Mk IV MOT Problem with 302 EFI Lamda value

[MkIV Lux]

My Mk IV with 302 EFI engine and catalytic converters.
   
   Car was at the MOT yesterday and was rejected because Lamda value was  too high. Measured at 1.2 - 1.25, should be around 1.
   CO2 values were alright though.
   
   What adjustments need to be done do get Lamda value down?
   
   Who can help?
   
   Constant

[nikbj68]

Hi Constant. Is there any chance of a slight exhaust leak? This could cause air to be drawn in & produce a high Lambda reading, (weak mixture in exhaust gas).

[AC Ace Bristol]

Constants..
   
   Couple of years back,  John Lewis had problem getting his CRS through Mot, Found it passed on exhaust readings if presented when engine and exhaust was hot.
   
   Not sure if this might help..  maybe some one with more knowledge can elaborate.
   
   Cheers
   
   Keith..[]

[Flyinghorse]

This link has some interesting info--perhaps the sensor need changing?
   http://www.picoauto.com/applications/lambda-sensor.html
   
   Graham

[MkIV Lux]

quote:Originally posted by nikbj68
   
Hi Constant. Is there any chance of a slight exhaust leak? This could cause air to be drawn in & produce a high Lambda reading, (weak mixture in exhaust gas).
   
   

   Hi Nik,
   we'll check this today. Will keep you posted.

[MkIV Lux]

quote:Originally posted by AC Ace Bristol
   

   
   ....... Found it passed on exhaust readings if presented when engine and exhaust was hot.
   
   ....
   Keith..[]
   

   Hi Keith,
   Glad I spotted you at Le Mans Classic before you all left.
   Yes above suggestion has also been indicated by a knowledgeable expert.
   We'll test it today. Will keep you posted.

[MkIV Lux]

quote:Originally posted by Flyinghorse
   
This link has some interesting info--perhaps the sensor need changing?
   ...
   Graham
   

   Thanks Graham,
   Very interesting article. It sure helps for the understanding.
   We'll make measures and see.
   Will keep you posted.

[Flyinghorse]

Constant,
   You have just completed a high mileage,and all that lovely Scottish petrol.
   Is the car running well in your opinion,or are there any symptoms linked in with the high Lambda (lots of oxygen) -ie The plugs might be worth checking to see if they show weak running on a particular cylinder to give you something specific to trouble shoot like a clogged injector.
   In the UK the car needs to be hot (minimum oil temp).
   I do have various 302efi parts lying about from my CRS engine that you are welcome to borrow,if finding parts is difficult.
   
   There is a lot of google info on this subject,but as stated exhaust leaks are favourite. One other that might be worth looking at is the air filter set up. If you have changed air filter to an oiled CDA set up appearently the MAF can get dirty,so that might just need a clean with solvent. Hot air into a CDA set up (Alfa forum) has also been an issue,so try with the bonnet up.
   Graham

[MkIV Lux]

Hi Graham,
   On the Scottish tour (3800 kms), on which I consistently ran on 97/98 octane fuel, the car functioned perfectly as usual.
   Thereafter, back home, I experienced some misfiring when the car was driven on low / idling revs. As a result, I had the plugs replaced before the car was taken to the MOT.
   Proceeding stepwise, I'll test it again today to see if that problem was solved.

[Wally]

An EFI engine with catalytic converters will be using a lambda sensor (or sensors) to measure any oxygen present in the exhaust manifolds. The ECU (computer_) will then make changes to the amount of fuel delivered to the engine in order to correct any errors. The waveform you have seen in the Pico website is actually showing that the fuel mixture is too weak - too rich - too weak - too rich etc etc as it makes adjustment either side of correct. Usually if the ECU has to make corrections beyond a certain limit it will store a fault code and illuminate the engine management warning light. If the vehicle appears to run correctly without any lights then the first impression is all is Ok.
   
   If you had the equipment the first check would be to see if the Lambda sensor is switching as it should proving the exhaust emmisions are correct as seen at the lambda sensor.
   
   For the Mot Test a gas analyser is used to measure the exhaust gases. In GB this measures four gases and then shows a calcalated Lambda value.
   
   As your reading is showing a high Lambda value this shows the mixture is weak and there is excess oxygen in the exhaust gases. This is why it has been suggested that there is a leak on the exhaust system. If you had a small leak on the exhaust before the Lambda sensor (engine side of sensor) the sensor would see this as to much oxygen and make corrections to add extra fuel, but if the leak is after the Sensor then it would not know anything about it, but the Mot test equipment is measuring at the tailpipe and would show excess oxygen in the tail pipe. You only need a very small leak to make a vast differance.
   
    It is a very complicated subject but I hope this helps explain what's going on.
   
   Wally

[MkIV Lux]

thanks Wally.. it sure helps..
   we will measure the Lambda sensors' signals,each side to detect if they do work correctly at all.
   As I have put already some 40 k miles on the car, the sensors might be feeling a bit old...

[MkIV Lux]

Good news today: the Mk IV passed the (luxembourgish) MOT. But I haven't got any feedback yet on what the mechanic had fiddled to bring the Lambda (calculated) reading to approx 1.1.
   Anyway, no part has been exchanged and the car drives as well as before, so I am fine to drive for the next 12 months.
   Next week I'll have one of our company's engineers measure the Lambda sensors and will keep you posted on eventual findings.
   
   Constant

[cobham cobra]

Great news - enjoy the weekend.
   John.

[Flyinghorse]

Constant-just out of interest do they test the Lambda for MOT up each exhaust pipe--perhaps he simply tried the other pipe?
   
   Graham

[MkIV Lux]

quote:Originally posted by Flyinghorse
   
Constant-just out of interest do they test the Lambda for MOT up each exhaust pipe--perhaps he simply tried the other pipe?
   
   Graham
   

   Could well be! I had not been myself, usually I delegate this to the garage that services my car, as I would have to bite my tongue to remain polite when I see the oh so often displayed nonsense that they impose on us! In my country the MOT is a public service in monopolistic situation, which makes most people (hence the economy) loose a lot of time! So every time I have to go there (altogether with 7 cars and 5 trailers makes it 12 times a year!) I am getting very upset. The general attitude here is: where can we find something that is not correct, to stop the car from being on the road, rather than how can we help to keep the car alive and on the road. Last time I took the Lotus Eleven, it was rejected because one light was beaming a bit too high!