All Things Mk5 > How to Guides / Troubleshooting
Revs dropping/hunting at idle.
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:
--- Quote from: reidy- on November 19, 2018, 08:49:07 pm ---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!
--- End quote ---
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:
--- Quote from: Pudding on November 23, 2018, 11:25:14 am ---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.
--- End quote ---
Thanks for confirming valve. As for OP problem... go with puddings answer
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