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Discussion Starter #1
How many Lambda sensors does a mgtf 115 54plate have?
I can see a post cat one but is there a pre cat one?
I'm experiencing problems at various instances whereby the car loses power when both hot or cold.
A car almost went into the sode of me today as it al.ost stalled pulling out of a junction.
Once it gets going its absolutely fine.
I would like to sell ot but feel like i can't like this.
Someone said it could be lambda sensor or air control valve.
Need help
 

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mg_tf
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If you've got a post cat sensor there should also be a pre cat sensor , the post cat sensor is a double check that the pre cat sensor and cat are doing their job. I don't profess to be an expert but it doesn't sound like a lambda sensor issue to me. A cheap OBD reader should be able to give the readings from the sensors.
 

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Firstly, I’ve got an O2 MGTF and it’s got the 2 cats one on the exhaust manifold and the other after the catalyst. What caught my attention was the symptoms you described. I spent months trying to cure mine-they were very similar- every so often- NO POWER. Occasionally it would stall. I took to the Forum and nothing helped or made sense. I even got Russell around with T4 and he found some slight adjustment were out - on the stepper motor and he also found the pre cat connector was a bit cruddy. I took the car for a long run and returned telling Russell it seems fine. The problem reoccured a few days later and it stalled in heavy traffic and was difficult to start. Right- it was now unreliable and needed fixing- so I took it off the road until it WAS!
I jacked it up, supported it and went underneath - the wiring to the pre-cat lambda was intact- but near the heat shielding was in bits- I took the sensor out and I could see it was potentially shorting out- at least 2, maybe 3 wires could short together. I replaced the sensor (from a reputable supplier- with an OEM) and the problem was solved!
It wasnt the stepper motor on the AIR valve, it wasn't the throttle pot, it wasnt so many other things Id considered or been advised to replace (you name it, someone had suggested it!)
Good luck- but worth a look!
 

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mg_tf
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A lack of power (but not all the time) suggests to me a lean condition. Whether that's caused by insufficient fuel or too much air, that's another thing, but the lambda may well be implicated. The Idle Air Control valve I would have thought would have only affected your idle speed. I don't think the IACV is a big deal when demanding power as the butterfly on the throttle body supplies nearly all the air when the engine is under load. I believe the lambda should jump between about 0.1V and 0.8V several times a second, after an initial 'open-loop' period after start-up when it does nothing and the car runs rich. I don't think that period is very long - I'd guess less than a minute, but I have no idea really, that's just an observation from another type of car. I'm not an expert on the engine control system though so take this with a pinch of salt. You might be able to intercept the Lambda probe signal with a pair of digital voltmeter (DVM) probes somewhere and see that the voltage starts switching when the car is going. Not ideal using a DVM to observe a switching signal, but it doesn't switch so fast you wouldn't see it, IMO.
 

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Discussion Starter #5
If you've got a post cat sensor there should also be a pre cat sensor , the post cat sensor is a double check that the pre cat sensor and cat are doing their job. I don't profess to be an expert but it doesn't sound like a lambda sensor issue to me. A cheap OBD reader should be able to give the readings from the sensors.
Cheers
Bought an obd2 reader but says no connection no matter how many times I've tried.
I've had full service incl coil packs
Cam belt change incl water pump ect
The garage says he didn't think it would be the lambda sensor but a mg mechanic says it might be a possibility?
The garage spotted a faulty injector on cylinder 4 but hasn't helped the situation at all.
 

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mg_tf
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Cheers
Bought an obd2 reader but says no connection no matter how many times I've tried.
Unless you're able to get some readings you'll be changing things by guesswork. You might get lucky or you might spot something hanging off but it's potentially an expensive way of doing things. You don't say if your EML light is on, if you have a O2 / Lambda sensor issue this should be on.

If you could get the OBD reader working you'd not only be able to monitor the sensors but you'd be able to read any fault codes, these might give you a clue as to the issue.
 

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Discussion Starter #7
A lack of power (but not all the time) suggests to me a lean condition. Whether that's caused by insufficient fuel or too much air, that's another thing, but the lambda may well be implicated. The Idle Air Control valve I would have thought would have only affected your idle speed. I don't think the IACV is a big deal when demanding power as the butterfly on the throttle body supplies nearly all the air when the engine is under load. I believe the lambda should jump between about 0.1V and 0.8V several times a second, after an initial 'open-loop' period after start-up when it does nothing and the car runs rich. I don't think that period is very long - I'd guess less than a minute, but I have no idea really, that's just an observation from another type of car. I'm not an expert on the engine control system though so take this with a pinch of salt. You might be able to intercept the Lambda probe signal with a pair of digital voltmeter (DVM) probes somewhere and see that the voltage starts switching when the car is going. Not ideal using a DVM to observe a switching signal, but it doesn't switch so fast you wouldn't see it, IMO.
Thank you so much for your input, beginning to see what each part does and for how long.
Firstly, I’ve got an O2 MGTF and it’s got the 2 cats one on the exhaust manifold and the other after the catalyst. What caught my attention was the symptoms you described. I spent months trying to cure mine-they were very similar- every so often- NO POWER. Occasionally it would stall. I took to the Forum and nothing helped or made sense. I even got Russell around with T4 and he found some slight adjustment were out - on the stepper motor and he also found the pre cat connector was a bit cruddy. I took the car for a long run and returned telling Russell it seems fine. The problem reoccured a few days later and it stalled in heavy traffic and was difficult to start. Right- it was now unreliable and needed fixing- so I took it off the road until it WAS!
I jacked it up, supported it and went underneath - the wiring to the pre-cat lambda was intact- but near the heat shielding was in bits- I took the sensor out and I could see it was potentially shorting out- at least 2, maybe 3 wires could short together. I replaced the sensor (from a reputable supplier- with an OEM) and the problem was solved!
It wasnt the stepper motor on the AIR valve, it wasn't the throttle pot, it wasnt so many other things Id considered or been advised to replace (you name it, someone had suggested it!)
Good luck- but worth a look!
Thank you so much for replying, going to take to garage and ask them to replace the lambda sensor at least then try it. MOT it if it's okay and sell it. I'm struggling to work on cars any more.
Unless you're able to get some readings you'll be changing things by guesswork. You might get lucky or you might spot something hanging off but it's potentially an expensive way of doing things. You don't say if your EML light is on, if you have a O2 / Lambda sensor issue this should be on.

If you could get the OBD reader working you'd not only be able to monitor the sensors but you'd be able to read any fault codes, these might give you a clue as to the issue.
Hiya, The EML is not on and I assume the garage found a fault hence why they changed an injector.
To be honest feel like just pushing it over the nearest cliff.
The person who sold it me had warmed it up previously but I can't bring myself to sell it like that knowi g it has a fault.
 

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Hi, I had a 2005 115 TF, which had am horrendous "flat spot" on acceleration when cold, and disappeared when warm, almost cutting out as accelerating away first thing. Felt like driving an old carburettor car from cold with no choke.
Never put the engine light (MIL) when it was occurring, and so no fault codes stored.
Eventually tracked it down to the Lambda sensor,from memory changed the 1 nearest the manifold, and no more problems.
 

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Discussion Starter #9
Hi, I had a 2005 115 TF, which had am horrendous "flat spot" on acceleration when cold, and disappeared when warm, almost cutting out as accelerating away first thing. Felt like driving an old carburettor car from cold with no choke.
Never put the engine light (MIL) when it was occurring, and so no fault codes stored.
Eventually tracked it down to the Lambda sensor,from memory changed the 1 nearest the manifold, and no more problems.
Thank you, think I'll change both as so many people are suggesting this as a solution 👍
 

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Changing both can be advised. They are awkward to change, but if the garage is doing the job no so much of a problem. I had the same problem as you, only a few weeks ago. Funnily enough after changing the o2 sensor. My tf failed its mot on emmisions. It was running in open loop all of the time. The garage swapped the o2 sensor -pre cat. Ran lumpy but no M.I.L. Stick with the Bosch one. Put a second one in and runs fine then passed its mot. You could disconnect the first o2 sensor and run it for a short while. It will stay in open loop mode with the ECU not referring to the o2 sensors. Then you would know if the o2 sensor was the problem.
My tf has had both o2 sensors cause problems. They are not that expensive to change. Good luck.
 

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Discussion Starter #11
Changing both can be advised. They are awkward to change, but if the garage is doing the job no so much of a problem. I had the same problem as you, only a few weeks ago. Funnily enough after changing the o2 sensor. My tf failed its mot on emmisions. It was running in open loop all of the time. The garage swapped the o2 sensor -pre cat. Ran lumpy but no M.I.L. Stick with the Bosch one. Put a second one in and runs fine then passed its mot. You could disconnect the first o2 sensor and run it for a short while. It will stay in open loop mode with the ECU not referring to the o2 sensors. Then you would know if the o2 sensor was the problem.
My tf has had both o2 sensors cause problems. They are not that expensive to change. Good luck.
Changing both can be advised. They are awkward to change, but if the garage is doing the job no so much of a problem. I had the same problem as you, only a few weeks ago. Funnily enough after changing the o2 sensor. My tf failed its mot on emmisions. It was running in open loop all of the time. The garage swapped the o2 sensor -pre cat. Ran lumpy but no M.I.L. Stick with the Bosch one. Put a second one in and runs fine then passed its mot. You could disconnect the first o2 sensor and run it for a short while. It will stay in open loop mode with the ECU not referring to the o2 sensors. Then you would know if the o2 sensor was the problem.
My tf has had both o2 sensors cause problems. They are not that expensive to change. Good luck.
Cheers, I've booked it in for a MOT plus they're going to look at it for me a hour before and give me some advice.
I will put what they say on here and you never know it might help someone.
 

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Cheers, I've booked it in for a MOT plus they're going to look at it for me a hour before and give me some advice.
I will put what they say on here and you never know it might help someone.
Remember what I wrote in my response because my pre-CAT (the one on the exhaust manifold downpipe) was short-circuiting- but not all the time- This gave me a problem only when hot and it caused flat-spots and very poor tickeover. It started off as not much a problem, it did not manifest itself as an issue on a long run- but at low revs it would come and go. Its easily disconnected but to pass the MOT you will need it fitted to pass emissions.
They (Lambdas) come out best when hot and with the correct tool- I actually got mine out with a spanner, but Id replaced it when I changed the flexi- tube 3 years ago and had copper-greased the threads.
 

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Remember what I wrote in my response because my pre-CAT (the one on the exhaust manifold downpipe) was short-circuiting- but not all the time- This gave me a problem only when hot and it caused flat-spots and very poor tickeover. It started off as not much a problem, it did not manifest itself as an issue on a long run- but at low revs it would come and go. Its easily disconnected but to pass the MOT you will need it fitted to pass emissions.
They (Lambdas) come out best when hot and with the correct tool- I actually got mine out with a spanner, but Id replaced it when I changed the flexi- tube 3 years ago and had copper-greased the threads.
The manifold heat shield hides the wires. When I took that off I could see wear, where the o2 had caught and melted.
 

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Discussion Starter #14
Remember what I wrote in my response because my pre-CAT (the one on the exhaust manifold downpipe) was short-circuiting- but not all the time- This gave me a problem only when hot and it caused flat-spots and very poor tickeover. It started off as not much a problem, it did not manifest itself as an issue on a long run- but at low revs it would come and go. Its easily disconnected but to pass the MOT you will need it fitted to pass emissions.
They (Lambdas) come out best when hot and with the correct tool- I actually got mine out with a spanner, but Id replaced it when I changed the flexi- tube 3 years ago and had copper-greased the threads.
Thank you, I'll show them all the replys and fingers crossed I'll get it sorted.
Glad you got your problem sorted and fingers crossed I'll get mine sorted too.
 

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You wil
Thank you, I'll show them all the replys and fingers crossed I'll get it sorted.
Glad you got your problem sorted and fingers crossed I'll get mine sorted too.
You will. They are pretty simple cars. Frustrating a lot of the time. I love mine more than I should and treat it like a member of the family.
 

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Discussion Starter #16
Aww lol.
Selling mine as soon as sorted especially with 12 months mot as just got disability car and can't do as much as used too. 😥
 
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