Drove car about 60 miles after I picked it up, drove again the next day, about 10 miles in EML came on, then went into limp mode. It hasn't gone into limp mode since in 100 miles of driving.

Have a friend with VCDS Lite and a ODBII cable, which we ran the diagnostic test with, it returned codes 16804/P0420 & 16687/P0303. Ross tech says they're catalyst errors and a misfire on cylinder 3. We then cleared the codes but today it came on again.

I have checked the exhaust system for leaks by blocking the exhaust and listening for hissing - doesn't appear to be leaky. Had a look at the cat from under the car - it's not got a big hole in and doesn't look corroded. The first lambda's wires all look OK too. I took the plastic casing off of the engine and cannot see any issues with the air system. I noted that at the end of the exhaust, it is quite sooty.

I also took the coil packs off of the spark plugs, the one furthest to the right, looked noticeably rusty, compared to the other two. Is the furthest right cylinder 3?

From what I can see the car has always been looked after, it's got a full service history too, so hopefully it's nothing major.

What would be the next step for me to do? Replace coil 3?

Get myself a full version of VCDS and test the sensors?

Anything else I can try?

Thanks

Edited 21 March, 201412 yr by chrsjcb

I have since replaced all three coils and all three plugs.

This appears to have removed the emissions error.

Ran car, everything OK until I put my foot down and light comes on at 5K revs, then limp mode. Restart car, all OK.

Got back, re read code - misfire on Cyl3 again. So I decided to swap the injectors, cyl1 injector for cyl3 injector.

Take car out again, everything OK until I put my foot down again to 5K revs, EML comes on and limp mode when I stop. Check code - misfire on cyl3, the misfire hasn't followed the injector so it's not them.

Since I've done all of the above, each time it appears to be performing better, sometimes the light even goes out straight away without clearing the error.

I'm at a bit of a loss about what to try next to be honest. Does anyone have any ideas?

My thought would be a compression test on cyl3? But it's strange that it only happens at high revs…. I've heard of the injector rail having issues, or the wiring? Have also heard of timing chain skipping a tooth, but wouldn't that cause a misfire on all cylinders?

Edited 26 March, 201412 yr by chrsjcb

Compression test would be the next logical step, might also be an idea to remove the "Knock sensor" and make sure the mounting surface is clean & rust free

Cracked exhaust valves???

http://www.vagcars.co.uk/forums/showthread.php?8599-Adam-s-Polo-9N3

PoloD.

Thanks guys, I work for a big garage chain so I'll pass those suggestions on when I drop the car down with him tomorrow. I don't have to tools for a compression test or checking the exhaust valves.

Edit: Compression test happening on Tuesday. Will update the thread when I find out the diagnosis. Hopefully it will help someone else.

Edited 27 March, 201412 yr by chrsjcb

Okay I had a compression test done. 12 bar across all cylinders. He said when he had the plugs out it looked like the car was running lean.

He suggested replacing the sensor for temperature as this can fool the car into thinking the cars very hot, and restricting the fuel.

This would also tie in with the reluctance to go at high revs accelerating hard.

Where should I be checking for vacuum leaks? I cant see many pipes that look like they carry air?

Any thoughts?

Edited 4 April, 201412 yr by chrsjcb

Replaced the temperature sensor today, gave it some gas, EML again.

Going to have a good look at the manifold tonight and see if there's any obvious leaks again, it does look quite rusty from what I can see without the shielding off.

Leaky manifold a probable cause? I cant think of anything else that it could be down to, any suggestions ?

Edited 7 April, 201412 yr by chrsjcb