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paulhide · Posted: Thu Nov 20, 2008 9:48 am Post subject: What's going on here?

Can anybody explain why a 16v motor that I'm working on (non-cat) is very slow to return from high revs to idle at any time?
It's not a sticking AFM flap.
It's not a sticking throttle cable or butterflies.
It's not an air leak.

So what does that leave. Are any of these possible?
1. Something to do with the throttle position sensor?
2. Sticking injector?
3. Flywheel sensor?
4. Anything else?
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bobbber · Posted: Thu Nov 20, 2008 10:26 am Post subject:

Hey Paul,

Sticking injector would, I'd have thought, produce rough running and over-fueling?

I'd go for the throttle position sensor myself. Check the continuity on the switches... I'd have thought the position (rotation) of the switch is out of alignment with the butterflies.

Hmmm... (thinks some more) - what about if you push the throttle rail to 'manually' close the butterflies? Perhaps the springs just aren't powerful enough anymore to close them quickly?

Bob Cool

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Admin · Posted: Thu Nov 20, 2008 3:01 pm Post subject:

Hi

Air leak after the throttle bodies would be my guess. Fair few places for that to happen (eg, damaged O ring around an injector).

All the best

Keith
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ZeNiTh-PbArM · Posted: Thu Nov 20, 2008 8:42 pm Post subject:

Hi,

On carb engines this happens a lot when both carbs are out of sync.
On the 16V due to the twin throttle bodies it can be the same situation.
Try to adjust the linkage between the throttle bodies and this should solve your problem.

Regards,
zp

paulhide · Posted: Fri Nov 21, 2008 10:18 am Post subject:

Thanks so much for the quick replies - great site this.
Butterflies are returning fine (and I think at the same time) and I don't think there is an air leak as the AFM flap opens to let the air needed through it as it slowly returns to idle. (The flap does not stick and moves freely.)
So would the throttle position sensor out of alignment cause this.
Thinking about it I think the motor is fine at first whilst it is cold.
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bobbber · Posted: Fri Nov 21, 2008 10:33 am Post subject:

Paul,

It takes two minutes to check the throttle position sensor. Take the connector off and hook a continuity tester up between the middle tag and one of the outside tags (if you're a skinflint, you can set up with a bulb, some wires and a battery instead).

One outside tag is for the full throttle and the other is for the idle. Check for continuity when the accelerator pedal is released. It should only be a complete circuit when the pedal is completely released but also instantly. On mine the springs are lazy so the idle switch only connects after the pedal is released for a while.

My idle switch is instantly off when you touch the accelerator, this is BEFORE the butterflies have opened enough. This causes the 'bumpy' spot when first accelerating. This adjustment can be quite fiddly I'm afraid.

When the engine is cold, the idle control reservoir (at the front of the manifold) is responsible for controlling the idle at a different level to when the engine is warm. It *may* be that the mechanism within it is sticking... try cleaning it out with parrafin and a bottle brush.

Hope that helps...

Bob
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paulhide · Posted: Fri Nov 21, 2008 1:11 pm Post subject:

Cheers! Will check it all out. Smile

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bobbber · Posted: Fri Nov 21, 2008 1:50 pm Post subject:

I can't talk much though Paul... can't get mine to start at the moment (although I haven't had a proper look yet! )

When is the first AROC meeting next year??? Might try to aim for that.

Good luck

Bob
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paulhide · Posted: Fri Nov 21, 2008 6:15 pm Post subject:

Not sure where SAD will be, but they've gone and located NAD in bleedin' Yorkshire, which is a fair old distance for me.
We are probably due to lose this place early next year, so if you want any parts, get in quick.
Reckon this motor (someone else's that I'm working on) is going to be throttle position sensor as it runs really well - just changed the gearbox to restore 4WD. You will love your P4 when it gets going Bob - very aesthetic and direct.
Mind you if we lose this place I may have to lose a couple of cars and Sue may end up with my P4!
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bobbber · Posted: Sat Nov 22, 2008 12:47 am Post subject:

Yorkshire is not too bad for me... but generally a(nother) stoopid AROC idea...

Feel free to crash at mine if you need somewhere to stay Paul.

Bob
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paulhide · Posted: Sat Nov 22, 2008 7:45 pm Post subject:

Cheers, may take you up on that. Smile

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bobbber · Posted: Tue Nov 25, 2008 2:05 am Post subject:

Jeez Louise...

It's in Ripon, which is like 130 miles north of me! Surprised

2 hours from where I live apparently... what *ARE* they thinking?!!???!?
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paulhide · Posted: Tue Nov 25, 2008 10:36 am Post subject:

They seem to think that it's very central. I guess that may be true for that large group of AROC members that live in Northern Scotland!
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bobbber · Posted: Tue Nov 25, 2008 10:41 am Post subject:

Ah yes... the AROC Lerwick Lot.
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