W12 High negative fuel trim
04-09-2018, 07:42 AM
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W12 High negative fuel trim
Hey. I've got a 2005 A8 with W12 and noticed that I have high negative fuel trim (~-15%) on one engine, but only like +1% on the other one. No error codes besides those from replacing CD changer with bluetooth module. Both MAFs are reporting around 170g/s on full throttle near redline and generally all sensor values seem reasonable.. So does anyone here know what could be at fault here? Any help or ideas are appreciated. I have VCDS so I can provide some values and logging if needed. And the issue here is that I failed my emission tests. Thanks in advance for any input!!
04-09-2018, 03:55 PM
#3
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Did it fail smog test due to actual emissions readings, or did it fail due to readiness codes not reading correctly? If check engine light had been on and then codes cleared to turn it off shortly before a smog test, it is usually a problem with either the readiness codes not yet resetting, and/or the underlying issue still lingering that triggered the check engine light to ultimiately turn on in the first place.
Also look for simple vacuum leaks on the side with the faulty readings--MAF related clamp points, the suck valve hose connection point near the brake booster /master cylinder connection area if the problem is on that side, and so on.
Also look for simple vacuum leaks on the side with the faulty readings--MAF related clamp points, the suck valve hose connection point near the brake booster /master cylinder connection area if the problem is on that side, and so on.
04-10-2018, 12:57 AM
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Would a negative fuel trim rule out a vacuumleak?
or am i looking at it the wrong way.
negative fuel trim reduces fuel delevery from ecu
and a vacuum leak would increase feul delevery?
Or am i really wrong here.
or am i looking at it the wrong way.
negative fuel trim reduces fuel delevery from ecu
and a vacuum leak would increase feul delevery?
Or am i really wrong here.
04-10-2018, 05:39 AM
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Sorry for being little bit unclear. So I went to take a smog test and it failed due to slightly high CO% (0.36%, and 0.20% is allowed here). All other values were fine. The readiness codes are not tested here (AFAIK), but I noticed I have "Secondary air injection" showing as failed or incomplete on VCDS. However this should not impact warm engine if I have understood correctly? CEL hasn't been on at any point.
So to clarify the issue: I am getting too rich mixture on the other side even though the fuel trim is -15%.
I'm with Dvorack here that vacuum leaks should cause the fuel trim to be positive instead of negative, but I could definitely be wrong aswell.
In addition I started experiencing very rough idling today, so I will have to check DTCs agian with VCDS as it almost feels like one of the cylinders is not 100% in.
So to clarify the issue: I am getting too rich mixture on the other side even though the fuel trim is -15%.
I'm with Dvorack here that vacuum leaks should cause the fuel trim to be positive instead of negative, but I could definitely be wrong aswell.
In addition I started experiencing very rough idling today, so I will have to check DTCs agian with VCDS as it almost feels like one of the cylinders is not 100% in.
04-10-2018, 07:21 AM
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Back up again, and explain the situation yet more fully. Where are you located as a very basic starter? Posts remain vague on that unless I am missing something. References in turn to passenger and driver's side that may come up in due course will vary depending on country. Then, which side of the motor is it with the issues? Geography also important to understand smog test. In CA for example they don't even sniff exhaust any more--it's all readiness codes. Some other states allow some codes to read not ready and still pass. For other countries, I have no idea offhand without more context from poster.
Did you clear codes in either (or both) ECU's shortly before smog test? CEL on would be a reason of course, but then when folks start poking at issues with VCDS, my experience is they often clear the ECU(s) trying to remedy something. That in turn is key to understanding the SAI issue, since readiness code for SAI is typically the slowest to reset among the 8 of them after any ECU clear. Can take a couple of cold hot cold mixed driving cycles to complete. And if you didn't clear the ECU(s) for codes in general, then if SAI says not ready you have a definite issue there, whether related to fuel trim or not. Yes, SAI only works when cold, but the heart of the system are the air pumps AND the combi (combination) valve. Combi valve is in "combination" with EGR, which is hot side. Not that likely to affect mixture (though there is a short pipe and some gaskets involved)--but again to the point of my original reply, I would take a holistic view and look at all the elements affecting smog. SAI issues are among the common issues for many brands of vehicles as they age, Audi's and W12 included.
The observable running issue from latest reply to me says that you have a definite issue in some basic part or system. Rich mixture tends to hide ills, so to me less likely that is the true cause as it relates to rough running. Regardless of trim, one common set of scenarios is connected to vacuum leak issues. Could certainly be ignition too as you seem to be suspecting; you should be seeing ignition though by cylinder with VCDS. That would likely affect CO too. SAI on W12 doesn't use vacuum hoses to drive the combi valves, so one area where it probably doesn't overlap; on 4.2 they are vacuum driven so the two can further interplay. While at it, when was last tune up and plug change? Within last 40K miles/60K km?
Did you clear codes in either (or both) ECU's shortly before smog test? CEL on would be a reason of course, but then when folks start poking at issues with VCDS, my experience is they often clear the ECU(s) trying to remedy something. That in turn is key to understanding the SAI issue, since readiness code for SAI is typically the slowest to reset among the 8 of them after any ECU clear. Can take a couple of cold hot cold mixed driving cycles to complete. And if you didn't clear the ECU(s) for codes in general, then if SAI says not ready you have a definite issue there, whether related to fuel trim or not. Yes, SAI only works when cold, but the heart of the system are the air pumps AND the combi (combination) valve. Combi valve is in "combination" with EGR, which is hot side. Not that likely to affect mixture (though there is a short pipe and some gaskets involved)--but again to the point of my original reply, I would take a holistic view and look at all the elements affecting smog. SAI issues are among the common issues for many brands of vehicles as they age, Audi's and W12 included.
The observable running issue from latest reply to me says that you have a definite issue in some basic part or system. Rich mixture tends to hide ills, so to me less likely that is the true cause as it relates to rough running. Regardless of trim, one common set of scenarios is connected to vacuum leak issues. Could certainly be ignition too as you seem to be suspecting; you should be seeing ignition though by cylinder with VCDS. That would likely affect CO too. SAI on W12 doesn't use vacuum hoses to drive the combi valves, so one area where it probably doesn't overlap; on 4.2 they are vacuum driven so the two can further interplay. While at it, when was last tune up and plug change? Within last 40K miles/60K km?
Last edited by MP4.2+6.0; 04-10-2018 at 07:36 AM.
04-10-2018, 07:44 AM
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Found two trouble codes...
Bank 1: Camshaft B (Exhaust) P0014 - 008 - Retard Setpoint not Reached (Over-Advanced) - Intermittent
Bank 2: Camshaft B (Exhaust) P0024 - 008 - Retard Setpoint not Reached (Over-Advanced) - Intermittent
Does this mean I have a faulty camshaft adjuster? I don't see how this would cause the engine to inject way too much fuel though
Bank 1: Camshaft B (Exhaust) P0014 - 008 - Retard Setpoint not Reached (Over-Advanced) - Intermittent
Bank 2: Camshaft B (Exhaust) P0024 - 008 - Retard Setpoint not Reached (Over-Advanced) - Intermittent
Does this mean I have a faulty camshaft adjuster? I don't see how this would cause the engine to inject way too much fuel though
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04-10-2018, 07:53 AM
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Back up again, and explain the situation yet more fully. Where are you located as a very basic starter? Posts remain vague on that unless I am missing something. References in turn to passenger and driver's side that may come up in due course will vary depending on country. Then, which side of the motor is it with the issues? Geography also important to understand smog test. In CA for example they don't even sniff exhaust any more--it's all readiness codes. Some other states allow some codes to read not ready and still pass. For other countries, I have no idea offhand without more context from poster.
Did you clear codes in either (or both) ECU's shortly before smog test? CEL on would be a reason of course, but then when folks start poking at issues with VCDS, my experience is they often clear the ECU(s) trying to remedy something. That in turn is key to understanding the SAI issue, since readiness code for SAI is typically the slowest to reset among the 8 of them after any ECU clear. Can take a couple of cold hot cold mixed driving cycles to complete. And if you didn't clear the ECU(s) for codes in general, then if SAI says not ready you have a definite issue there, whether related to fuel trim or not. Yes, SAI only works when cold, but the heart of the system are the air pumps AND the combi (combination) valve. Combi valve is in "combination" with EGR, which is hot side. Not that likely to affect mixture (though there is a short pipe and some gaskets involved)--but again to the point of my original reply, I would take a holistic view and look at all the elements affecting smog. SAI issues are among the common issues for many brands of vehicles as they age, Audi's and W12 included.
The observable running issue from latest reply to me says that you have a definite issue in some basic part or system. Rich mixture tends to hide ills, so to me less likely that is the true cause as it relates to rough running. Regardless of trim, one common set of scenarios is connected to vacuum leak issues. Could certainly be ignition too as you seem to be suspecting; you should be seeing ignition though by cylinder with VCDS. That would likely affect CO too. SAI on W12 doesn't use vacuum hoses to drive the combi valves, so one area where it probably doesn't overlap; on 4.2 they are vacuum driven so the two can further interplay. While at it, when was last tune up and plug change? Within last 40K miles/60K km?
Did you clear codes in either (or both) ECU's shortly before smog test? CEL on would be a reason of course, but then when folks start poking at issues with VCDS, my experience is they often clear the ECU(s) trying to remedy something. That in turn is key to understanding the SAI issue, since readiness code for SAI is typically the slowest to reset among the 8 of them after any ECU clear. Can take a couple of cold hot cold mixed driving cycles to complete. And if you didn't clear the ECU(s) for codes in general, then if SAI says not ready you have a definite issue there, whether related to fuel trim or not. Yes, SAI only works when cold, but the heart of the system are the air pumps AND the combi (combination) valve. Combi valve is in "combination" with EGR, which is hot side. Not that likely to affect mixture (though there is a short pipe and some gaskets involved)--but again to the point of my original reply, I would take a holistic view and look at all the elements affecting smog. SAI issues are among the common issues for many brands of vehicles as they age, Audi's and W12 included.
The observable running issue from latest reply to me says that you have a definite issue in some basic part or system. Rich mixture tends to hide ills, so to me less likely that is the true cause as it relates to rough running. Regardless of trim, one common set of scenarios is connected to vacuum leak issues. Could certainly be ignition too as you seem to be suspecting; you should be seeing ignition though by cylinder with VCDS. That would likely affect CO too. SAI on W12 doesn't use vacuum hoses to drive the combi valves, so one area where it probably doesn't overlap; on 4.2 they are vacuum driven so the two can further interplay. While at it, when was last tune up and plug change? Within last 40K miles/60K km?
04-10-2018, 07:58 AM
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Apparently I can't edit my posts.. anyway, forgot to say that I'm not sure which side the problem is. My guess is that it is on the driver's side (left) since the fuel trim issue is on Engine 1.
04-10-2018, 08:32 AM
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Hmmm... More info and ideas are coming out.
A W12 has two ECU's as it sounds like you know. BUT, the motor is NOT just divided "in half"--one ECU owns all of some tasks, while some are spilt. You have to get into exactly which issue it may be to get into what ECU plays in. Meantime, you definitely want to know what side of the motor is generating the trim issues, since it gets at where to look for physical inspections. Also be sure you are looking for codes inside of ECU II (2). Sometimes folks--including me--just forget to look in there, and on generic scanners it can't even reach it. ECU II often puts a code into ECU I saying check ECU II if there is an issue, but if I recall correctly I have seen some codes just sit in ECU II that don't generate a check engine light (CEL).
Your cam codes are on (at least) the continental Europe/USA passenger side. Honestly I forget if the cam banks are divided in two or four. For those divided in four like the oxygen sensors, bank 1 and 2 are both on the passenger side. For banks divided only in two (like for the heads and cams?), bank 2 is then the driver's side. If the latter (I would need to look up but likely true since cams are whole side of motor), then you have issues on both sides. The usual failure point on a W12 is not the adjuster, but the cam position sensor. There are separate sensors for the exhaust and for the intake cams. On my 2006, I first lost the drivers side at about 100K miles and then the driver's side a year or so later. The exhaust ones are up above the very hot exhaust at extreme back of motor so seem to take a lot of heat. Painful to access, but it can be done w/ motor in car once intake comes off. For most cost effective approach, intake also comes off for plug or ignition coil changes, so jobs can be combined. Intake removal is surprisingly easy on W12, even for its apparent huge size--only weighs like 7kg--and way easier than V8 or many other larger motors. If/when you get to that, I got the sensors in OE Bosch rather than dealer, so they cost left than half the dealer price.
Smog tests aside, you want to fix cam system issues. The car goes into a protective mode when it sees cam issues so the motor integrity is protected. BUT, it is at the expense of both performance and fuel economy. I'm not sure without a bunch of research, but yes I think it could affect your readings. The exhaust cam timing could affect exhaust content since depending on when it opens combustion cycle may be more complete or less complete. It also affects cross cylinder scavenging, thought that will be very motor, exhaust and tuning specific. Net, just because of performance and fuel use, you want to get to bottom of it anyway. On running FWIW, you may feel a slight difference if cam system has gone to its safe mode, but typically nothing like a cylinder miss in terms of obviousness--other than maybe right at the moment it cuts to safe mode where engine shakes a bit as rest of fuel and ignition system recalibrate.
Also on the readiness codes, if the issues are persistent, even with a code clear a month ago, they will likely never reset until issue is fixed, or at best will reset until the issue comes up again if it is intermittent. That s my personal experience with SAI on W12, and on exhaust cam sensors, even with a code clear they would come back in a few weeks. At a deeper level, the cam system runs a self check in background from time to time. It occurs at low engine loads, fully warmed up, like just driving on a country road or expressway. Almost sort of random, but each time it happened I would get a code (and typically a CEL). Sometimes days, sometimes weeks. Only with sensor change was it resolved.
One other dumb corner case given uncertainly about tune up: If air filters are clogged badly, you could have issues I suppose too, though it would have be pretty extreme. I found even at 20K miles (half the change cycle), they would be obviously dirty. Won't hurt just to open top half of air boxes to inspect and blow out boxes and filters with an air nozzle. What I try to do mid tune up. The air intake on a W12 is virtually straight in from the grill and only about 12"/300mm long, so a lot of stuff gets to the filters, especially sucking 6 liters of air even with two boxes. SAI also pulls its air from the continental Europe/USA drivers side box--the hose at the extreme bottom inside of filter center so it gets filtered air. FWIW, my SAI issue was ultimately a bad pump, which likely started with a kinked drain hose in the bottom of that air box so it ate water. It has two pumps, so again pump side of SAI is a little trickier to diagnose than just listening for what is normally only one pump to turn on/off when car is first started cold.
BTW, if from Finland, see if volvopentaman checks in. Small world--he hails from Hanko Finland, is now with his sccond W12 and does deep dives on a variety of W12 issues. He is currently posting about half a dozen threads down on his W12 coolant and heater issues.
A W12 has two ECU's as it sounds like you know. BUT, the motor is NOT just divided "in half"--one ECU owns all of some tasks, while some are spilt. You have to get into exactly which issue it may be to get into what ECU plays in. Meantime, you definitely want to know what side of the motor is generating the trim issues, since it gets at where to look for physical inspections. Also be sure you are looking for codes inside of ECU II (2). Sometimes folks--including me--just forget to look in there, and on generic scanners it can't even reach it. ECU II often puts a code into ECU I saying check ECU II if there is an issue, but if I recall correctly I have seen some codes just sit in ECU II that don't generate a check engine light (CEL).
Your cam codes are on (at least) the continental Europe/USA passenger side. Honestly I forget if the cam banks are divided in two or four. For those divided in four like the oxygen sensors, bank 1 and 2 are both on the passenger side. For banks divided only in two (like for the heads and cams?), bank 2 is then the driver's side. If the latter (I would need to look up but likely true since cams are whole side of motor), then you have issues on both sides. The usual failure point on a W12 is not the adjuster, but the cam position sensor. There are separate sensors for the exhaust and for the intake cams. On my 2006, I first lost the drivers side at about 100K miles and then the driver's side a year or so later. The exhaust ones are up above the very hot exhaust at extreme back of motor so seem to take a lot of heat. Painful to access, but it can be done w/ motor in car once intake comes off. For most cost effective approach, intake also comes off for plug or ignition coil changes, so jobs can be combined. Intake removal is surprisingly easy on W12, even for its apparent huge size--only weighs like 7kg--and way easier than V8 or many other larger motors. If/when you get to that, I got the sensors in OE Bosch rather than dealer, so they cost left than half the dealer price.
Smog tests aside, you want to fix cam system issues. The car goes into a protective mode when it sees cam issues so the motor integrity is protected. BUT, it is at the expense of both performance and fuel economy. I'm not sure without a bunch of research, but yes I think it could affect your readings. The exhaust cam timing could affect exhaust content since depending on when it opens combustion cycle may be more complete or less complete. It also affects cross cylinder scavenging, thought that will be very motor, exhaust and tuning specific. Net, just because of performance and fuel use, you want to get to bottom of it anyway. On running FWIW, you may feel a slight difference if cam system has gone to its safe mode, but typically nothing like a cylinder miss in terms of obviousness--other than maybe right at the moment it cuts to safe mode where engine shakes a bit as rest of fuel and ignition system recalibrate.
Also on the readiness codes, if the issues are persistent, even with a code clear a month ago, they will likely never reset until issue is fixed, or at best will reset until the issue comes up again if it is intermittent. That s my personal experience with SAI on W12, and on exhaust cam sensors, even with a code clear they would come back in a few weeks. At a deeper level, the cam system runs a self check in background from time to time. It occurs at low engine loads, fully warmed up, like just driving on a country road or expressway. Almost sort of random, but each time it happened I would get a code (and typically a CEL). Sometimes days, sometimes weeks. Only with sensor change was it resolved.
One other dumb corner case given uncertainly about tune up: If air filters are clogged badly, you could have issues I suppose too, though it would have be pretty extreme. I found even at 20K miles (half the change cycle), they would be obviously dirty. Won't hurt just to open top half of air boxes to inspect and blow out boxes and filters with an air nozzle. What I try to do mid tune up. The air intake on a W12 is virtually straight in from the grill and only about 12"/300mm long, so a lot of stuff gets to the filters, especially sucking 6 liters of air even with two boxes. SAI also pulls its air from the continental Europe/USA drivers side box--the hose at the extreme bottom inside of filter center so it gets filtered air. FWIW, my SAI issue was ultimately a bad pump, which likely started with a kinked drain hose in the bottom of that air box so it ate water. It has two pumps, so again pump side of SAI is a little trickier to diagnose than just listening for what is normally only one pump to turn on/off when car is first started cold.
BTW, if from Finland, see if volvopentaman checks in. Small world--he hails from Hanko Finland, is now with his sccond W12 and does deep dives on a variety of W12 issues. He is currently posting about half a dozen threads down on his W12 coolant and heater issues.
Last edited by MP4.2+6.0; 04-10-2018 at 08:51 AM.