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Polo Mk2F 1.0 AAU - Crankcase ventilation and inlet cold/hot air feed

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Hi Gents,


I have a few questions below, first some background.  I have a 1991 Polo Mk2F with AAU engine, 126k mi, and it's been off the road for 5 years. Decided to get it back on the road so have repaired some mice-chewed indicator and heater-blower wires under the dash (I now own a cat!).  Checked all fluids OK and fired her up, started first time :-), getting up to temp she smells a bit rich, but I've only driven modern cars recently so maybe had forgotten how older cars smell.  The oil filler cap hole has always smelled a bit petroly in previous years, but all previous MOTs have been well within emissions.  I decide to take off the  air filter housing lid and end up looking at inlet parts:


1)   What is a normal amount of smoke from the crankcase vent hole, where the pipe enters the floor of the air filter housing?  Mine looks like someone has lit a cigar.  I know the purpose is to evacuate crankcase vapour but was surprised at how it was puffing along.  I'd previously sucked the hole (ooh matron!) with the engine off and was surprised at getting a lungful of vapour when the car hadn't been fired up that day.  Is this normal or have I got bad piston blowby - the car has done only 1k mi after its last MOT 5 years ago before being off the road so I don't suspect wear?  The throttle body and filter housing look very lightly glossed with oil - nothing horrific.


2)  Is it normal for the throttle (when closed at idle), and the hedgehog underneath, to have drips of fuel?  I know it deliberately runs rich at startup, but I was surprised to see some puddling around the hedgehog.  The injector seemed to be squirting less when warm idling vs cold idling, and opening the throttle lead to a healthy squirt - seems reasonable behaviour.


3) The warm air-cold air intake with the flap inside (before the air filter housing)... With engine off the flap was open about 1cm, and pushing it down by hand made it pop up 1cm again.  With engine just started, the vacuum line successfully lifted the flap and gave pure hot air, when engine well up to temp the flap was totally closed (all sounds good).  Should it ever show partial behaviour?  It felt like a spring was holding it part open.


The car smells rich but as it's not (yet) road legal I can't take it for a burn and test wide throttle.  At full water temp, the car runs OK, throttle blips seem decent, and the Blue Temp Sender is outputting 250 Ohm which I believe to be correct.  Tempted to just MOT it and hope for the best - just wanted to make sure I've not missed anything blatant.


Cheers in advance

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  • Bread


  • Pete1


  • mk2tastic


  • zob


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If it’s been stood 5 years you could have seals and hoses that have dried out, hardened and cracked, hence what seems like drips.If it’s fuel you need to investigate further as you clearly don’t want any sort of leaks, especially as the exhaust runs directly below the engine.


Again with the engine, if it’s been stood I’d be tempted to give it an oil and filter change as an absolute minimum.Oil does deteriorate, especially if it’s old, plus it’ll be full of all sorts of contaminants from normal engine running, which can affect emissions. 


My HZ engine puffs out of the breather pipe, but not what I would call excessive(120k miles). If you see loads of oil in the air cleaner then that would be a sign of trouble.


Check other breather hoses too to make sure they are in one piece and not blocked.There’s one one connected to the cam cover that can become sludged up.Mine ‘kinked’ as it passed round the air cleaner as the rubber became soft.



Edited by mk2tastic
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Occasionally when I have the breather off it definitely puffs, but as said above if it hasn't had new oil in a while that won't help. Petrolated oil stinks. To add to the final point above - there is an oil trap inline with the big breather hose that can get real gungy. I rinsed mine back and forth in a pot of petrol and got it good and clear - stopped the spray of filth into the top of the inlet which can only be good!


If it passed its emissions last its relatively unlikely but its worth checking that the resistance across the blue temperature sender is low when at operating temp (~400 ohms) and high when cold (a few kohms @ <10C). But in my experience its equally important to check that the earth to the sender (brown wire i think) is seeing a good strong earth - check the pin on its plug against an earth point with a meter. Rich running usually starts and ends with the temperature sender.


Having said that it should run rich while warming but if you're in white smoke, chugging or generally boggy territory then its probably over the line.

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Have you established that the hedgehog actually works? If it's not warming up that will exacerbate any pooling of fuel, that's kinda what it's there for.

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Thanks for the replies everyone.  Just found out my exhaust silencer has pinholed from corrosion - Gun Gum and repair patch are at the ready.  If there's water in there causing corrosion, then when warming up, water vapour would explain some of the white smoke.


Cam cover vent hose, and cam cover metal spur sucked/blown through and all clear and OK, though there was a one-off gurgle from the hose which then stopped.  I'll check later for a filter, didn't think AAUs had them but I've never looked in earnest.


Blue temp sender 2.5 kOhm at ambient, 250 Ohm at 90*C.  Earth continuity in sender plug brown lead is good (nice suggestion to test - I previously tested my sender at every service, but never the earth).


This weekend I hope to get the filter housing off and will then be able to access the hedgehog.  Does anyone know if it heats with the ignition turned the first click (when the dash lights fire up)  or does the engine need to be fired?  Was hoping I could feel the hedgehog underside with engine off to save removing it.

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20 minutes ago, Bread said:

Does anyone know if it heats with the ignition turned the first click (when the dash lights fire up)

I can't remember, but it seems logical that it would.  I think there's a thermoswicth in series with the relay's coil so it will only come on below a certain coolant temperature, so test from cold engine.


You can probably test its health with a multimeter measuring resistance from its feed wire to engine metalwork/battery negative though. Should be well under 1 Ohm at ambient temperature, from memory.

Edited by Pete1
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Hedgehog tested, tracing the wire from under intake mani lead to a connector at the rear of the engine towards the dizzy end, with a fat wire going into it.


I had 1.5 Ohm resistance between the "hedgehog's side" electrical connector terminal and engine block.  Possibly not the cleanest earth though I was scratching around on the block to try to get a good line.  Hmmm...


Testing Voltage at the the "from battery/relay" side of the hedgehog connector: Ign off gave zero volts (duh), Ign first click gave about 0.3 V which I think must be leakage at the relay and no heat at the hedgehog after reconnecting.  Firing the engine gave instantly over 13V so it looks like the hedgehog is only activated on full ignition.  Why they do this I don't know, unless its to save battery if someone left the key on first click for some time - the hedgehog is basically like a longer duration glow plug and these come on at first click to preheat.


Hedgehog reconnected, and then with the engine running feeling under the manifold, bottom of hedgehog became lukewarm.  A little hard to tell as after a couple of minutes the block, and coolant pipes start heating up the inlet manifold in general.  So the hedgehog is (at least partially) working, though the resistance was not ideal.


Cleaned lambda earth onto inlet mani while I was in there, the outside surfaces of the connection were dirty as hell, mating surfaces had smears of grease or oxide on both surfaces - not nice - so sandpapered, solvent wiped and then refitted.  Gonna do all my earths in due course.

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  • 2 weeks later...

Update with info for anyone that follows:


1) Spark plugs out, good colour, no signs of rich running.


2) Dizzy contacts/rotor  cleaned after mild pitting visible


3) Compression test, warm, dry (but like a numpty I forgot to hold throttle open - so done at idle throttle)

1-2-3-4: 144-150-150-150 psi.  haynes "new from factory" spec is 10-15 bar, wear limit 7 bar, max diff between cylinders 3 bar.  All good (after 126 k miles!).  I think at open throttle they'd be even better.


4)  Followed the specs in this excellent guide here for testing various throttle body and sensor resistances (the pics in this thread would be a great sticky):

 I also cleaned the TB connector plug contacts with contact cleaner, as some were a little oily looking.


All good except my TPS readings don't match, pins 1+5 are within spec, however I get pins 1+2 giving 1.8/2.25/1.5 kOhm (0/25/100% throt opening).  Pins 1+4 give 1.8/0.9/0.9 kOhm (0/25/100% throt opening).  The constant-to-variable changes arent as per the specs.   BUT on a spare throttle body I get the same readings as my actual TB.  I'm inclined to think they both cant be wrong in the exact same way.  Hmmm... maybe typo by VW specs or an AAU to AAV difference?


Cleaned all car earths too.


Anyway, gonna take a punt at the MOT in the next few weeks after giving her a pre-emptive oil & filter change.



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  • 1 month later...

Info for anyone that comes across this thread:


Oil changed, car passed on emissions (roughly one third of the limit - pretty clean).  There is still a somewhat fuely smell under the oil cap.... guess modern cars are just that much cleaner.

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