Keep adding Antifreeze

I have to add about every 40 days but I can't find where it's leaking. How do you tell if it's a head gasket? I don't see any in the oil. 4.8L 77,000

How much do you add?

It drops down about an inch from the full mark. I don't see it on the ground or anywhere under the hood.

Does not sound like a head gasket.
You may have a small coolant leak cold or hot or warm.
Check rad sides and waterpimp seap hole and all hoses rad and heater and around intake gasket and watch borronm of heater core housing and also coolant tank and tank return hose.
And last but not least a bad rad cap will let coolant out.
If GM pink coolant it will leave a little pink stain where it is leaking.

A pressure tester on rad cold and hot amd also watch on warm up or about half warm may show the leak.
Different leaks will leak at different temptures.
I had a rad one time that would pour out a stream at one certain tempt on warm up and not leak at any other tempt.

They make a leak dye for the coolant system.
Helps spot small leaks.

Good Luck and let us know how it goes.
MT

Thanks I will check all areas suggested. Encouraging news that it's probably not a gasket.

Stupid as this may sound,I seen some vehicles for whatever reason that will not stay at the full mark . An inch from full was not a problem, but ya I had keep an eye on things just incase .........lol

Interesting I just had a mechenic tell me the same thing. Instead of topping it off I'm going to let it go but keep and eye on it.

i think it's the water pump. Is aftermarket ok and is it a big job? I've done them in older vehicals but not this one.

Not hard.
Older and newer GM truck v6 and v8 water pump replace are all about the same.
Some have ps and air pumps and ac belt stuff.
Yours may have a seperate AC belt. Check it real close.
I HAVE HAD A LOT OF THEM BREAK.
A couple of hrs should get it.
Check all hoses and belts when you are replacing it.
And a good time to flush coolant system and new thermostat.

Yes aftermarket is OK on water pumps.
But I would go a good brand of brand new not rebuilt with aftermarket water pump.
Have you found water pump coolant seapage at the weap hole?
MT

I have not found any leaks on my sil 1500 5.3 but it is possible that the thermostat housing gasket is leaking. being as you know in a location that is hard to inspect. i found that the gasket is not very thick and it could easily leak. check for leaks after vehicle cools down over night before you start up the next day. as mentioned weeping coolant out the pump vent hole is another area to check especially if the engine runs ok. on my vehicle the only way you can check all this is to remove the air intake ducting. another area observed that was designed poorly is the radiator drain valve . good luck ...........

I have not found any leaks on my sil 1500 5.3 but it is possible that the thermostat housing gasket is leaking. being as you know in a location that is hard to inspect. i found that the gasket is not very thick and it could easily leak. check for leaks after vehicle cools down over night before you start up the next day. as mentioned weeping coolant out the pump vent hole is another area to check especially if the engine runs ok. on my vehicle the only way you can check all this is to remove the air intake ducting. another area observed that was designed poorly is the radiator drain valve . good luck ...........

On the hard to find leaks the coolant dye leak test kit works good.
MT

[quote=MT-2500]On the hard to find leaks the coolant dye leak test kit works good.


If i can't find leak buy the methods i explained before, i use the pressure test method. this i use not only on coolant but have used to locate and test for leaks on mercruser outdrives before mounting on engine, and after water pump impeller replacement.

[quote=MT-2500]On the hard to find leaks the coolant dye leak test kit works good.


If i can't find leak buy the methods i explained before, i use the pressure test method. this i use not only on coolant but have used to locate and test for leaks on mercruser outdrives before mounting on engine, and after water pump impeller replacement.


Pressure test and looking at it is good.
But on the hard to find ones or the ones that leak psrt time or at certain temptures.
The old coolant leak dye will tell you where the hard to find leaks are.

[quote=j cAT]


Pressure test and looking at it is good.
But on the hard to find ones or the ones that leak psrt time or at certain temptures.
The old coolant leak dye will tell you where the hard to find leaks are.
yes I have used the dye to find freon leaks but i have never needed this on engine coolant leaks. i guess i'm lucky on that failure.

[quote=MT-2500]
yes I have used the dye to find freon leaks but i have never needed this on engine coolant leaks. i guess i'm lucky on that failure.

The coolant dye works just like the AC dye test.
Not needed often but on the real hard to find or strange leaks it does help.
Some in intake cover up lines and water pump leaks and hard to get to stuff can be a pain to get to leak and I have had leaks that you would have to drive a few hrs or a day to get them to leak and others that would only leak at a certain tempture.
The dye will show the area of the leak even after it has leaked and dryed up or quit leaking.
MT

Pretty weird, checked my reserve bottle and it's now on the full mark.Can anyone explain why?

Hard to say but staying full is good news.

I would try a new rad cap to start with and keep a eye on it.
Good luck
MT