New build toilet flushing issues

[craigap]

Hi

Moved in to a new build property about 8 months ago but noticed the downstairs toilet hasn't been flushing correctly for the last couple of months, i.e. the water doesn't go at once and often tissue is still left in the bowl.
The toilet cistern butts up to a boxed out section which contains the incoming water supply and the soil pipe down to the sewer. The soil pipe has a return on it pointing upwards, which has a grey cap on it. I assume this is to stop smells?
Whilst investigating this I removed the grey cap and the toilet flushed as expected and everything cleared with no issues. However the down side was the smells from the sewers, which wasn't liveable. So I replaced the cap and we are back where we started as clearly the toilet isn't blocked and neither is the pipework.
I wasn't sure if it would affected this, but the drain pipe from the hand basin has been fitted directly behind the toilet in to the top of the soil pipe before it drops down.
I am trying to resolve this with the builder/plumber but based on previous experiences its going to take time and unsubtly I will be bullsh*ted about the issue.
Any ideas as to the issues or what else I could try?

 

[Newboy]

Sound like a partial blockage. By removing the cap you are allowing the pressure build up to vent.

Either partial blockage or (quite possible in a new build) a belly in the underground pipework.

Insist that a camera check is done from the external inspection chamber back up towards the house.

 

[craigap]

OK, so are you saying the actual sewer past the soil pipe could be blocked? Suppose this makes sense. I will pull up the drain cover and try to rod it myself and see if I hit anything.
With regards to the sink drain going in to the toilet soil pipe, is this usual?

Thanks.

 

[Nige F]

With regards to the sink drain going in to the toilet soil pipe, is this usual?

.

Yes

 

[Burnerman]

This grey cap......is it blanking the pipe off completely or is it a durgo valve, i.e an automatic air vent?
John

 

[Newboy]

I will pull up the drain cover and try to rod it myself and see if I hit anything.

If it's a belly in the pipe then you won't feel that through rods - you may clear the blockage but the run needs to be checked.

If it's a new build then get them to check the drain for bellies.
I had exactly the same problem on a new build 20 years ago repeated blockages until the run was finally exposed to show two bellies and inadequate drain laying - if the foul run is at fault then it won't cure itself.

 

[Newboy]

This grey cap......is it blanking the pipe off completely or is it a durgo valve, i.e an automatic air vent?
John

Either a Durgo or an access cap - the description suggests a fault in the run. The Durgo is to allow negative pressure to be balanced, a blockage is likely to produce positive pressure causing the Durgo (correctly) to shut