Car idling rough replaced head, pistons

[Zish5595]

Hi, I have a Vw golf Ed30 my timing chain had snapped whilst driving so I replaced the head with new valve stem oil seals I replaced the pistons as these were also damaged. Having replaced the engine I also had water pump cambelt snd also a new oil pump fitted to the car. When the mechanic has swapped heads and built the car back up on idle it’s revving rough when the revs drop below 1k. A smoke test and also a air leak and vacuum test has been done but there are no leaks. When you drive the car it’s running fine it’s only when stationary there a are at idle is when it shakes the car and idles rough? Can anyone help me with what this could possibly be.

[LC5F]

How does it feel while driving at higher engine speeds?
-does it produce the expected amount of power or feel gutless?

First thing to do is look into the engine timing, all the timing marks need checking on timing belt and chain

[Zish5595]

Thanks for your reply, I’ll get the timing checked.

Yes the car makes expected power when driven.

One thing that may make a difference is that the aftermarket pistons I used had small reliefs cut away almost to give the valve more room to travel downwards. The oem pistons weren’t like this. Could this have affected compression or something and cause the issue?

Also, as the car is mapped for no extra power but just to account for the decat turbo back exhaust,  could it needs a custom remap to sort idle?

[doylebros]

If that was my problem my approach would be getting it checked with VCDS and look at the values of reply #43 within this thread

https://www.mk5golfgti.co.uk/forum/index.php/topic,106158.30.html

[ROH ECHT]

Timing chain stretch needs to be checked with VCDS - MB 93...should be no more than 3° at warm idle.
Valves in the PCV and rear crankcase breather tube need to be checked.

I am leaning toward it still being an undetected leak due to the behavior because a leak doesn't get bigger when revs go up. So it will affect idle but its effect will lessen when revs go up.

[LC5F]

Ah - thought you had fitted standard pistons
They sound like high compression pistons, and are a big change, your engine still thinks its standard and may not be fuelled correctly - speak to a tuner