1.2 BME 2007 Burnt Valve, Bottom Pulley, Chain Jump, Bent Valve.

[guv.c]

Hi All,

I'm just posting this to share my experience and makes sure no one makes the same mistake!

I bought a Polo a few months ago for my sister, 2007 plate with the dreaded 1.2 engine however it only had 45k on the clock, started well, sounded great and drove well too. However once a week it would come up with a misfire fault with the engine management light coming on. A quick code read came up with misfire with CYL number 2. Now after having already owned a 1.4 engine, my immediate thought was coils and sparkplugs but after having changed these, it was not that.

A compression test later 1-190psi 2-120psi 3-210psi. So it seemed like the standard worn guide burnt exhaust valve problem. We took the head off and confirmed it was indeed a burnt valve. We bought a reconditioned head with new valve guides to save some time. Put it on, timed it all up, everything was perfect. Put it all back together and the last job was getting the crank pulley tight - we didn't do it tight enough though.

So when we started the engine (it started first time and sounded tappety), the bottom pulley was slipping and was not spinning as fast as it should have been. The first sign was the battery light on, then the oil pump light on. Before I managed to turn the engine off - clunk. and it died. The car was only on for 30 seconds and at idle. I went to inspect the bottom pulley and it was almost off.

Another compression test later 1-160psi 2-230psi 3-0psi. Head off and 2 very noticeably bent valves in cyl3. While we were reseating all the other valves we noticed two more bent ones in cyl2 even though it had good compression. So after changing 4 valves, lapping all valves, checking guides for damage and then testing compression using water in the chamber and blowing air into ports, we managed to get the head in a good position.

We put it all together and this time, the bottom pulley was done with 90NM +90 degrees as per spec. Last night we managed to get it going and it stayed on, running as smooth and as silent as it is supposed to!

So it was my mistake for rushing and non getting the pulley bolt tight. What I believe happened is that the oil pump gear on the crankshaft is not keyed to the crankshaft - therefore when the pulley bolt got loose, the oil pump stopped (hence the warning light inside). The lack of oil pressure in the tensioner caused the chain to jump at the crankshaft causing the timing to be out and bent valves. This all happened at idle!!!

So anyone attempting this job, make sure that bottom pulley bolt is tight! We had to make a custom tool so we could hold the pulley while getting it tight. We had to use the same tool for the camshaft sprockets too as they also require 90 degrees!

I know there is probably a good reason, but why did VW not design the crankshaft bolt with a left handed thread so that this doesn't happen?

Ah well - you live, you learn.

[zollaf]

vw didn't use a lh threaded crank bolt because they couldn't predict the level of mechanics that would tinker with their cars. in this case, a lh bolt wouldn't help anyway, and his is also why mechanics such as myself have to make sure everything is done properly.

[guv.c]

In hindsight, I should have given it to a mechanic rather than give it a go myself (I am far from a mechanic, just a very keen DIYer!). But nonetheless, it was an interesting learning experience.

Just out of interest, why would a lh bolt not help?

[zollaf]

if a lh bolt was used then it would stand less chance of coming undone if done up tight, but then a rh bolt doesn't come undone anyway. a lh bolt isn't going to tighten itself and the same would have happened.

[guv.c]

That makes sense - It's done up tight now... I'll never make the same mistake again, you can be sure of that!

[zollaf]

even the best have made major **** ups.