[FirebirdHank]

I have a coolant leak that is baffeling me. After the car ('69 Firebird 350) sits for a while coolant shows up under the intake manifold by the thermostat housing. It could be the thermostat housing gasket, the O-ring @ the front of the intake manifold or the manifold gaskets (the upper rad hose is new and the clamp can't be any tighter). While the system is pressurised there is no leak to be seen. After a couple of days sitting in the garage there is collant on the floor and the source can be traced to the area under the thermostat housing but no hint as to where it is coming from. I don't want to replace the intake manifold gaskets if I don't have to.
How can I detemine where the leak is? Pressure testing will not work since it doesn't leak under pressure.
It isn't a major leak by any means but it is starting to p--- me off.

Help

[Pontirag]

you nailed it with the list of possible sources or the leak

now it becomes a process of elimination

not for nothing but a leak is aleak is a leak. if it leaks with no pressure, its bound to leak more under pressure.

usually one leak is the tip of the iceberg you my be chasing a few leaks, one after the other over time. better to re seal the cooling system all at one time and watch those freeze plus too

[MNBob]

My bet is the thermostat housing gasket. The aluminum water necks/housings can warp and will not seal even with a new gasket.

[mchell]

I have cracked those old aluminum t-stat housings while tightening them down...

the o ring between the water crossover and water pump housing is also a likely place to leak...

...a small leak will sometimes not show while the car is hot and running due to the fluid evaporating quickly on a hot surface and getting blown around by the fan etc...

[68 Firebird]

Had a t-stat housing crack after changing thermostat. thought it was gasket and
when housing was removed you could see it clearly. new housing solved issue.
and tightening bolts carefully!!!

Gerry

[indymanjoe]

Mine leaked at the top of the water crossover(pass side),then it would travel along the underside of the intake drip onto the valley pan and pool up on the top of the valley pan under the t-stat housing.Drove me nuts as i thought it was the t-stat housing lol.use an inspection mirror and look for water tracks..

[rexs73gto]

My guess would be the cross where the intake & the water pump housing come together . I just put mine together & it leaked there as the new rings that come in most of the intake kits are now a little thinner then the old ones. I pulled my intake & put in an older left over one from an old kit & the leak stopped. So if you just installed a new intake gasket , that would be the first place to look as after the car is off the pressure in the system goes up before it goes down.

[lust4speed]

Take a rag and rap it around just one of the areas in question. The puddle of coolant should come back until you get the rag around the leak.

[rexs73gto]

I was in the process of breaking in a new cam with mine & it started to leak after it built p the pressure & then it started to blow steam out of where the seal was leaking. I had to wait until the pressure came down & then had to mop up all the coolant that leaked out. As soon as it got cool enough to take apart I pulled it apart & what I found was not what I liked. The reason it did leak at that seal between the intake & water pump housing was from the CHEAP seal the came with the Mr. Gasket intake set. When I looked at it close I found that the seal was about 50ths thinner then the old one that I took out. The new seal was thin enough that if you don't get enough sealer (silicone sealer) to help fill the gap between them the ne seal won't seal . I got lucky I looked through my stuff & found an older seal from an old gasket set & it was thicker by a lot just by looking at it you could tell it would work. I guess what I'm saying is look close at your seal before you install you intake to make sure it's thick enough to take up the distance between the intake & water pump housing. I put the older style in & it no longer leaks.