Will a clogged cat converter cause carbon build up?

[flanagaj]

Just trying to work out why the head I put on the polo only lasted 5 months before I had the dread low compression due to an exhaust valve. The valve wasn't burnt just was sealing and as a result really low compression.

Just trying to work out what I should also replace as I really don't want to be doing it again in another 5 months.

Questions I have to date.

1) Would a cheap lambda sensor have potentially caused an incorrect reading causing the vehicle to run rich. My daughter did say her fuel consumption was crap.
2) Could the previous head issue which I know was present, but not sure how long have damaged the cat. Could a clogged cat cause a reduced flow in exhaust gas and therefore making a sooted head more likely?

Thanks

[littlepolo]

Any error codes?

[flanagaj]

Before the engine management light came on and the valve failure the only light that did keep coming on, but I stupidly ignored was the EPC light.

No error code to hand though as when I hooked it up it was just reporting the P0301 misfire code.

[littlepolo]

Codes should remain until cleared. That's a misfire on cyl 1 which could be caused by multiple problems including electrical, fuel, or engine internals problems. You don't say why you fitted the head 5 months ago. I presume you checked the valve seating before fitting the replacement head. Did you fully inspect the head and valve gear?

I think you'd notice a clogged cat in terms of performance. I can imagine a cheap (low quality/mismatched) lambda might cause problems with fueling but I think you'd notice if it was way out (smell of fuel, idling, etc.) Fuel consumption could be caused by a variety of problems too. Are the plugs sooty now?

I guess you mean that the old valve wasn't sealing properly? Rings could also contribute to compression issues. What is the mileage? What engine/gearbox is it and what is the mpg now?

[flanagaj]

5 months ago it was the exhaust valve on cylinder 3 that went. This was deduced by an initial compression test and then a leak down test. Having removed the head I poured water in the upturned head and the water flowed out through the exhaust port. This time around it was the same problem on cylinder 1.

First time around I had a local cylinder repair shop put new valve stem seals in and fix it. This time around I have ordered a whole reconditioned head as Iv'e lost faith in the one I took off.

It's not rings as I did a wet test and the psi remained the same.

60k on the clock with full service history.

Just want to understand why the head could have gone so soon. Have also purchased an NTK lambda sensor this time instead of the cheap ebay one that I fitted last time.

New head has not gone back on yet, but yes, the plugs were sooty and the lambda sensor was very black.

[littlepolo]

That's a low mileage to have such problems. Usual problems are with valve stem seals and rings.