Hi
I loose coolant and i don't know if it natural because the summer . (250 ml to weak)
I searched for leaks and don't found nothing .
Maybe i have leak from the intake manifold ? How can i be sure ?

Thanks

250 mls (roughly 1/2 pint) lost in one week? No that is not normal or due to evaporation in the summer heat. Are we talking about a 3800 here? If so the LIM gaskets are probably on there way out. I don't think you're losing enough coolant yet to be in danger of hydrolock, I had the same issue on a 04 Bonneville SLE at around 140,000 miles. I had Blackstone Labs do an UOA to confirm the presence of coolant in the oil, came back with 0.8% coolant in the oil, caught it in time and had the repair done.

I had one of the 3800 (mine is a 199 with the upper intake/plenum burn through at the EGR stove pipe. I posted about it on here. If I can find the link to that post I will add it.

https://www.gmforum.com/buick-172/co...intake-308386/

If that is your problem; you will have to inspect to see... If you are not seeing any external coolant leaks, such as the radiator; drips anywhere, etc; and just park the car on a sheet of corrugated board after letting the engine reach normal temp to find drips, then it has to be an internal leak. You can check the oil but it may be burning it but not enough that you see it in the exhaust.

However, what I was going to say is that if that is your problem then it is an easy fix; not a lot of effort. The only thing I ran into was the elbows; I was working alone and I finally decided that removing the belt tensioner was better than risking damaging the lower intake gaskets and yes, you will do 90% of the labour to replace the LIM gaskets so it makes sense to do so and they are not that expensive; look on RockAuto or Ebay.

You will need an inch pound torque wrench because the torques are low; especially on the plastic Upper as it is easy to crack.