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RPM dips below idle when I come off the throttle

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Car is a '93 'Clipper' Cabriolet.

When I am driving and pull up to a stop sign or traffic light, and let off the throttle and disengage the clutch the rpm dips very low to about 300. So much so the battery charge warning light blinks for a moment and it feels like the car may stall. This is especially worse if I have the lights on. Same thing happens when revving the car stationary.

I've done the following (both for this problem and others):

 - Checked for vacuum leaks on hoses and boots
 - Replaced IAC valve
 - Replaced both fuel pumps, relay and fuse
 - Cleaned earthing point on top of rocker cover
 - Replaced alternator
 - Checked battery health

Would anyone know what could be causing this issue?

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I would have to ask, Carbed, or Digi.

Timing.
Butterfly closing as it is maladjusted on a Carb.
Air leak on the intake hose for Digi.
Throttle body Air By-Pass Screw, o-ring digi maladjusted.
Throttle Butterfly in the throttle body not set correctly, or the throttle body throttle position sensor a little too closed.
ISV not functioning or Dirty DIGI.
Bad Grounds, Battery to Frame and Frame to engine.  

Just some of the things I have seen.

Relaying your headlights may improve the quality of illumination, but also off load high current o the headlight switch as well.

What do Divorces, Great Coffee, and Car Electrics all have in common?

They all start with GOOD Grounds.

Where are my DIY Links?

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If k-jet injection system adjust the idle screw and increase the idle speed to 1200 rpm rev the car a couple of times and re adjust to 1200 rpm then once settled lower idle speed to 1000 rpm.
What may be happening is the idle screw is coming lose or the rubber O ring is shot and the car is idling on the anti stall device which increases the idle if needed when things like the lights and blowers are put on.

1988 Mk1 Golf GTi Cabriolet 1.8cc DX, K-jet. Daily drive. 317,000 miles and counting
1978 Mk1 Scirocco GLS 1.6cc FR, Webber carb. Weekend toy.

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Sorry all, should have mentioned: this is a K-Jet model (1.8lt)

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Have a look at the idle screw.

It lives here.


Unscrew it and if the o ring is flat and hard fit a new one.
My old one, the screw would work its self lose and the idle would change and drop every month or so.


New O ring next to the idle screw.


New O ring fitted, a little grease helps it fit on and makes it's easier the screw it back into the throttle body, the old one broke removing it.






1988 Mk1 Golf GTi Cabriolet 1.8cc DX, K-jet. Daily drive. 317,000 miles and counting
1978 Mk1 Scirocco GLS 1.6cc FR, Webber carb. Weekend toy.

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Sorry for the late reply, all.

Yep, the idle screw adjustment did the trick!
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