Revs rising and falling when brake pressed

T

thecremeegg

Active member
North Hampshire
Hi guys,

Got a new issue with my 3.0i in that at idle, when pressing the brake pedal the revs rise and fall constantly until I let off the brake.
Is it a problem with the servo?

Thanks
 
Could well be an air leak somewhere in the vaccum lines across the engine, a smoke test would find them if you can't find them by a visual check. Common leaks can be at the back edge of the inlet manifold, there are rubber blanking caps situated there and they rot out and fall off allowing unmetered air to get in that cause running issues. Dip stick O rings can do the same if you have a dip stick. Most common leaks are splits in the intake bellows after the Maf, usually the small take off branch that leads to the idle control valve. Had one car that developed an air leak from the sucking jet valve sited just below the idle control valve on a 2.2 Z3, that was found using a smoke test. Same car also had a leak from the plastic cam cover around No4 plug hole, the plastic had gone hard and brittle and was craked and starting to crumble apart. Replacement cam cover solved that one. If no leaks are found then turn your attention to the Maf, only replace with oem Bosch or Siemens brands do not use cheap ebay Maf's they dont work.
 
Any holes or cracks in those vaccum hoses will be the cause of your fluctuating idle, it will also be affecting your fuel trims which it will be seeing as a lean condition at the exhaust sensors due to the unmetered air being picked up in the exhaust as the extra air has not been through tye Maf sensor so not measured. To correct what it sees as a mismatch it will increase the fuel at the injectors to richen the mixture up in effort to correct what it sees as a lean mixture. The engine management will keep trying to correct it until it gets to be about 20% adding more fuel, if that dosn't correct it it will eventually give up and set the EML on the dash. Code reading will reveal error codes pertaining to the overfueling issue. You mpg will also fall dramatically with air leaks due to the overfueling its doing. Replacing any duff rubber hoses or plastic pipes should sort your issues.
 
Some really good input from colb there. I'm going to check mine as I've noticed a definite drop in mpg this year. No other issues though so I assumed it was just my heavy right foot :driving:
 
colb said:
Any holes or cracks in those vaccum hoses will be the cause of your fluctuating idle, it will also be affecting your fuel trims which it will be seeing as a lean condition at the exhaust sensors due to the unmetered air being picked up in the exhaust as the extra air has not been through tye Maf sensor so not measured. To correct what it sees as a mismatch it will increase the fuel at the injectors to richen the mixture up in effort to correct what it sees as a lean mixture. The engine management will keep trying to correct it until it gets to be about 20% adding more fuel, if that dosn't correct it it will eventually give up and set the EML on the dash. Code reading will reveal error codes pertaining to the overfueling issue. You mpg will also fall dramatically with air leaks due to the overfueling its doing. Replacing any duff rubber hoses or plastic pipes should sort your issues.
Click to expand...

Very interesting, thanks!
 
As an update, replaced the split intake pipe, no change! I then changed the MAF and it's now fixed.
It actually seems to have gained some power at the higher end of the rev range so that's a bonus! Thanks all!
 
Sounds like you had a combination of faults splits and holes in any of the vaccum pipes will affect the fuel trims and a bad Maf will also confuse the fueling if its misreporting whats passing through it. Mafs don't always set codes and can be difficult to diagnose, a sympton of a bad Maf will be the feeling that the engine is being held back, had this with my 1.9, cleaning the Maf killed it so new oem Maf obtained and puit on, cure was instant, car back to full power. Do not be tempted by cheap ebay Maf's they don't always work or last long.
 
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