High idle.

As above my engine is idling at 1600rpm.
Engine is an aum in my mk3.5 cabrio.
Has no fault codes. It’s had new coils and plugs all new pcv pipes and valve, new rocker gasket and a decat exhaust.
fuel trims say 0% and -2.3%
When maf is unplugged it stays the same.
Not sure what could be causing this issue. Any ideas?
TIA

 

Air leak? Brake servo hose/servo itself.

Is it a cable throttle I'm not sure what AUM is? if so is cable allowing idle.

 

good shout. Will unplug the pipe and block the end to see if it changes the idle.
aum is electric controlled throttle so no cable to adjust.

 

Right so i disconnected the brake booster at the inlet manifold and blocked over the hole. No difference to idle. Wish it would throw a code so I have some idea of where to start.
What about injector seals? Inlet manifold gasket?
Would it pass an MoT with a high idle?
TIA

 

Spray around the inlet gasket with WD40 or whatever, if the note changes you have a leak

Shouldn't pass an MOT as it states the car should be at 'natural idle' when doing the emissions.

 

It has to be unmetered air causing it, or the throttle body not closing the throttle plate enough?

 

I agree it has to be unmetered air. when I took the pipe off the throttle body the plate looks closed. Have realigned it several times.
will have to spray around the inlet manifold at the weekend and see if I can find where it is.

 

Does it surge at all or just stay at high idle ?

Have a look at this vid about fuel trims too.

 

Just stays at a constant high. 1600rpm
No surging. Seems to drive fine too

 

This may sound stupid,and a long shot as our green mk2 had high idle, but its cable operated carb, but thought was cable as car had been stood a while, lubricated and checked sleeve, turned out to be the pedal mechanism itself was sticking inside the car, good squirt of wd and all good.
Is the mk3.5 a pedal operated actuator similar to the caddy mk2? May be worth a squirt.

 

Thanks for the reply. It’s a drive by wire electric throttle so not sure. But will defiantly be worth a check.

 

also that video is handy. I will get it hooked up to Vcds at the weekend and look in to the fuel trims

 

Check in vagcom what % is throttle pedal.

 

ok what measuring block is that? will this tell me if it thinks the pedal is depressed slightly.

 

Don"t know for sure, maybe 2 or 62/63.

 

thanks mate. Yes think it’s block 2. Will check that out as well.

 

Right update on this issue.
Measured block 032 and 033.
033 short term fuel trim stays between +3% and -3%
Doesn’t rise stays at that level.
032 long term fuel trim has a reading of 0%. Doesn’t rise at all. Which is strange.
also when I’m the engine controller and select readiness it says oxygen sensor heating failed or incomplete and oxygen sensor failed or incomplete.
Then I decided to pump the brake pedal and this increased the revs considerably.
So I guess this points to brake booster? but strange when I removed the pipe and blocked it revs stayed the same.
so I think new break booster and o2 sensor and go from there? Any other suggestions welcome.
TIA

 

Had another quick look. When pumping brake pedal short term fuel trim goes up to 21%. So got to be brake booster. Just strange that when it was removed rpm stayed the same.

 

Right need to bump this thread up.
car has been in garage and my mechanic has confirmed there is zero vacuum leaks. Has a new brake booster, Still idles at 1600rpm.
I had a thought the other day if the n249 was coded out of my ecu but left on the engine could this be the problem?
My mechanic said his next port of call is throttle body. It’s all very strange.
Any help greatly appreciated.

 

Definitely I'd be looking at the TB, making sure that the butterfly is closing, it's clean, and properly calibrated after.