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Erratic idle and stalling

Engine mods, conversions, turbos, gearboxs, drive shafts

Erratic idle and stalling

Postby unomatt » Sun Mar 17, 2013 4:00 pm

Hi guys, was wondering if you could help me. I have a '91 GTi with a problem. It starts fine on cold start and the idle isn't too bad, but if I drive it for a minute or 2 and stop, the idle will be up and down from around 500-1500 rpm and then will cut out. Starts up again straight away but happens again everytime unless I can get on a run with no stops for a bit, any ideas? Thanks.
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Postby MAD-PUG » Sun Mar 17, 2013 4:50 pm

The SAD valve would be a prime suspect.
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Postby unomatt » Tue Mar 19, 2013 10:11 pm

Ok, silly question but, what's that?! Is it a cold start valve? I googled it and that came up. Are they expensive/easy to fit?
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Postby miles1234 » Wed Mar 20, 2013 12:44 am

SAD is the supplementary air valve or cold start valve . When the engine is cold it allows extra air into the inlet manifold .It sits roughly under the dizzy and the first thing to do is remove it from the car and test it . When cold if you look through it you can see a small wedge shaped aperture , when the engine is started a voltage is applied which heats an element inside and the hole closes within a couple of minutes .Rig up some wires to a 12v battery and check that it closes .
These engines are famous for erratic tickover and problems like you have .There are loads of oil vapour hoses under the bonnet that can split or have leaky connections .Check all jubilee clips , cut hoses back a bit to get a better seal if they are long enough .Be careful it is possible to mix the connections up .
Its worth doing the standard Haynes manual checks on the TPS setting and the multimeter checks on sensors .
If the vacuum advance on your dizzy has perished that will allow air to be sucked through so plug the vacuum pipe or try to get a new vac advance diaphragm .
Make sure the dipstick is a snug fit in its tube ( the orange end bit ).Make sure the bolt that holds the black plastic oil filler body to the plenum chamber is not missing .
Do the obvious stuff first , clean and gap the plugs , check rotor arm, dizzy cap and HT leads are ok , ignition timing etc .
If you come to any of the shows this summer post it on here and I will stick my head under your bonnet and see if there is anything obvious wrong , I am no expert but I have had the same problems , you just have to work through all the standard tests .
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Postby 309jazzpanda » Wed Mar 20, 2013 8:53 pm

don't block the vac advance off get it fixed if the diaphragm is buggered, timing advances to 30 degrees at 3500rpm
why do i keep finding holes? where's the galv?
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Postby unomatt » Thu Mar 21, 2013 9:36 pm

Thanks guys! If the weather ever sorts itself out I will start the checks!
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Postby MAD-PUG » Mon Mar 25, 2013 10:33 pm

You don't actually need the SAD valve. We removed it from the rally car and threw it into the neighbour's garden. It means nursing the engine a wee bit from cold, but it runs fine when warm, and you don't get any warm start difficulties.
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