High fuel trim adaptions, lumpy idle. Not a vac leak...

[shalmaneser]

Hi All,

Having trouble with the above on my '00 Porsche 911. Values up to about +8/+7 on each bank.

Motor is lumpy at idle, and a bit reluctant to rev. This is not to say that it's really knackered, it just doesn't have the zing that other cars do. Still feels plenty quick enough, but missing that last 10%. Passed emissions tests just fine a few months ago. Uses no oil.

I've checked for induction air leaks and found one which I fixed with a replacement O-ring. Now the induction system will hold ~20psi when I use a smoke tester on it so I am confident that there is no air leak.

I replaced the fuel pump with a used good one which has made no difference.

Gasket between manifold and cats has been replaced (what a mission that was...) to ensure a exhaust leak isn't causing the o2 sensors to report a lean condition.

Spark plugs and coils are all new. Air filter is new. Previous owner has fitted a new fuel filter which would be my next port of call - spark plugs and coils were also reported as changed by the last garage but were clearly not when inspected - hence I changed these.

The strange thing is when the battery is disconnected (as I just did to change the fuel pump) and the adaptions are reset to 0 the car feels much smoother and more willing. This suggests to me that there is a sensor fault rather than a mechanical issue as then while being driven over the next few hours reverts back to the poor running condition and high adaption values.

I noticed the previous owner fitted a new MAF - could this be a poor copy? The garage which fitted it was also the same place that failed to replace the spark plugs....

Thinking about cleaning the injectors next.

Any suggestions?

[E-bmw]

I would be trying a known good MAF if at all possible as they can cause what you say & many vehicles (BMW are well known for it) do not like non oe copies.

Can you see if it is a Bosch or similar?

Do you have a code scanner that can read live data, if so the air flow should be in there somewhere.

[Huff]

Agree with post above.

If it is a bosch MAF on many BMWS the first thing to just try is unplugging it and starting the car. If it's smoother (without resetting adaptation)- that has to go near the top of the list. you can drive around like this lightly, - the ecu just defaults to a nominal value for airflow.

(I had ta similar experience on my Alpina a few years ago. Owner had a receipt for a new MAF - the one actually fitted just had a Maplin 1p resistor inside it, no hot film element at all! The correct siemens/VDO unit (£160 on sale) cured it all.

Edited by Huff on Monday 16th November 16:42

[shalmaneser]

It's funny that sometimes writing these things out can sort of help lead to to the problem. The fact the car runs worse over time following a reset does rather suggest it's a sensor failure.

I'm going to go and have a good look at the sensor. Finding a known good one might be a bit tricky though, needs to be the right car and nearby too!

EDIT: I've actually been logging data using my bluetooth scan tool and the car scanner app - highly recommended. Airflow as far as I can see is being well reported but it only needs to be a bit off to cause the issues I've been experiencing:



This is the log straight after fitting the replacement fuel pump. You can see the adaption values climbing over a couple of hours worth of driving. They do tend to rise as the airflow increases and drop as the airflow decreases (see - the 15 minutes stuck in traffic) which lead me to a low fuel pressure diagnosis initially. But if the sensor is proportionally under reporting it might cause behavior like this.

Edited by shalmaneser on Monday 16th November 17:12

Edited by shalmaneser on Monday 16th November 17:58

[GreenV8S]

Can you borrow a WBO2 kit to validate the O2 readings the ECU is seeing? As long as the exhaust doesn't leak you can use a tail pipe sensor.

[shalmaneser]

I'm pretty confident about the o2 sensors. I've had to replace one that went bad a while ago on one bank and they both give very similar readings. The ECU logging speeds aren't really good enough to be able to make head or tail of the actual output.

[stevieturbo]

And presumably this hasn't happened overnight ? Some background ? Has it ever ran right ? And when did it last run correctly ?

You mention an emissions tests months ago....well..that was months ago.

Can you get another ?

The MAF, do you suspect it is a cheap copy or something ? This item is critical. You could scope it's output voltage, but without a known good to compare against...even very small voltage differences can make a huge difference.

What do the plugs look like ? Fuel pressure ?

[shalmaneser]

stevieturbo said:

And presumably this hasn't happened overnight ? Some background ? Has it ever ran right ? And when did it last run correctly ?

You mention an emissions tests months ago....well..that was months ago.

Can you get another ?

The MAF, do you suspect it is a cheap copy or something ? This item is critical. You could scope it's output voltage, but without a known good to compare against...even very small voltage differences can make a huge difference.

What do the plugs look like ? Fuel pressure ?

All good questions.

I've owned the car for a year, it's never been quite right. I should make clear that it's not running really roughly, just a bit lumpy and lacking a little bit of pep.

They're fairly notorious for knackering coil packs so when I bought it I assumed swapping these our and a bit of a tune up would sort things out. It's low miles but been a bit unloved recently. Spark plugs were a good colour if a bit worn:



Fitted new plugs and coils all round. Not much change.

Then I started searching for air leaks. Made a smoke machine and found a significant leak caused by a missing O-ring on the brake booster vacuum take off. This stabilised the idle which was hunting up and down a lot, but still didn't feel 100%.

Thus lead me to suspect the fuel pump, low fuel pressure would explain lack of go at higher revs. Given these seem to be very reliable I risked a second hand one which has made no difference at all. However the car was running a lot better after disconnecting the battery (hence clearing the adaptions) then reconnecting as discussed above.

I've just been for a quick spin with the maf completely disconnected - not something I've tried before - and I'd say that the engine definitely felt better. It was popping and gurgling a bit more though which would indicate the engine running rich (I think - is this right?) which would indicate that the current adaption of 4.7 on both banks is over fuelling?

The maf was changed by the previous owner a year ago when the air-oil separator was changed by clearly a slightly dodgy garage - those plugs should have been 3montys old and the coils were clearly not new. I've checked the part number which is correct and it certainly looks genuine but the fact that the car runs better with it disconnected does rather point to it being faulty somehow. I'd rather not spend £140 on a hunch that it's somehow bad, though.

[nsa]

shalmaneser said:

Having trouble with the above on my '00 Porsche 911. Values up to about +8/+7 on each bank.

I assume 8/7 refers to camshaft deviation readings. Those are quite high. I think at 9 you should get a CEL. Have you checked the oil pan for plastic, to see if the cam chain guides are worn?

[shalmaneser]

nsa said:

shalmaneser said:

Having trouble with the above on my '00 Porsche 911. Values up to about +8/+7 on each bank.

I assume 8/7 refers to camshaft deviation readings. Those are quite high. I think at 9 you should get a CEL. Have you checked the oil pan for plastic, to see if the cam chain guides are worn?

No I'm referring to the fuel trim. Cam deviation will be getting attended to during the next oil change, though. Quite a big job, not looking forward to that one...

[stevieturbo]

nevermind fuel trims....are the o2 sensors suggesting rich or lean when driving ? They may just be narrowbands, but can still give useful info.
And presumably there are sensors for both banks ?

Why waste time changing fuel pumps without actually testing fuel pressure at the rails ?

Plugs....are they 100% the correct plugs ? Must admit, I hate multi-pronged plugs ! Even more so Bosch lol
Although if they are the absolute correct plug from Porsche....I'd accept that. Unless many recommend a more sensible normal type plug from NGK or Denso.

MAF certainly looks like a proper part...is it installed correctly and in the standard housing and all pipework etc ?
Whether it's faulty or not, harder to say, and unplugging it doesn't really give a definitive notion as to whether it's faulty or not.
But if it does make it run rich, it will be very obvious on the o2 sensors from before.

[shalmaneser]

nevermind fuel trims....are the o2 sensors suggesting rich or lean when driving ? They may just be narrowbands, but can still give useful info.
And presumably there are sensors for both banks ?

- Will log the o2 sensors when driving.

Why waste time changing fuel pumps without actually testing fuel pressure at the rails?

- I did buy a fuel pressure sensor which showed marginally low fuel pressure but I'm not sure I believe it frankly. It was a cheap one from Ebay. Buying a decent one cost the same as a decent second hand pump so I thought I'd just risk it. No change as discussed. Will see if I can get this confirmed by someone with a good gauge!

Plugs....are they 100% the correct plugs ? Must admit, I hate multi-pronged plugs ! Even more so Bosch lol
Although if they are the absolute correct plug from Porsche....I'd accept that. Unless many recommend a more sensible normal type plug from NGK or Denso.

- Porsche specified plugs (14FGR 6KQU) and generally recommended to be the ones to use. I don't like multi prong ones either but here we are!

MAF certainly looks like a proper part...is it installed correctly and in the standard housing and all pipework etc ?

- Yep totally standard engine apart from stainless back boxes.

Whether it's faulty or not, harder to say, and unplugging it doesn't really give a definitive notion as to whether it's faulty or not.
But if it does make it run rich, it will be very obvious on the o2 sensors from before.

- I might try logging the o2 sensors then resetting the adaptions and seeing where it leads me.

[shalmaneser]

I gave in to temptation and ordered a new Bosch AFM.

It's so easy to change and thus far I've not spent much money on chasing this problem. At an LTFT of 4.6 for both banks the car was running very nicely, if a little on the rich side going from the o2 voltages which trended higher. It was good to see both sensors reacting similarly though, they were pretty much in sync which indicates that they're both seeing the same thing and also both reacting at the same time.

I reset the adaptions to 0 and ran the car without the AFM and the o2 voltages were noticeably lower indicating the car was running lean. More shuddering at idle, too. I might reconnect the AFM to get the adaptions to ~2 and see how that looks on the o2 sensors. It's a 20 year old car so I would expect the car to need to overfuel slightly as the injectors are no doubt a bit dirty.

What's noticable is how much more willing to rev the engine is and how much smoother at idle it is with the AFM disconnected. Should have tried this trick sooner, but I was to focussed on other issues and I knew the AFM had just been replaced so wrote it off as a potential failure point.

Interesting to note that the ECU so far is completely happy, no lights on the dash after disconnecting the AFM! Quite surprised by that but it is very basic - a Bosch Motronic 5.2.2 I believe which is pretty old school.

[Darkslider]

Are you 100% confident in your DIY test to rule out any more air leaks? A lot of MAFs are replaced because they fail the 'unplug it' test, where in actuality unplugging the MAF results in the pcm utilising MAP sensor readings instead, creating a richer running condition for safety and bringing all the fuel trims back into line. This gives the illusion that the MAF must be at fault but it's still an air leak somewhere.

On the face of it the smoke and pressure test seems conclusive but have you double checked its output and calibrated the pressure gauge even roughly? Tried spraying carb cleaner at all the pipework and gaskets you can just as a secondary test?

Faulty MAFs even from genuine manufacturers aren't unheard of though so if you're confident you've no more leaks I'll keep my fingers crossed the new one fixes the problem!

[shalmaneser]

Darkslider said:

Are you 100% confident in your DIY test to rule out any more air leaks? A lot of MAFs are replaced because they fail the 'unplug it' test, where in actuality unplugging the MAF results in the pcm utilising MAP sensor readings instead, creating a richer running condition for safety and bringing all the fuel trims back into line. This gives the illusion that the MAF must be at fault but it's still an air leak somewhere.

On the face of it the smoke and pressure test seems conclusive but have you double checked its output and calibrated the pressure gauge even roughly? Tried spraying carb cleaner at all the pipework and gaskets you can just as a secondary test?

Faulty MAFs even from genuine manufacturers aren't unheard of though so if you're confident you've no more leaks I'll keep my fingers crossed the new one fixes the problem!

One of the issues with the position of the engine is that almost all the vacuum connections are very very well hidden.

I've used a home-made smoke tester connected to a track pump which pressurised the intake assembly pretty effectively, so much so that undoing the oil fill cap (which is attached to the intake via various means) gave an audible 'whump' noise. I'll retest though, as it's fairly easy to do, just need to buy some more baby oil for the smoke machine.

The problem with an air leak though is that it doesn't explain the lack of top end, generally air leaks affect the idle much more as the throttle is closed so proportionally more air is getting in through the leak and hence screwing up the fuelling. Clearly this is less of an issue when the throttle is open.

EDIT: According to the service manual the airflow at idle (680rpm) should be 17kg/hr +/- 2.5. I'm reporting around 21kg/hr which does seem on the high side.

Edited by shalmaneser on Tuesday 17th November 17:20

[stevieturbo]

shalmaneser said:

I gave in to temptation and ordered a new Bosch AFM.

It's so easy to change and thus far I've not spent much money on chasing this problem. At an LTFT of 4.6 for both banks the car was running very nicely, if a little on the rich side going from the o2 voltages which trended higher. It was good to see both sensors reacting similarly though, they were pretty much in sync which indicates that they're both seeing the same thing and also both reacting at the same time.

I reset the adaptions to 0 and ran the car without the AFM and the o2 voltages were noticeably lower indicating the car was running lean. More shuddering at idle, too. I might reconnect the AFM to get the adaptions to ~2 and see how that looks on the o2 sensors. It's a 20 year old car so I would expect the car to need to overfuel slightly as the injectors are no doubt a bit dirty.

What's noticable is how much more willing to rev the engine is and how much smoother at idle it is with the AFM disconnected. Should have tried this trick sooner, but I was to focussed on other issues and I knew the AFM had just been replaced so wrote it off as a potential failure point.

Interesting to note that the ECU so far is completely happy, no lights on the dash after disconnecting the AFM! Quite surprised by that but it is very basic - a Bosch Motronic 5.2.2 I believe which is pretty old school.

When you're viewing o2 data....is it in closed loop or open ? If closed, unless it has very fast logging, what you're seeing in terms of voltages wont be much use ( assuming the o2's are switching correctly and CL is operating correctly )
If open loop you should get a good view at the raw voltages which will be of more use.

Certainly with the MAF unplugged, I would expect it to be open loop because of a fault.

And you're saying there is no CEL with the MAF unplugged...that is odd. IS there a working CEL at all ?

And no, there is no reason at all for it to run rich, most of the time because of the o2 sensors when all is well, it should run at stioch...that's their job. That's what the fuel trims will be trying to do, assuming the o2's are reading correctly, and working correctly so the ecu can make changes.

[LarJammer]

What does the air pressure reading show? An odd reading would indicate a faulty ECU ( surprisingly common). Also worth noting that the early MAFs had a different calibration. A software update is available for the ECU to suit replacement MAF voltages (a decent specialist should be able to do this).
A couple of other ideas - try removing the oil cap at idle. If the engine stumbles / runs rough, its airtight. If there is no change you probably still have an air leak or faulty AOS. And check the cam deviation reading (only on bank 1 on an early car). Closer to zero the better, if its nearer to 10 then stop trying to fix the problem... you need a rebuild.

[shalmaneser]

How do I read the air pressure signal? I know there is a pressure sensor on the ECU for altitude adjustment but I don't know how to access it.

Cam deviation is 5 degrees, not bad but not great hence why it's on the list at the next oil service. Needs the exhaust manifolds to be dropped to repair though which is a mission in itself and will be done in the next 6 months or so.

Removing the oil cap makes a big difference to the way the car runs.

stevieturbo said:

When you're viewing o2 data....is it in closed loop or open ? If closed, unless it has very fast logging, what you're seeing in terms of voltages wont be much use ( assuming the o2's are switching correctly and CL is operating correctly )
If open loop you should get a good view at the raw voltages which will be of more use.

Certainly with the MAF unplugged, I would expect it to be open loop because of a fault.

And you're saying there is no CEL with the MAF unplugged...that is odd. IS there a working CEL at all ?

And no, there is no reason at all for it to run rich, most of the time because of the o2 sensors when all is well, it should run at stioch...that's their job. That's what the fuel trims will be trying to do, assuming the o2's are reading correctly, and working correctly so the ecu can make changes.

I'll check whether the car is open loop or not with the MAF unplugged, I suspect it's open as you say. Voltages correlated closely with the performance of the car - dropped to zero when off the throttle, aligned with eath other when on the throttle or idling.

Edited by shalmaneser on Wednesday 18th November 09:27

[stevieturbo]

You should be able to read the raw voltages though.

narrowbands output is very non linear, but should still be usable to get some sort of indication of fueling and also whether the sensors are working and responding quickly etc. And it is good that both banks are behaving the same.

[shalmaneser]

Just to draw a quick line under this - it was the AFM.

I really don't know how, but somehow the existing new one was either bad/fake or because faulty. It looks genuine but I guess that's the point of a fake...

Engine is feeling better than ever. With the issue it dyno'd at 260bhp, will take it back to the same place (Surrey Rolling Road) and see how many horses I've released! Will be interesting to see.

One thing I did read was about stiffer engine mounts causing the knock sensors to think there was knocking and pulling timing. I have 'RS' engine mounts - so not ridiculous ones - and am wondering if that, along with a resonator bung (now removed) which is a common mod to increase the induction noise could have together been creating a strange frequency which caused the issue. It certainly felt like the engine was pulling timing somehow. I did run the old AFM without the bung for a long time though.

If I'm sufficiently motivated I'll pop the old AFM back without the resonator bung and see what happens. It's such a common mod that it can't affect all cars, as are the RS engine mounts, but maybe the balance of my engine and these bits worked out wrong?

Will continue investigations if the mood takes me.