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just_jack2018

Rough idle and missfire on cylinder 3 fault

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just_jack2018

hi guys, new here,

bought my other half a rather loverly 2006 VW polo 1.2, loverly car, and a dam decent first car if you ask me!

 

bought it from a mechanic friend who had recently rebuild the head with the polo GTI valves as it had suffered from the dreaded valve cracking symptom, all valves were replaced with the much better ones, he also replaced the timing chain, all gaskets and seals and all the oils and lubricants you can imagine, proper decent bloke, I bought it for what it cost him to rebuild the thing! only has 68k on the clock!

 

anyway! drove it 270 miles home completely fault free and only used half a tank (result!) handed it over to my other half which you can expect she was bouncing off the walls (passed her test last week) and she clocked up another 300 miles in 3 days!

 

unfortunatly it then developed a intermittent fault,

the fault only occurs when the car is fully at temperature, and won't always happen, but when it does it's only at idle and it has to have been idling for at least 30 seconds to a min, it will then bring the engine warning light up and have a rough idle, cars shaking around, and goes into "get me home mode" the fault can be rectified by turning the car off and back on again, no issues restarting it ever, the engine warning code is only ever missfire on cylinder 3, as this is her first car I'm not messing around with parts, I have ordered 3 new coil packs (went with the upgraded ones from ebay, the blue ones?) took the spark plugs out and all 3 have a thin layer of white power on them (believe this indicates the combustion cycle is to hot) any pointers or things you guys know could cause this? car drives loverly with no hesitations even after you turn it off and on again after the fault

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nige8021

First Welcome to the forum.

 

I would start by getting a wet/dry compression test done, even though its had recent head overhaul

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just_jack2018

thank you! hopefully going to be around for a while!

 

okay so your thinking could be loosing compression?

 

it seems so odd that its only when its at idle and has been idling for a while, and even then its not every time?

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nige8021

The other thing to try would be swapping the injectors around and see if it moves with the injector or stays on #3

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just_jack2018

okay, well hopefully I'm collecting the coil packs this weekend (been delivered to my ex's address, this should be interesting) if that doesn't cure anything then I will move to the injectors next just to rule them out, will be about a week before I can get a compression test done

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vwfan3
12 hours ago, just_jack2018 said:

....... if that doesn't cure anything then I will move to the injectors next just to rule them out, will be about a week before I can get a compression test done

Hi

The better way is compression test first. If that is low, then you save money by not buying coil packs and injectors.

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just_jack2018

I have attached a copy of the most recent obd2 fault code scan

Screenshot_20180120-111209.png

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just_jack2018

okay just checked the mechanic who did the head work and top end rebuild did a compression check and it was fine, it's done 800 miles since the top end rebuild

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dvderlm

or a leak down test... could identify a burnt valve. 

 

What GTI valves for a 3 pot? Do you mean TSI?

And how better than 214n austenitic stainless steel?

Sodium filled?

Cobalt coated?

 

If they are larger valves cylinder will fill faster and empty quicker so rev higher, but that means more heat to transfer from exhaust valve to valve seat in less time, in an engine that's doing 4/3rds the work of a 4 cylinder - without supercharger efficiency gain help.

 

Was the Ecu remapped to account for the change in airflow? Using same octane fuel? Was the knock detector resonance filtering modified? Ignition point moved to retard more at onset of pre-ignition from hot spots? All guesses for things an engine designer would do on a highly efficient engine with strict emissions targets.

 

 

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dvderlm
2 hours ago, just_jack2018 said:

okay just checked the mechanic who did the head work and top end rebuild did a compression check and it was fine, it's done 800 miles since the top end rebuild

Sorry, do you mean he's just done a compression test, or did it 800 miles ago?

 

 

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just_jack2018

he did it after he finished the head rebuild which was 800 miles ago / 1 week ago

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dvderlm

I had a Ford Sierra 1.8Cvh that got a burnt exhaust valve about 6000 miles after a head rebuild and skim by totally competent trustworthy engineer. Think it happened in cold weather. First fail was seized waterpump after cambelt change by Ford. Resolved to do almost everything myself after that.

 

Engine outlasted the body after that single valve was redone. 

 

Edited by dvderlm

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just_jack2018

nice! the only thing making me think it might not be a valve is because the GTI valves are tungsten coated due to it being a turbo, he did the change as he has done a fair few before this with the head rebuilds and better valves

 

I will have another compression test done as soon as I can and see where to go from there

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dvderlm

Tungsten! Cor.

But they would need lapping into the seat, which would presumably take off some of the coating. 

 

Not sure what the white powder on plugs might be. Running very lean (too hot) gives very white ceramic insulator on spark plug and tip has no soot or oil at all.

When it's sparkly on the plug insulator engine is combusting far too hot and damage is happening, normally pitting in cylinder crown.

 

 

So after comp test or leak down test check the electrical connections for injectors.

Swap injectors.

And depending on car's history before you got it MAP sensor might be dirty. Cat might be clogged ("Oh, something's wrong with the engine I'll add oil". Had that before.) 

 

Wonder if the EGR valve is sticking open?

That would make idle bad. 

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