Hey everyone. It seems that every few weeks to a month I notice that the Jimmy needs coolant (reservoir level goes down to near "ADD" line) added. This is very surprising since I don't drive it that much nowadays now that I have the Trailblazer. I just drive it around town on short trips before work. Not even a few hundred to a thousand miles between 3-month oil change intervals nowadays, but I don't drive that much in terms of mileage on either truck anyway. However, I cannot seem to find out why it keeps disappearing. But here is what I have so far in terms of info:

1. Does not appear that there is any coolant in the oil. Normal color/smell
2. Coolant was changed about 1-2 years ago and is green
3. Water pump is 2 years old and no leaks
4. Intake manifold gaskets were replaced in 2009 and while there is a tiny bit of oil on the front of the engine, 3 mechanics said it is fine.
5. Hoses appear to be in good shape.
6. No antifreeze smell unless I just added some.
7. Oil cooler lines just replaced, mechanic didn't see anything else wrong around there
8. Exhaust gives off some steam for a couple minutes on startup especially if it hasn't been driven in days and it is cool out, but seems to be normal. Cannot tell if it smells different

What else should I be checking? At 128k miles, could this be something bad like head gaskets? Thanks

 

The thermostat housing has a additional length of neck added to it because the radiator hose can't clear the T/B. Check for "smaller than a pin hole" leak/leaks where they are joined. Found one on mine a few weeks ago after dealing with a situation like yours.

 

Check the overflow. Even if there's no signs of antifreeze leaking from it, it still could be. Once it's hot and pressure is built up, antifreeze could leak from a tiny crack that's near impossible to see. My truck was leaking antifreeze and I couldn't find anything, so I took a chance and replaced the overflow despite not seeing any leaks in that area... fixed the problem.

 

Small intake leaks can cause coolant to disappear yet not show visible signs in the oil, at least 50% of your coolant is water and if the engine runs up to operating temp and has a functioning PCV system all of the water gets steamed off, leaving only the concentrated silicate based antifreeze that destroys these engines, it's only when you have a major leak that you will definitely see the chocolate milk looking oil, im not saying that this is your problem but dont rule it out just because the gaskets have been changed before, lots of shops dont take the time to really clean the mating surfaces or torque things properly and most use the cheapest parts available, which means they replaced them with the same 20 dollar plastic junk the factory installed, except this time the surfaces have oil/chemical residue and minor warpage so the gaskets will have an even shorter life, the fact that there is oil seepage is a sign that the materials or practices used in that repair job were sub par. A quick way to find out is by having a chemical oil analysis done, you can buy the oil analysis kit at most major auto parts stores or order one online, i used a WIX 24077 not too long ago to verify a cracked head on a friends tahoe (There was a service bulletin about some bad cylinder head castings and he was having coolant disappear slowly), sure enough the results showed high levels of sodium and potassium and said to STOP running the vehicle, also reported water as less than 0.05%, theres proof that water evaporates out of the system, sure beat pulling the heads and having them pressure tested. The kits are $13 on Amazon, it will cost you $5-$10 to ship it in after you have the oil sample but i got results emailed to me within a week. All you have to do is pull the plug and fill the jar while the oil drains, i think they suggest the engine be warm or recently shut off so the oil is mixed up, if heavier contaminants have had time to settle out you may not get an accurate report. If you do this you can rule out anything internally engine related and focus on external, Pump Gaskets, hose connections, thermostat housing, cracked radiator tank....Also dont forget the heater core and hoses, check the passenger carpet for wetness and the A/C drain for green drips, you would likely smell that inside though.

Good Luck!