Cat, exhaust sensor and rich running
Discussion
I have noticed a great thirst on my 2 litre mondeo recently so took it in for a check over. I was told the cat was ruined so the exhaust sensor was making the engine do strange things which was resulting in overfuelling the engine (running way too rich). Having taken it in elsewhere to have a new cat fitted, it struck me that because the sensor is in front of the cat, how can it tell what is going on down there and why would it overfuel.
The garage have just told me they don't know how the sensor works (its just a national tyres place to be fair) but that if the ovefuelling is an engine problem, it could ruin the new cat.
So does anyone reckon the new cat will let the engine sort itself out or should I be taking it straight to the Ford garage to check it is now running properly?
The garage have just told me they don't know how the sensor works (its just a national tyres place to be fair) but that if the ovefuelling is an engine problem, it could ruin the new cat.
So does anyone reckon the new cat will let the engine sort itself out or should I be taking it straight to the Ford garage to check it is now running properly?
Lambda sensor is there to protect the cat, rather than the other way round. If you just replace the cat without curing the underlying problem, the new one will most likely go the same way. Could be as simple as a duff lambda sensor, but rather than guessing you need to take it to somebody who can access the ECU diagnostics.
the ford garage stuck it on the diagnostics and said the lambda sensor (replaced in the last MOT about a month ago) was compensating for the damaged cat - so the car apparently still passes emissions control but is doing so by running very badly.
Doesn't make much sense to me though - you would have thought more fuel than usual = more emissions?
Doesn't make much sense to me though - you would have thought more fuel than usual = more emissions?
Aha - just phoned Ford again - the Lambda sensor takes readings in the cat and their diagnstics check showed no problems with engine management etc but for the lambda sensor trying to compensate for the damaged cat.
My concern now is that why did National tell me the sensor was BEFORE the cat, when Ford reckon it is in (or at least takes readings from) the cat???
My concern now is that why did National tell me the sensor was BEFORE the cat, when Ford reckon it is in (or at least takes readings from) the cat???
Why not try looking?zebedee said:Because national tyres people are trained to fit exhausts, batteries and tyres and the accuracy of anything they tell you pertaining to diagnosing problems with your engine will very much depend on who you talk to and just because the person you spoke to believes themselves to be right has no bearing on whether they are or not.
My concern now is that why did National tell me the sensor was BEFORE the cat, when Ford reckon it is in (or at least takes readings from) the cat???
There are some very knowledgable people working at these places but unless you know as much as these people you will have trouble differentiating between the knowledgable people and the people who know
all. Mark
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--"I have noticed a great thirst on my 2 litre mondeo recently so took it in for a check over. I was told the cat was ruined so the exhaust sensor was making the engine do strange things which was resulting in overfuelling the engine (running way too rich). Having taken it in elsewhere to have a new cat fitted, it struck me that because the sensor is in front of the cat, how can it tell what is going on down there and why would it overfuel.
The garage have just told me they don't know how the sensor works (its just a national tyres place to be fair) but that if the ovefuelling is an engine problem, it could ruin the new cat.
So does anyone reckon the new cat will let the engine sort itself out or should I be taking it straight to the Ford garage to check it is now running properly?"
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Ok, assuming the car you're talking about is the 1994 Mondeo on your profile, it IS NOT OBD emissions compliant. This means that it DOESN'T have TWO HEGO or LAMBDA sensors (usually fitted upstream of the cat in the cone and between the two bricks).Two Hego sensors (heated gas oxygen sensors) are fitted on OBD emissions compliant cars to MONITOR the condition of the emissions performance of the car.
You're quite right with YOUR reasoning, the Lambda sensor fitted upstream of the cat can't know the condition of the catalyst and the garage that told you this are talking out of their moist flaps.
Secondly if the car is running rich ( which is easy enough to check) it WON'T ruin the catalysts, as long as it doesn't go righer then about 9:1 Air fuel ratio. It's ONLY if RAW unburned fuel gets into the cat and is burned off there- that this can destroy the catalysts. That's why misfires are so dangerous on modern cars and thats why OBD emissions systems are so intent on detecting misfire. So again, this place is talking out of their chocolate starfishes.
Many cars, especially boosted engines run very rich as full throttle and high speeds to KEEP the CATALYSTS cool, protecting them.
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"the ford garage stuck it on the diagnostics and said the lambda sensor (replaced in the last MOT about a month ago) was compensating for the damaged cat - so the car apparently still passes emissions control but is doing so by running very badly.
Doesn't make much sense to me though - you would have thought more fuel than usual = more emissions?"
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The car will most likely still pass a MOT emissions test as this is so much less stringent then what manufacturers have to meet. If the mixture runs rich, CO2 emissions will rise, HC emissions will also go up but NOx will actually come down. A 3 way catalyst is designed to run at stoichimometry (14.7:1) during part load operations and during the emissions test window to keep a general low balance of these three emissions.
This single upstream lambda sensor won't compensate in this way they suggest.
I know and understand that your lambda sensor was just changed, and it's danegerous to diagnose over the net in a forum but it sounds more like your lambda sensor.
Can you scribble down and tell me what readings the garage are getting for HC, NOx and CO- pre cat at , say idle or some other part load site?
More helpful would be readings taken BEFORE the cat. Some cars have a sampling port there, Im not sure the Ford Mondeo does.
This is an example of a simple job that could be tackled by engineers, but given to mechanics who get paid almost as much as engineers in this country anyway.
>> Edited by Marquis_Rex on Thursday 21st October 17:36
--"I have noticed a great thirst on my 2 litre mondeo recently so took it in for a check over. I was told the cat was ruined so the exhaust sensor was making the engine do strange things which was resulting in overfuelling the engine (running way too rich). Having taken it in elsewhere to have a new cat fitted, it struck me that because the sensor is in front of the cat, how can it tell what is going on down there and why would it overfuel.
The garage have just told me they don't know how the sensor works (its just a national tyres place to be fair) but that if the ovefuelling is an engine problem, it could ruin the new cat.
So does anyone reckon the new cat will let the engine sort itself out or should I be taking it straight to the Ford garage to check it is now running properly?"
--------------------------------------------------
Ok, assuming the car you're talking about is the 1994 Mondeo on your profile, it IS NOT OBD emissions compliant. This means that it DOESN'T have TWO HEGO or LAMBDA sensors (usually fitted upstream of the cat in the cone and between the two bricks).Two Hego sensors (heated gas oxygen sensors) are fitted on OBD emissions compliant cars to MONITOR the condition of the emissions performance of the car.
You're quite right with YOUR reasoning, the Lambda sensor fitted upstream of the cat can't know the condition of the catalyst and the garage that told you this are talking out of their moist flaps.
Secondly if the car is running rich ( which is easy enough to check) it WON'T ruin the catalysts, as long as it doesn't go righer then about 9:1 Air fuel ratio. It's ONLY if RAW unburned fuel gets into the cat and is burned off there- that this can destroy the catalysts. That's why misfires are so dangerous on modern cars and thats why OBD emissions systems are so intent on detecting misfire. So again, this place is talking out of their chocolate starfishes.
Many cars, especially boosted engines run very rich as full throttle and high speeds to KEEP the CATALYSTS cool, protecting them.
-----------------------------------------------------
"the ford garage stuck it on the diagnostics and said the lambda sensor (replaced in the last MOT about a month ago) was compensating for the damaged cat - so the car apparently still passes emissions control but is doing so by running very badly.
Doesn't make much sense to me though - you would have thought more fuel than usual = more emissions?"
-----------------------------------------------------
The car will most likely still pass a MOT emissions test as this is so much less stringent then what manufacturers have to meet. If the mixture runs rich, CO2 emissions will rise, HC emissions will also go up but NOx will actually come down. A 3 way catalyst is designed to run at stoichimometry (14.7:1) during part load operations and during the emissions test window to keep a general low balance of these three emissions.
This single upstream lambda sensor won't compensate in this way they suggest.
I know and understand that your lambda sensor was just changed, and it's danegerous to diagnose over the net in a forum but it sounds more like your lambda sensor.
Can you scribble down and tell me what readings the garage are getting for HC, NOx and CO- pre cat at , say idle or some other part load site?
More helpful would be readings taken BEFORE the cat. Some cars have a sampling port there, Im not sure the Ford Mondeo does.
This is an example of a simple job that could be tackled by engineers, but given to mechanics who get paid almost as much as engineers in this country anyway.
>> Edited by Marquis_Rex on Thursday 21st October 17:36
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