Corsa - Rough idle , unstable drive. Some soot from exhaud - Markval
Hi guys ,

I have a 1.2 petrol corsa 2010.

A few months ago it started with rough idling. Eventually engine management light came on. I took to garage and it was diagnosed as o2 sensor which I had replaced. Fine for another week and then same thing. Had another sensor replaced. Fine again for a while. Then idling went so bad that when I was driving in heavy traffic it stalled. Rac were called. Diagnosed o2 sensor again and said the fuel was too rich. Upon revving a black sooty substance was firing out of exhaust. Not as smoke, it was just hitting the floor. Not heavy , just a dusting. Rac mechanic removed the coil pack and said it's got a few very minor burn marks but it's ok. Sparks were full of black sooty deposit but only slight wear.

Changed air filter and cleaned up sparks and I was able to drive home.
He cleared the codes so I had to wait for light again. It took a day and there it was. So back to garage and diagnosed as air flow meter. At this point I am sick of throwing money at it. But I agreed and got it replaced.

Drive well for a couple of days and now I am starting to feel the rough idle again!

Also when driving I can feel little bursts of power like I am lifting foot and putting back on gas. Sort of jerks a bit.

This is what I have had done....2 o2 sensors, air flow meter. New air filter, used egr cleaner and I am still having problems.

Sorry for long winded post but don't know how else I can explain problem.

Service is about due now but rac mechanic said it's probably not that causing problem

Help

Corsa - Rough idle , unstable drive. Some soot from exhaud - elekie&a/c doctor

First thing I would do is take it to a garage/mot station that is able to run an emission test and give you a 4 gas read out of what is actually coming out of the exhaust. This will give an indication of how good/bad of the engine combustion process .

Corsa - Rough idle , unstable drive. Some soot from exhaud - hardway

Agreeing with the "Doc" and suspect as you do it's massively overfueling.

You can unplug the MAF's on these and they will start/run,

Reason being I've come across some very bad MAF's brand new out the box.

unplugging it is a good way to test if it runs better without one plugged in.

So I'd say you need a comprehensive scan done.

by someone who knows how to read STFT,

LTFT and o2 sensor outputs.

CTS and so on.

No offence to your RAC guy but there's no mention of any of the above.

The Short term fuel trim is one of the first things I would want to know,

Corsa - Rough idle , unstable drive. Some soot from exhaud - Markval
Thanks for the input guys.....

So getting a service isn't going to eradicate any of the problem?

Garage were talking about replacing the catalytic converter next. That's an expense I can do without

Guy feelings guys?

Also, will ask for emission test at garage
Corsa - Rough idle , unstable drive. Some soot from exhaud - hardway

It's overfueling and though that "will" kill a cat it might not be there yet.

Again you need a good diagnostic guy.

The overfueling might be something as simple as an air intake leak so the ECU is adding fuel.

It's all just a guess here.

Get it live data read by somebody who understands the system.

NOT someone who reads codes.

I much prefer to find out what's wrong before buying ANY parts.

Corsa - Rough idle , unstable drive. Some soot from exhaud - Gibbo_Wirral

Hardway is spot on. The garage is misinterpreting the codes. Faults are being picked up at certain components, but it doesn't mean that the component is faulty.

It sounds like they have no idea of the steps to take in fault diagnosis and are just throwing parts at the problem. But since its your money they won't care.

Instead of a generic scanner a proper marque specific tool must be used that can read live data.

Corsa - Rough idle , unstable drive. Some soot from exhaud - Railroad.

The reason why an O2 sensor fault was generated is that's where the fault is being reported from. Anything mechanical could be wrong, but how would the ECM know that? It sees a rich mixture reported by the O2 sensor, but the cause could be absolutely anything from a stuck open fuel injector to a blocked exhaust or air intake. Unfortunately this is what often happens when you let any old garage loose with a fault code reader. You need to do some fuel trim checks using live data to identify the root cause of the problem.

Corsa - Rough idle , unstable drive. Some soot from exhaud - Markval
Thanks guys. I will see if the garage can carry out those tests. If not, I guess I go elsewhere.

Thanks again
Corsa - Rough idle , unstable drive. Some soot from exhaud - Markval
Ok had live data readings. Aparantly the pre cat o2 sensor is sending lean constantly. Going to give egr valve a thorough clean tomorrow and take it for a long drive again to see if that stops the problem.

Mechanic said judging by the readings it's possibly a clogged cat.
Corsa - Rough idle , unstable drive. Some soot from exhaud - hardway

You'd be better of looking for the induction leak,

Air is getting in AFTER the MAF.

It's un measured air so the ECU's only option is enrichen the air/fuel mixture.

So if you had the relevant live data you's see that both longterm and short term fuel trims were high to compensate.

A professional garage would smoke test the induction system.

This test as the name implies forces manufactured smoke into the intake and where it flows out from there's a leak.

On these cars my first guess would be the composite inlet manifold and pipes.

If you'd ever had one of these manifolds off you'd see just how complex the plastic/composite construction is.

Taking it for a run will do squat.

Corsa - Rough idle , unstable drive. Some soot from exhaud - Markval
Smoke test was carried out before the maf was changed.

So they said today.

See how we go tomorrow now.
Corsa - Rough idle , unstable drive. Some soot from exhaud - Railroad.

Your petrol engine will either have a mass air flow (MAF) sensor to measure air flow, or it will have manifold absolute pressure (MAP) sensor to measure inlet manifold pressure. It will not have both as there is no need. The inlet manifold is actually under vacuum, but the ECM doesn't understand vacuum so it interpretes it as negative pressure instead. With the engine not running the MAP sensor reading should be around 100KPa. This is atmospheric pressure, and will depend on your altitude above sea level. The higher you're located the lower the reading within a few KPa. With the engine idling and the throttle closed the MAP sensor reading should be around 40KPa. This figure will be higher than this if you have a manifold air leak. If you snap open the throttle the reading should initially go back to around 100KPa because there will be no pressure difference either side of the throttle valve. When the throttle is released the MAP sensor reading should fall below 40KPa before settling back at around 40KPa. This is because manifold pressure drops as the engine revs down with a closed throttle, and the pressure difference either side of the throttle valve is greater. So the scantool measuring the MAP sensor is very useful.

Short term fuel trim is the O2 sensor reporting to the ECM the state of the fuel/air mixture. Long term fuel trim is the the ECM's corrective response to STFT. So if the LTFT reading is 10% for example it means the engine is running lean and the ECM is adding fuel by 10% to maintain the correct fuel/air ratio to achieve Lambda 1.

If the ECM has to correct too much a fault code relating to the O2 sensor will eventually be generated because that's where the ECM is receiving the information from, and not necessarily because the component is at fault.

You need a garage who understands the system, and not one who's just going to throw parts at it.

Corsa - Rough idle , unstable drive. Some soot from exhaud - hardway

Any other tests done that you "forgot" to mention?

Have you had the engine coolant temperature sensor read in live data?

Corsa - Rough idle , unstable drive. Some soot from exhaud - Big John
Then idling went so bad that when I was driving in heavy traffic it stalled. Rac were called. Diagnosed o2 sensor again and said the fuel was too rich. Upon revving a black sooty substance was firing out of exhaust. Not as smoke, it was just hitting the floor. Not heavy , just a dusting. Rac mechanic removed the coil pack and said it's got a few very minor burn marks but it's ok.

I've seen this sort of thing in the past on a Fiat Punto where all sorts was replaced - in the end it was the coil pack. Basically if one half of the pack fails it drops down to two cylinders (stalling!) and injects two cylinders worth of neat fuel into the cylinders that won't be burnt in the cylinders although it will cause havoc in the exhaust and catalyst and coat oxygen sensors

It' quite a cheap part and easy to change -change anyway

Corsa - Rough idle , unstable drive. Some soot from exhaud - Markval
Right, didn't get the cat gutted.
Found a problem with oil pressure switch. Was leaking oil which was travelling along loom and contaminating components. One of the new o2 sensors was damaged which has been replaced under warranty(lucky I know). Some had made its way to the ecu which they have cleaned up. They said every wire they unplugged has oil deposits. Anyway, all cleaned and fingers crossed ecu doesn't fail.
Been told that it still might have oil in loom and will probably need to get it cleaned another couple of times.

New oil pressure switch installed also.

Luckily the work didn't cost me any more money.

I just can't trust this car anymore.

Anyone heard of this before?
Corsa - Rough idle , unstable drive. Some soot from exhaud - elekie&a/c doctor
Not a common fault,but certainly not unknown.there was a service recall on some Astra models where a leak from the switch caused oil to run onto the hot exhaust manifold and cause a local fire
Corsa - Rough idle , unstable drive. Some soot from exhaud - Markval
Don't tell me that haha

Edited by Markval on 21/07/2017 at 10:38

 

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