All Things Mk5 > How to Guides / Troubleshooting
Revs dropping/hunting at idle.
slash79:
--- 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 ---
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:
--- Quote from: slash79 on November 28, 2018, 03:29:31 pm ---
--- 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 ---
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
--- End quote ---
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
Sent from my iPhone using Tapatalk
slash79:
Cheers dude. I’ll have a look
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