146 Lambda Sensor Oddness - Alfa Romeo Forum
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(Post Link) post #1 of 9 Old 02-11-11 Thread Starter
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146 Lambda Sensor Oddness

Hi Folks,

Bought a 146Ti with some interesting problem.

It failed the MOT, principally on emissions (don't they all), which the garage duly chased up the exhaust, replacing the CAT and Lambda sensor. They also replaced the cam sensor and a cam sprocket (with giveaway tipex marks, !!guys!!), did some welding and fixed up the handbrake cable. Took 4weeks. Passed the MOT, charged me 1,000 for the work.

But despite the cost, fuel economy dropped like a stone, where it had performed brilliantly before, it was now driving like a bag of hammers, and chewing almost double the fuel.

Took it to a different garage for an investigation, and none of the sensor readings make any sense. The combined sensor readings were increasing the injection pulse to 8.5ms.

Checked out the MAF sensor, which is OK, have swapped that out, but with no improvement.
Lambda sensor seems to be the culprit. If I unplug the Lambda, the ECU uses a default value, and the injection pulse drops to a respectable 4.3ms. Broken sensor? But it was new, not 1,000 ago!

There are 4 wires to the Lambda - a heating circuit, and a sensor circuit. If I apply a voltmeter to it, it appears to return a signal. If I measure the ECU sense input wire (on the connector) when the lambda is disconnected, the sensor wire is reading 4.8V. The garage electrician seems to think that the ECU is creating a voltage to read because the sensor is not providing a signal - I am highly sceptical of this explanation.

To replace the lambda sensor AGAIN seems to be sending me around in circles. What am I missing? I need the sensor in the equation but the ECU seems scrambled when it's attached. Without it, 4.3ms timing is driveable. It's not great below 2k rpm, and feels like I am dragging an anchor sometimes. Then it seems to clear and drive sweetly (think an oil change and sparks may solve that).

One possibility I suppose is that the ECU is screwed. The key to the car is a replacement Fiat key, not the standard red one, so I have no control over that. And the alarm LED is always pulsing above the steering wheel.

The exhaust system could be replaced. But I worry about spending money on a second CAT and Lambda without a better understanding of WHY the readings are messed up.

Anyone with any wisdom would be appreciated. I would prefer a reasoned approach to solving the issues, not just buy and swap in case approach.

For all that, I (and the kids) are enjoying it. When it goes, it goes well.

Thanks in advance.

Last edited by DDS9; 06-11-11 at 22:28. Reason: Ti not TS
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I am aware there were a dodgy batch of Boosch products went out so could be a case of contacting them, the product will contain a batch coding along with a part no. this would be my first point of call to determine if this is the case, that costs nothing.

Emmisions.... a big area to cover really, but like you say, i would suggest checking the spark plugs and and i would have a look at the coil packs. (NOT just replace these parts, IVESTIGATE these parts).

I would definately be asking questions of the people who did the work on the car, as to why your car is now not comsuming fuel at a good rate and thus how the car has past its emmisions, because for sure if a car is churning fuel like that then the emisions on at least one reading would be high.

I hope you kept your original cat as they are worth scrap alone and it gives you a reference point along with all the other parts. I think that is my 10pence worth.
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Do you want to try a different second hand lambda sensor to see if it makes any difference?

Also, did you get a pattern cat converter? They are normally crap and barely last through their warranty period.
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Thanks to you for replying.

The garage kept the parts and are 350miles away. Not likely to have the CAT around at this point.

The ECU does look like it has been opened - there is a missing nut on one corner. The ECU seems to give mixed messages - engine temperature sensor was missing on some readings, then popped up later in the process, for example. I am told if I want the alarm and ignition sorted, a new ECU is the way to go. I just can't help but wonder if that is the root cause - perhaps this is not the original part and we are working with a replacement already. If I replaced that, I suspect the sensor readings would come right.

Another Lambda is what the local mechanics want to try, gut instinct is that this is not going shed more light on it, without understanding what voltages we ought to be seeing. Normal is supposed to be around 0.1V so an input voltage of approaching 5V is well out of kilter. Although, trying to remember by electronics - does a chip input measure 5V when open circuit?

Thanks for the heads up about the coils and sparks.
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PS. Is it worth shelling out on the manufacturer's specific oil or is there a suitable equivalent? I have been told that alternatives kill Alfa engines owing to viscosity differences even if the grade is right. Advice sought!
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Hmm... the ECU being "opened" sounds ominous. There's all sorts of mischief that could have been going on in there.

Anyhow... I would look at the lambada first.

You can get a good brand (e.g. NTK) on fleabay for not tooo much.. and if it doesn't solve the problem then at least you have a spare sensor. Apart from the catalyst, nothing else affects emissions.

The lambaba circuit at 4.x volts sounds right.. I remember reading something (I'm not leccy-techie unfortunately) that the signal drops to 1.x volts when it's doing it's thing.. so if it's stuck around 4v and not changing, then something isn't happening.. <Hopes someone can explain it better>

So... first check the lamda relay. There is a relay that controls the lambda heater and the signal circuit... if the connection is shagged, then there's your problem... the lambo won't work.

Your pattern part cat might not be great quality (have a look what brand it is - Bosal? Timax etc. will be as good as anything) but don't get hung up on that. It's new, so it will work... even if that's "just for now". It only has rare-metal honeycomb in it.. quite difficult for it to suddenly fail.

If you still don't get anywhere, take the lid off the engine and one at a time, remove the coils and examine the end-connectors. All that fiddling about may have damaged a corroded end.. either into the coil or into the spark plug boot.

While you're there, peek into the plug wells and make sure they're not full of oil, though if the garage had the rocker cover off, then it should be okay.

Take the plugs out and make sure they're in good condition. I'm always depressed how often garages just put crappy plugs back in..

If you get desperate, take the rocker cover off and turn the beast over by hand until you get TDC on #1 cylinder (closest to the cam-belt) and check the timing. If it's out, you could get wonky running even if everything else is fine.

Your other questiones... Don't worry about the oil. If they used any 10W40 semi-synthetic then it's good enough for your old donkey. Alfa's like oil.. but they're not oil quality fussy. The minimum spec' is impossible to buy these days. Any 10W40 semi-synthetic is good enough.

Don't worry about the key. If it starts the car then it works and has nothing to do with the problem. From memory, the flashing light on the dashboard means that the car thinks that the signal from the key (alarm button) is weak and your battery needs replacing. I presume your Alfa key was a red one with the rubbery alarm button built into it. Your Fiat one might be similar.. ? but stick a new battery in there and see if that sorts out the flasher.

Ralf S.

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check sparks

had similar issues check plugs, easy job .good luck
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Depends if it's zirconia or titania sensor, zirconia are most common. A zirconia sensor doesn't need a supply voltage since it generates it's own by the chemical reaction of the oxygen in the exhaust. The only supply voltage is for the heater element (2 white wires) the other 2 are for the sensor signal and when working should typically switch between 0.2 - 0.8 Volts. Heater voltage is battery voltage. It's usually the black wire for the sensor output.

A titania sensor DOES require a voltage to work and this is typically around 5 Volts from the ECM. Typical titania sensor wire colours are red, white, black & yellow (other colours are available) with the red + white being the heater & black & yellow the sensor wires. The black wire is the sensor output (what the ECM sees) and should read between 2 - 5 Volts.

You can only really check the operation of a lambda sensor when it's reached it's operating temperature (approx 300 degC) since it does NOTHING until then (hence why they're heated). If you've got 5 or more wires then it's a totally different kettle of noodles and requires the laptop plugging in.

The cat won't really affect the economy unless it's blocked or too restrictive (providing there's no post cat lambda). Engine temperature sensor will have an affect since the ECM will have a 'cold' fuelling strategy.

The cam bits may have something to do with it if the valve timing is incorrect then you'll never have a smooth running engine.

Last edited by monkeyspanker; 12-11-11 at 07:26.
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I would like to thank you all for your generous writing contributions. Good to know that in the absence of a manual there is help at hand.

Rather than comment on each thing "in theory" I shall go through the points you have each mentioned one by one (with a little help from the mechanic) and see if we can get to the bottom of what is going on.

Has to be a solution, it's a machine after all !

Wish me luck...
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146 , ecu , exhaust , lambda , oddness , sensor

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