Revs dropping/hunting at idle.

[slash79]

Morning guys.
Would anyone be able to point me in the direction of an annoying fault with my car.
It’s a 2006 dsg by the way. Sometimes, well more than half the time, when sitting in traffic, the revs will sit where they are supposed to on idle, then they’ll start hunting. They drop and go back up, until I blip the throttle or move off again.
The car has a Forge PCV blanking plate and large Ram Air induction kit.
I read on the internet that they’re the symptoms of a busted PCV, but I don’t have one lol
Any help greatly appreciated
Thanks
Alex

[suf]

I've had similar before and they've always been a air/boost leak - usually pinpointed out using a smoke test or if you have a good look through your bay, try and spot out any torn/cracked plumbing.

[slash79]

Thanks mate. I’ll have a look. Seems like the proverbial needle in a haystack lol

[slash79]

So got back from a 70 mile round trip earlier, pulled up on my driveway and let the car idle for a little and it was really dropping and then rose to 1000rpn and stayed there, so I read somewhere to unplug the maf and if the revs go back to normal, it’s that..... well the revs dropped to the normal 700rpm ish and sat there perfectly, so I’m guessing my issue is this!
Question is, I have the large RamAir induction kit (fitted by the previous owner) so is the maf sensor the little box on the intake pipe on the top, with the cable plugged into it? Cos that’s what I unplugged so I’m guessing it is.
Is this removable and cleanable? Is there something else I can do to sort this?
Cheers folks

[Dan_FR]

It doesn't mean its the MAF at all... it shows that you have an air leak that needs resolving, anywhere between the MAF and the throttle body, which includes all of the PCV system and includes the cam cover and the chambers within. Find the leak

[pudding]

Does the climate control work OK?  A dying/knackered AC pump can put a lot of load on the engine at idle, and since the engine only makes about 10hp at idle, it will slow it right down.

It could still be the MAF but measuring the values with a scan tool first is wise before throwing £140 at a new one.   In fact a scan tool to check the long term trims is a good idea anyway.  There's a few conditions were you won't get any trims (Partial throttle % locked to 0.0), such as faulty carbon can purge valve, bad MAF, cam sensor bad etc etc.

[reidy-]

I had a very similar problem with random rev hunting and eventually getting to a level causing a CEL light for running too lean, and in short it was a total nightmare to isolate and resolve.


A method to prompt my symptoms was that if I pumped the brake pedal heavily and repeatedly the revs rise (it should only ever bog down) indicating a vac leak & faulty 1 way valve in the vacuum system

Odds are you need a new vacuum hose as these degrade over time and sit next to a very hot HPFP

https://www.ebay.co.uk/itm/New-VAI-Brake-Vacuum-Hose-V10-3631-Top-German-Quality-/202140395482

- make sure everything is very clean when fitted as any dirt can break the vac seal relatively easily (as I found out first hand and makes trouble shooting even more of a nightmare)


Note other options as said it could be the MAF, but unplugging the MAF and the revs dropping back to normal doesn't necessarily mean the MAF is at fault - the car just defaults to a 'safe' mode which may mask the original problem but point you in the direction of it being a faulty MAF as it seems to eliminate the problem.

Good luck!

[muff1991]

I had a very similar problem with random rev hunting and eventually getting to a level causing a CEL light for running too lean, and in short it was a total nightmare to isolate and resolve.


A method to prompt my symptoms was that if I pumped the brake pedal heavily and repeatedly the revs rise (it should only ever bog down) indicating a vac leak & faulty 1 way valve in the vacuum system

Odds are you need a new vacuum hose as these degrade over time and sit next to a very hot HPFP

https://www.ebay.co.uk/itm/New-VAI-Brake-Vacuum-Hose-V10-3631-Top-German-Quality-/202140395482

- make sure everything is very clean when fitted as any dirt can break the vac seal relatively easily (as I found out first hand and makes trouble shooting even more of a nightmare)


Note other options as said it could be the MAF, but unplugging the MAF and the revs dropping back to normal doesn't necessarily mean the MAF is at fault - the car just defaults to a 'safe' mode which may mask the original problem but point you in the direction of it being a faulty MAF as it seems to eliminate the problem.

Good luck!

Hi Reidy, is the 1 way valve you talk about... part of those Vac lines in the link? cheers.

[pudding]

Yep it's the plastic part, top left with the stubby rubber hose attached.  Just remove yours and blow through it.  Air will only pass one way if it's working correctly.

Have you got a scan tool, or VCDS?

You need to do some proper diagnosis instead of throwing parts at it.  Check your MAF numbers (should see around 3 g/s at hot idle), check your short and long term fuel trims, check the up and down stream lambda sensors and see if the numbers change when squirting brake cleaner into the intake.   Gotta get your hands dirty fella.

[muff1991]

Yep it's the plastic part, top left with the stubby rubber hose attached.  Just remove yours and blow through it.  Air will only pass one way if it's working correctly.

Have you got a scan tool, or VCDS?

You need to do some proper diagnosis instead of throwing parts at it.  Check your MAF numbers (should see around 3 g/s at hot idle), check your short and long term fuel trims, check the up and down stream lambda sensors and see if the numbers change when squirting brake cleaner into the intake.   Gotta get your hands dirty fella.

Thanks for confirming valve. As for OP problem... go with puddings answer


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[slash79]

I had a very similar problem with random rev hunting and eventually getting to a level causing a CEL light for running too lean, and in short it was a total nightmare to isolate and resolve.
A method to prompt my symptoms was that if I pumped the brake pedal heavily and repeatedly the revs rise (it should only ever bog down) indicating a vac leak & faulty 1 way valve in the vacuum system

Odds are you need a new vacuum hose as these degrade over time and sit next to a very hot HPFP

https://www.ebay.co.uk/itm/New-VAI-Brake-Vacuum-Hose-V10-3631-Top-German-Quality-/202140395482

- make sure everything is very clean when fitted as any dirt can break the vac seal relatively easily (as I found out first hand and makes trouble shooting even more of a nightmare)


Note other options as said it could be the MAF, but unplugging the MAF and the revs dropping back to normal doesn't necessarily mean the MAF is at fault - the car just defaults to a 'safe' mode which may mask the original problem but point you in the direction of it being a faulty MAF as it seems to eliminate the problem.

Good luck!

Thanks mate. That hose you’ve posted on the eBay link, where abouts is that situated in the engine? Can’t seem to find where abouts it goes on any diagrams.
Thanks

[muff1991]

I had a very similar problem with random rev hunting and eventually getting to a level causing a CEL light for running too lean, and in short it was a total nightmare to isolate and resolve.
A method to prompt my symptoms was that if I pumped the brake pedal heavily and repeatedly the revs rise (it should only ever bog down) indicating a vac leak & faulty 1 way valve in the vacuum system

Odds are you need a new vacuum hose as these degrade over time and sit next to a very hot HPFP

https://www.ebay.co.uk/itm/New-VAI-Brake-Vacuum-Hose-V10-3631-Top-German-Quality-/202140395482

- make sure everything is very clean when fitted as any dirt can break the vac seal relatively easily (as I found out first hand and makes trouble shooting even more of a nightmare)


Note other options as said it could be the MAF, but unplugging the MAF and the revs dropping back to normal doesn't necessarily mean the MAF is at fault - the car just defaults to a 'safe' mode which may mask the original problem but point you in the direction of it being a faulty MAF as it seems to eliminate the problem.

Good luck!

Thanks mate. That hose you’ve posted on the eBay link, where abouts is that situated in the engine? Can’t seem to find where abouts it goes on any diagrams.
Thanks

If you look at the inlet manifold... on the right hand side there’s a 2-3inch piece of rubber hose, follow that and you’ll see the whole lot in the link


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[slash79]

Cheers dude. I’ll have a look