel economy isn't bad for 'autobahn' speeds ;o) And my car always has a black tail pipe (typical of any car running unleaded fuel).

Regarding the Lambda - yes, it is possible to damage it - either the loom itself can touch something hot like the manifold itself, causing a short (as JT found - and more recently Tim who discovered that a fuse had blown as a consequence), or the shell of the sensor can be damaged.

The K-series uses a four-wire Lambda: one for signal, one for ground, the other two for the sensor's heating circuit. Therefore if you are planning to interogate your lambda, you can't do it on a cold sensor, and moreover, you need to identify the correct wires.

Tim and I figured out the wiring code a while back - can you remember it Tim?

When the engine/ MEMS is running in closed loop, the fuel mixture should be stoichiometric (14.7:1) and show a voltage in the range of 0.4 - 0.6 volts.

So the checks to perform, if you are concerned, are:
1. inspect the lambda wiring harness
2. remove the lambda (what an easy job that is - not) and inspect
3. warm engine, attach your voltmeter the lambda harness, and measure the potential difference across the signal/earth leads (hopefully will show 0.4-0.6V at idle)
Rob Bell

From top of head, white & white is the heater, black is ground and 'the other wire' is the output. But you will need a digital voltmeter.

I thought the range was 0.7->1.2 and it should osscilate between the two in 1 second cycles (slower as the lamda wears out)

I wouldn't do 2 before doing 3 - lamda removal can destroy either or both of the lamda and exaust manifold!

Of course, an MOT station can do 3 for you!
Will Munns

I *think* that it should be 0.45V for 14.7:1 Will from my general reading of Lambda sensors - see which admittedly is the curve for a 5-lead broad band lambda, but the general relationship seems to hold true for 'universal' type sensors.

Must have a look in the archives, as Carl has written about this in the past...
Rob Bell

From the archives:

Posted 08 November 2001 at 21:19:29 UK time
Carl, Sweden

Well then , here we go : Among all the multicoloured cables entering the MEMS unit there are for the 1.8 I as well as for the VVC one solid grey and one light-green with a grey stripe. These are the 2 cables from sensor part of lambda sonde. Measuring over these 2 cables with a digital voltmeter, (2 Volt range or whatever close on Your meter )will on a healthy lambdasonde give heavily fluctating readings between almost 0 Volts and around 1,3 Volts maximum. This should be measured after warmup at idle. A partly faulty unit will show a more or less steady reading within the range of 0 to 1,3 Volts or never reach beyond something like 0,5 - 0,8 Volts.. Heavy rapid changes is the good thing to look for. As the digital instrument doesn't update it's readings as frequent as the sonde changes its voltage the readings will be erractic but this is NOT a sign of a faulty lambda sonde ! The corresponding 2 cables for the measurement can as on my car be fitted with a small mating contact for future readings. Do not use too long unscreened cables (several meters long) as there is a possible pickup of inteference that can interfere with the readings and work of MEMS.

Regards , Carl
Rob Bell

Agrees with you
Will Munns

Ok testing this weekend then. Why does it require a digital voltmeter? Is the voltage ossilation too fast for an analog voltmeter?

The sensor was removed when I changed the manifold over from the origional rover to the Janspeed ~ so getting it out again should not be too much of an issue. BUT getting it out the first time was a real pain, two men a 22mm ring spanned and a long extention bar!

Steve Ratledge

>Why does it require a digital voltmeter?
>Is the voltage ossilation too fast for an analog voltmeter?

Nope, quite the opposite (may be too fast to be clear on DVM) , but the Lamda sensor is actually a voltage generator, it uses the oxygen differential against the outside to create it's voltage.

As you may guess this voltage* doesn't have much in the way of ampage** and so actually driving a magnetic field instrument - like an analoge meter will overload it and you won't see the output. A digital meter works in a diffrent way, and shouldn't cause any load at all.


*Voltage is like pressure in a water system
**Ampage is the rate of flow.

Consider a very narrow pipe and a very wide pipe leading to a water tank in the loft. Both pipes have the same pressure when capped off, but let water flow and there is a lot less from the narrow pipe. DVM's compare pressure they generate with the capped off pressure, AVM's work by measureing the flow thru a restriction, so if the system is very restricted beforehand then the AVM will not give the right reading.
Will Munns

I have not yet managed to get hold of a tester :-( but I did find this data over on Seloc. Add value to the archive if nothing else.

mv A/F
20 16.0
40 15.9
50 15.7
60 15.4
80 14.8
120 14.7
160 14.6
200 14.5
500 14.4
600 14.3
640 14.2
680 14.1
720 14.0
760 13.9
800 13.2
840 12.5
900 11.0
940 9.9
960 9.3
1000 8.5

Quote: "if you are watching it *live* I would expect it to bounce around a bit either side of 0.06 and 0.7V that equates to ~15.4 - 14.0A/F

this is how they work, by constantly sweeping up and down that area of the table."
Steve Ratledge

This thread was discussed between 21/04/2004 and 27/04/2004

MG MGF Technical index

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