Slow return to idle and hunting revs

[dub-disaster]

My engine will returns to idle normally until around 2-1.5k where it hesitates for a second like you've blipped the throttle it will then return to idle slower than usual.

Second symptom is sometimes it will rev up to 2k on it's own and stay there regardless of if I rev the engine over 2k it will just keep settling to 2k after a couple of minutes it will settle down on it's own.

Final symptom is when returning to idle usual idle is a constant 950rpm but occasionally it will hunt up and down between 1-1.5k, if the throttle is partially open whilst this is happening it still does the same, it will continue to do the same if I rev it and let it settle again it will settle on it's own after a few minutes.

Could this be a vacuum leak ? Or a poor sealing throttle body. I have checked both of these and the 51mm body I've got seals as well as an oem one (not perfectly but good enough) wouldn't think this would be to much of a problem as the idle screw let's air past anyway.

Checked the vac system over and replaced all pipes so can't see it being this either. Also of there's a vac leak surly the map sensor would read any pressure changes and adjust accordingly ?? Can't see any strange fluctuations in vac reading on guages.

Any ideas or pointers ??

[Andy]

Very common problem with 51mm throttle bodies.

Jezza's car does it, but if you snap off the throttle from a wide throttle opening it's fine - presumably because the butterfly is snapping shut with enough force to seat and seal pretty well. If you ease off the throttle gently though, it seems to stick a little bit, creating exactly the symptoms you describe.

The hunting revs is Digifant trying to do its best to stabilise the idle - it does this by swinging the ignition timing around. Obviously this only works up to a point, and if the throttle plate is stuck slightly open it'll be fighting a loosing battle. You can't send air down a wire!

[dub-disaster]

Cheers for that Andy I'll grab another throttle body and see if it cures it.

[dub-disaster]

Ok tried new throttle body today and it actualy made it worse !! Had both bodies side by side and the seal is identical both operate fine on and of the car to be honest the 51mm one is slightly better.
  Still having hesitation returning to idle and this erratic up and down hunting could this be caused by anything other than a leak vacuum side of the throttle plate? could it be the ecu maybe ? Any other areas to look in.
I've checked over and over again for vac leaks and all is good. Can hear a slight hissing from around the top plastic boost pipe I'm assuming this is just air traveling through it I did take it of and there are no cracks and it is air tight checked all connections above and below boost pipe. Still can't solve this problem.

[Jezza-7]

We should compare bays mate. Swap bits over and see if it sorts anything out. You using the standard bendy boost pipe?

[xandyx]

how about trying to put a stiff spring for when you release the pedal, if full close the tb..

just an idea!

[Andy]

Is the ignition timing set 100%? And idle speed adjusted okay?

[LightDiodeNEAL]

 Just to check if its ECU in panic, does the same happen if you pull the water sensor off? Just check see If there's no change with it off or on. Just something I found once because there wasn't an ECU light on my 1st one (I not sure now if this one has either)

Neal

[dub-disaster]

right, ignition timing is set on the z mark, it does flick around a little but 80% of the time bang on the z marker, the idle is set to 950rpm and it does idle at that apart from when its hunting around. it only hesitates for a second or so at 1,5k then will fall to 950rpm some of the time and other times it will hunt up and down.
   the ecu seems to be "happy" as it will go into service mode no problem with the blue temp sender unplugged. when in service mode i checked but didnt adjust the timing as it was ok, i wound the idle screw up and down and the idle did change reset that to 950rpm again then replugged the blue sender before switching of.
   yes im using the standard plastic top boost pipe. jezza drop me a pm n let met know when your about see if we can worksomthing out over the weekedn ??
  could the ignition alone rev the car up to 2k and hold it there for a few seconds or cause these problems or should i still be looking for leaks in the inlet system somwhere??
    with the standard throttle body on it reved up to 2k and stayed there immediatly stoped the engine and took the top part of the body of and the butterfly was shut tight.im assumimng it would have to be a pretty large leak for it to rev to 2k and stay there ??

[LightDiodeNEAL]

I'm sure its the ECU, can you check the idle sensor and wide open sensor lines? Ones not stuck (w/o) or not working is it?. Bet you've done a quick avotest check they change.
Cool mate
NEAL

[dub-disaster]

Idle switch is working fine can hear it click and when driving i can see the fuel cut when I close the throttle on my afr guage wot switch is a 6psi boost switch now. These would surly cause fueling problems anyway and the fueling is all fine as it should be.

[PeteG40]

you sure the battery and alternator are pumping out good voltage? sometimes revving etc can be down to sensors cutting in and out with rubbish voltage (so i'm led to beleive)

[dub-disaster]

you sure the battery and alternator are pumping out good voltage? sometimes revving etc can be down to sensors cutting in and out with rubbish voltage (so i'm led to beleive)

Both alternator and battery are brand new and putting out good voltages

[Andy]

with the standard throttle body on it reved up to 2k and stayed there immediatly stoped the engine and took the top part of the body of and the butterfly was shut tight.im assumimng it would have to be a pretty large leak for it to rev to 2k and stay there??

Did you adjust the idle screw on the standard TB to set it up?
Are you using Polo CL pulleys and timing markers?

[Jezza-7]

What about a new ISV?

Not around this weekend mate. Got monday off if your around?