Misfire on 3 cylinder Polo

[spelly 123]

Hi,
Your expert advice would be gratefully appreciated.
My Daughter has a 2005 Polo with the 1.2, 3 cylinder engine.

It has developed a misfire. The engine management light first begins to flash but there is no real noticeable symptoms. The light then comes on permanently and the engine runs on no 1 and no 3 only.
Once the engine cools down the vehicle is driveable again (no noticeable misfire)
The fault code it throws up are :- 01314 (passive/ sporadic) and 16686 (also marked passive / sporadic) cyl 2 misfire detected.

I have first changed the plugs and coil-pack's , no change. I swapped both the plugs and coil packs around from no1 to no 2 cylinders, I then cleared the fault codes and road tested, fault stayed on no 2.
I then did the same with the injectors, swapped no 1 and no 2 around cleared the codes and the fault re occurred again on no 2 cylinder.

I understand from reading this forum there is a issue with the exhaust valves on this engine, so I did a compression test. With a hot engine I got readings of no 1 155 , no 2 135 and no 3 185.
The no 2 cylinder looks a little low but I am not sure it is low enough to cause the problem. Can anyone give me any advice on this problem?

Am I right to assume that the serious misfire is caused by the ECU detecting the sporadic misfire and shutting off the injector to protect the Cat?

I am reluctant to take the head off until I have eliminated simpler causes.
Any pointers of advice as what to check next would be appreciated.

Thanks

Spelly

[union22]

I have had a few 1.2 polo's over the last ten years, its very common for the coil packs to fail and many of the aftermarket ones only last 2 years, I have had them fail both intermittent and completely once one fails I tend to treat the car to three new ones. I have also had them fail hot. Although recently I have bought Bosch ones which appear to be lasting much longer.


But the fault should move when you put them in a different place. Yours don't which is strange......


One of my polo's had low compression the readings were 1=165 2=200 3=200 the only clue was uneven tick over. No fault codes or other issues. I never took off the head so can't say why it was low. But think it may have been caused by driving the car with a failed coil pack maybe bore wear and not a valve.


The other things I have had that cause a misfire were a stuck egr valve and a broken lander sensor. But both showed appropriate fault codes.


Back to your car if your convinced its not the plug, coil pack or injector it must be something else I would not suspect an internal engine problem. So that leaves wiring or whatever drives the coil pack or injector which I presume is the ecu.


Its not easy to test the ecu as you would need an electrical brake in harness and an oscilloscope and if your right with your theory that the ecu is closing down your number 2 cylinder, that is what your going to see!!!!


I would probably treat it to a set of plugs and coil packs maybe even a set of injectors if I were feeling flush. Then look into trying a different ecu. Or if your convinced about the status of plug, coil pack and injector go for an ecu.


Good look and happy fishing, let us know what you find.

[zollaf]

your compression test readings are all over the place. do them again, and if they come out about the same then off with the head it is. if they were within 10psi then i would look elsewhere, but as you already know, the 1.2 eats valves, especially if you run on low octane fuel and not the expensive stuff.

[spelly 123]

Thanks guys I really appreciate the input.

Zollaf, I have checked the compression readings 3 times now and I get very little variation, maybe 10 psi either way for any particular cylinder.

Having done a little bit of surfing the internet for help to understand the problem I came across a very interesting web site giving advice on how to read compression test results.
It suggested that any result above 135psi were good. However it also said that readings should be all within 21 psi of each other, which mine are not.
So I am not sure how to interpret these result are they likely to indicate the source of the problem or not?

Technical Articles at Greg's Engine & Machine (http://www.gregsengine.com/cylinder-compression-testing.html)

[spelly 123]

Hi All,

Just thought I would bring this thread up to date.

The apparent fault has been traced to the exhaust valve guides. I made the final diagnosis by removing the exhaust manifold, inserting one plug at a time and listening for air leaking past the valves on the compression stroke.
no 3 was fine and held compression well. no 1 & 3 would not hold compression and it was easy to hear the air passing the valves.
Having removed the head the exhaust valves were out of the stated 0.8mm sideways movement tolerance but there was no sign of burning of the valves.

I have now dropped the head off for machining. so hears hoping that is going to be the end of the problem.

Just on a side issue. I spoke to Derek from Ilexa as to whether VCDS would be able to help in the diagnostics. He was incredibly helpful.
The diagnostic backup they provided was very impressive. And that was with someone who was not even a customer yet !!!!

I will be buying VCDS from them as soon as I have finished with this expense of this repair.

As I am typing just got a call from the machine shop. The head only needed the guides replacing, the valves were still ok
£100.00 great job.

Just need to put it all back together now.

[niall campbell]

my head was £110 so good to hear your news

[spelly 123]

Hi all,

Just to wind up this thread.

Problem now sorted. As a take home point to help anyone else with the same misfire problem and thinking they may have a exhaust valve problem.
The simplest way to check is to simply turn the engine by hand using a socket on the crank-pully nut and feel the individual cylinder compressions.

A good cylinder will kick back as the compressed air in the individual cylinder pushes back against the piston. However a cylinder with a poorly seated or damaged exhaust valve will not.

Granted this could also be caused by gas passing the piston rings so the next step is to remove the exhaust manifold to see if you can hear the air passing the valve.
With mine, the hissing of the gas passing the exhaust valves were very plain to hear.
It is possible that you may be able to hear this from the end of the exhaust pipe, although i did not try that myself.

Just thought I would pass this info on. Might help someone.

Thanks again for your input.

Spelly

[niall campbell]

my oil rings were away, had to take the engine out to get the pistons off

Not a drop of engine oil is now used on the 110,000 mile Polo

I am just away to get my new £45 knock sensor from VW.

If I was to do it again, I would put new exhaust studs and nuts on the exhaust manifold. As the car is 3 cylinder, the engine does shake a wee bit & kept slackening the exhaust manifold to bits.

I will take a video before & after to see if the knock sensor cures my shaking at tickover.

Out of interest, there was no blue/ black smoke in my exhaust fumes to think that my piston rings and oil rings were away, but stripping the engine showed the oil rings stuck in with no springyness