My Lupo misfires under steady running (e.g. at a steady 30mph) when it is cold, but is OK once warmed up (petrol engine).
My garage said it was the MAP sensor and but replacing this didn't cure the problem.
I have seen some info on the web indicating that this fault can be caused by failure of the heater element in the lamda sensor, causing the ECU to think the engine is running too rich when it is cold, so it makes the mixture leaner until the mixture till the engine misfires. Once the exhaust gas has heated up the lambda sensor, the problem goes away.
What do you think?
If I could find out which wires on the lamba sensor connect to the heater element then I can easily test it myself. There are 5 wires: grey, white, yellow, black and blue. The garage won't tell me which is which and I can't find a reference on the web which has this info.
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Rather than guess it needs to go on diagnostic gear with someone that knows how to use it. Check the breather pipes as they are prone to cracking & allowing too much air in!
When an engine is cold the sensor is out of loop!
I Doc
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Thanks I Doc, I understand the need for proper diagnostics. I'll have a look at the breather pipes (when it stops raining).
Further info - my wife tells me that the Lupo is now misfiring when warm as well. It's going back to the garage on Tuesday to see if they can find anything else with their diagnostics.
I would be interested in the lambda sensor wire colours if anyone has them.
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What's the engine code? Why did the garage change the MAP sensor - low signal?
If so; check the cam belt tensioner hasn't come apart and jumped the belt.
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Screwloose
There is a number stamped on the engine: BBY231168. Is this what you mean by engine code?
The garage changed the MAP sensor because they thought that it was causing the misfiring though I don't know how they came to that conclusion.
I've checked the cam belt tensioner - it looks fine.
Also the breather tubes look OK too.
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That makes it a 75hp, coil-over-plug BBY motor. Should have a 6-wire wide-band mixture sensor pre-cat and a conventional oxygen probe post-cat. [Neither match your wire colours.]
If you have a heating element problem with either of those; there WILL be a code stored. Personally; I'd check the coolant temp sensor, the two pins furthest from the square end should be around 2.5K-Ohms @20C and around 200 hot.
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Thanks Screwloose,
It's odd about the lambda sensor. I've checked again and there are definately 5 wires: grey, white, yellow, black & blue. Thinking about it, the colours on the harness are different though - ones with stripes. Shall I get you these? It's the pre-cat sensor I think I am looking at (presuming that's the one in the front of the engine bay just before the exhaust pipe bends under the engine). I can't see the post-cat one, is this underneath the car?
I'll check the coolant temp sensor out when I get the car back from the garage - should be tomorrow evening. The coolant temp sensor was replaced by the garage in a previous attempt to fix a problem with stalling. That didn't fix the stalling and on a second attempt they cleaned the throttle body which did fix the stalling.
This is is proper VW service centre I am taking the Lupo to so they should know what they are doing?
I've ordered myself a diagnostic kit so I can read the ECU codes myself in future.
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I've looked at the lambda sensor again.
There are 5 wires between the sensor and the connector: grey, white, yellow, black & blue, all the same thickness.
There are 6 wires in the wiring loom to the connector: white/red, red/brown, green, black, grey/white, grey/red. The white/red and red/brown are much thicker than the rest so I would guess these are the heater wires? What I'll have to do is work out how they route through the connector as they don't line up in either side.
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The white/red and brown/red are indeed the heater circuit. I wouldn't give that heater theory another thought without a code to back it up.
Although the 75hp isn't normally affected; this fault is well-known on the 100hp versions at around 50K and usually requires a new engine. Anyone done a compression test?
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The sorry saga of my daughter's misfiring 2001 1.4 Lupo was described here:
www.honestjohn.co.uk/forum/post/index.htm?t=47157&...f
I sincerely hope your problem is something simpler to fix...
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Hi, I had looked at the sorry saga and I hope this is simpler to fix too. Not so far though.
I now have a diagnostic adapter and have been driving the car for the last couple of days to see what it tells me. I also did a 1 hour test drive with a technician from the garage with his diagnostics plugged in during which the car behaved flawlessly.
I now have a better picture of what is going on. The car misfires apparently at random for periods anything up to a few minutes. This doesn't seem to be related to warm, cold, uphill, downhill etc but does seem to be worse under light load rather than under firm acceleration.
The diagnostics are not registering any fault codes. What I have seen however is that the long term fuel trim graph shots up when the engine misfires to around 38% and stays there until the misfiring settles down.
What I don't know is what this means e.g.
Does the ECU think (wrongly) that the engine is running lean and is over-enriching the mixture making the engine misfire?
Is the engine actually running lean (and misfiring because of it) and the ECU is trying to enrich the mixture but failing due to some other fault e.g. injectors not responding properly?
Is this just a side effect of me pressing the throttle to try to get more power whilst the engine is misfiring?
Any ideas gratefully received.
The oxygen sensor current graph doesn't seen to deviate greatly at the same time - just blips when you change gear and maxes out when you throttle down.
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You have to be careful of fuel trims [surely that was the short-term trim reacting that fast ?] when you have a misfire, as there will be large quantities of unburned oxygen passing through the engine and upsetting the oxygen sensors' feedback.
It's almost a knee-jerk reaction; but could this be something like sticking exhaust valves? It's not getting picked-up by the, fairly good, self-diagnosis; so, whatever is happening, it's outside the ECU's monitoring range.
Injector faults are rare on these; but would fit the symptom - as would a lot of other things.... Genuine plugs fitted?
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I'll have a look at the plugs. I think they changed them at the last service.
I'm pretty sure it was the long term fuel trim I was looking at. The short term one seems to go up and down all the time.
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Progress: The engine check and EPC lights have come on and I now have a diagnostic code.
P0106 Manifold Absolute Pressure/Barometric Pressure Circuit Range/Performance Problem.
If this was the same fault that it came up with before (which I didn't see) I can see why it led the garage to replace the MAP sensor but it seems that wasn't the correct diagnosis.
Talking about sticking valves, could this be a sticking inlet valve causing the misfire and also upsetting the pressure in the intake manifold?
What would be the best way to check?
Are these engines sensitive to supermarket petrol? We recently switched to supermarket petrol during the recent price increase after normally using Texaco before.
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Are you 100% sure about the breather pipes? This could be a sticking inlet valve - but....
Can you see the MAP and BARO [atmospheric] pressure with your kit?
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I was wondering whether you managed to work out what the problem with your Lupo was? My 2001 Lupo seems to be suffering from similar symptoms - it splutters everso slightly when I'm going at 30mph in 4th gear, but at no other time. I've had the car a year and it's a problem that's only developed in the last three months. I've taken the car to two different garages four times, and none of them can work out what's wrong with it. Do you have any pointers? I have changed the spark plugs and plug leads but it's no better. The problem started after my car was serviced at 60,000 miles 3 months ago.
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