Any VW polo owners or experts out there

[RonA]

Just giving the daughters 08 polo (90k on clock) the once over for its mot & it’s got a rough tick over.
Checked the engine with my code reader & it’s indicating a misfire or to ground on No1 cylinder.
Have had problems previously with the coil units before & suspected No1 might have gone again. To be sure changed the unit over in turn with No2 & 3 cylinders but fault still on No1. New set of plugs purchased & running has improved albeit still a bit rough on tick over but engine seems ok at higher revs.
My next port of call is a compression check, anybody had a similar experience.
RonA

 

[Wendelspanswick]

If you've swapped ignition parts around my next suspicion would be a burnt exhaust valve.

 

[RonA]

If you've swapped ignition parts around my next suspicion would be a burnt exhaust valve.

Yes, tend to agree with you now I’ve just done a compression test Cyl 1 80 psig, cyl2 175 psig , Cyl 3 240 psig. With all this high tech emission gear on cars these days tend not to think old school & look for something exotic. Looks like the heads coming off!!
Thanks for the spot on analysis.
RonA

 

[Ruffian]

1.2 3cyl?

Worth changing the timing chain kit also if the head is coming of as do stretch and throw the timing out also.

 

[Fazerruss]

I'm no expert but thinking logically I would buy a new coil pack to replace the faulty one. Although the code reader says misfire on cylinder 1 still I wonder if this is just a generic misfire code regardless of which cylinder.
How does the ecu know which cylinder missfires? I've not yet come across an engine that has a lambda sensor on each exhaust outlet on the manifold so how does it know if the misfire could have moved to another cylinder when you moved the coil?
I may be talking bull so this is not and experts view but I would go for the cheapest easiest parts first before pulling the head.

 

[james butler]

the eco is using the circuitry to each cylinder to measure the resistance and what not to determine if a plug is firing correctly saab has been doing this for years https://en.wikipedia.org/wiki/Saab_Direct_Ignition
did make me scratch my head trying to work out how it worked!

 

[pedrobedro]

Not a Polo but a Golf my friend had and it melted the wiring to a coil pack. New piece of wiring and plug fixed it.

 

[the snooper]

I'm no expert but thinking logically I would buy a new coil pack to replace the faulty one. Although the code reader says misfire on cylinder 1 still I wonder if this is just a generic misfire code regardless of which cylinder.
How does the ecu know which cylinder missfires? I've not yet come across an engine that has a lambda sensor on each exhaust outlet on the manifold so how does it know if the misfire could have moved to another cylinder when you moved the coil?
I may be talking bull so this is not and experts view but I would go for the cheapest easiest parts first before pulling the head.

The ECU will know because of crank sensor cam position sensor and which coil it's firing

 

[Fazerruss]

Thank you, I always wondered.

 

[anto44]

Yeah vw cross reference crank position with knock sensor signal to determine cylinder miss fires. Going by them compression readings you have burnt valve seats due to worn valve guides. Common problem with petrol vws. I do at least one head a month cause of this

 

[kcchan]

I think the cam angle sensors are prone to going on the VW 3 pots.

May be less hassle than taking the head off.

Although that doesn't go to explain the low compression.

 

[RonA]

After a bit of digging on the net, it appears the VW 3 cylinder engines are prone to burning out valves so my compression tests seem only to be expected at advanced mileage.
RonA

 

[dannyp]

most engine now have wideband lambda sensors, they are fast enough to detect 02 present in the exhaust from the misfire, you can datalog it but it forms a very distinctive trace the ECU can recognise it, diffrent systemes work out whitch cylinder is at fault thorugh diffrent methods but they all get the result pdq