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Took the car for a drive today and it's developed a bit of a weird problem . When driving and the clutch is depressed ( for example coming to a junction or traffic lights ) and the car is still moving, the revs drop to about 500rpm and jump about a bit before settling or on some occasions the car will cut out. When stationary the car idles with no issues . Still appears to drive fine .....problem is only when car is moving and no gear selected. Few days ago I removed the o/s auxiliary radiator , removed the flap in the airbox and installed a cold air feed. Plugged the vacuum line using a screw that is tight in the hose. Obviously I had the airbox/ maf off whilst doing the work so wonder if I've done something then. No warning lights on and no fault codes showing ( have checked with Vag Com). Thinking possibly faulty maf and just coincidence it's happened now? Any suggestions greatly appreciated.

As you said, I think the MAF is the first thing to look at.
Check connections etc.
Mike

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I had a similar issue a few years back.

viewtopic.php?f=35&t=109770&p=672269&hilit=Maf#p672269

In the end I had a full carbon clean but with hindsight it could have been a leak in the vacuum system.

I would replace the MAF sensor first as this is the cheapest option.

coffey555 wrote: ↑

Sun Oct 20, 2019 12:05 pm

I had a similar issue a few years back.

viewtopic.php?f=35&t=109770&p=672269&hilit=Maf#p672269

In the end I had a full carbon clean but with hindsight it could have been a leak in the vacuum system.

I would replace the MAF sensor first as this is the cheapest option.

Cheers will try maf first. Has done 128k miles so similar mileage to yours when you had the issues . It's also going on a rolling road in two weeks to see what power it's at just now. No history of it ever having had a carbon clean .

Does your cruise control still work?

It might just be the new intake

Vacuum is highest at idle. Therefore when you dip the clutch it could be unmetered air getting in somewhere.

Check your fuel trims in block 032 on engine measuring blocks. Also in the second engine ecu by going to module 11 first. See if there is a significant value in the idle readings. Make sure you have not recently cleared all fault codes as that resets these values and so give incorrect readings. They take time to adapt.

I would not waste money on a maf until you do this personally. If you do buy one, check out my thread for the Bosch part number and get it from opie oils cheap.

One last thing to try. Switch off ac completely and drive it. See if it still does it. I had a similar issue and it doesn’t happen with ac on, implying it could be a ac /ac clutch issue.


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ArthurPE wrote: ↑

Sun Oct 20, 2019 10:36 pm

Does your cruise control still work?

It might just be the new intake

Cruise control works fine. Checked everything I had apart and it's all secure and no obvious signs of where there would be a leak.

JamesBaby wrote: ↑

Mon Oct 21, 2019 7:21 am

Vacuum is highest at idle. Therefore when you dip the clutch it could be unmetered air getting in somewhere.

Check your fuel trims in block 032 on engine measuring blocks. Also in the second engine ecu by going to module 11 first. See if there is a significant value in the idle readings. Make sure you have not recently cleared all fault codes as that resets these values and so give incorrect readings. They take time to adapt.

I would not waste money on a maf until you do this personally. If you do buy one, check out my thread for the Bosch part number and get it from opie oils cheap.

One last thing to try. Switch off ac completely and drive it. See if it still does it. I had a similar issue and it doesn’t happen with ac on, implying it could be a ac /ac clutch issue.


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Tried with air con off and no difference. Lambda value on engine 1 are -1.5% & -2.0%. Engine 2 showing -2.0% & -0.8% . Also measured airflow which peaked at 141g/s.

If you had a vac leak unmetered air would be causing a lean condition resulting in positive fuel trims. You are opposite.

Could be a gradually failing maf. For engine safety they can cause a rich running when they start to fail. Safer than a lean condition.

Only other thing I can think is coolant temperature sensor if the cutting out is mainly when engine is hot.


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JamesBaby wrote: ↑

Mon Oct 21, 2019 8:00 pm

If you had a vac leak unmetered air would be causing a lean condition resulting in positive fuel trims. You are opposite.

Could be a gradually failing maf. For engine safety they can cause a rich running when they start to fail. Safer than a lean condition.

Only other thing I can think is coolant temperature sensor if the cutting out is mainly when engine is hot.


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Cheers. Noticed today that it smelt rich as well. Think I'll replace the maf and take it from there. Emailed opie as they don't list it on their site .

The reason I asked is that a bad clutch switch has similar symptoms. I would put the oem box back on and see if that clears it up before springing for a new MAF.
Did you clean it when you had it out?

The 140 g/sec seems low
Should be closer to 180 (per bank)

1/2 bank x (8000 rpm/2)/60 sec/min x 4.2 liter x 1.25 g/liter (depending on temp) x 1.05 vol eff = 185 g/sec

If you went to 6500 rpm then 6500/8000 x 185 ~ 150 g/sec

ArthurPE wrote: ↑

Mon Oct 21, 2019 10:02 pm

The reason I asked is that a bad clutch switch has similar symptoms. I would put the oem box back on and see if that clears it up before springing for a new MAF.
Did you clean it when you had it out?

It has the oem airbox fitted . Has a k&n panel filter which I cleaned whilst it was removed but it's ran this for a few years . All I've done is remove the flap in the airbox and auxiliary rad. Idle has been slighlty lumpy before this but hadn't noticed if it did the cutting out as I usually don't put it into neutral unless car is stopped . Replaced the clutch switch about 5 months ago as the cruise control stopped working then.

Maf value was at 7800rpm in 3rd gear . Going to do some more runs tomorrow and see what readings I get. Searched on here and seems to be about 155-160 on a standard car . Car has done 128k but not been carbon cleaned as far as I know . 180g/s not high for a standard car as that would work out as roughly 450bhp?

neil_f wrote: ↑

Mon Oct 21, 2019 10:17 pm

ArthurPE wrote: ↑

Mon Oct 21, 2019 10:02 pm

The reason I asked is that a bad clutch switch has similar symptoms. I would put the oem box back on and see if that clears it up before springing for a new MAF.
Did you clean it when you had it out?

It has the oem airbox fitted . Has a k&n panel filter which I cleaned whilst it was removed but it's ran this for a few years . All I've done is remove the flap in the airbox and auxiliary rad. Idle has been slighlty lumpy before this but hadn't noticed if it did the cutting out as I usually don't put it into neutral unless car is stopped . Replaced the clutch switch about 5 months ago as the cruise control stopped working then.

Gotcha
Thanks

I meant clean the MAF

Does it run smooth under load at higher rpm?
When the flap is supposed the be open

What rpm did you measure 140 g/sec?

Beat me to it lol
140 g/sec at 7800 seems low
Take readings at intervals
3000, 4000, 5000, so on (20-22 g/sec per 1000 rpm)
If linear sensor is likely ok