My truck is beginning to lose quite a bit of coolant and I can't for the life of me figure out where.

When I say quite a bit, I mean about a quart every 5,000 miles - which given my commute of 100 miles a DAY, is far too much for my comfort.

It doesn't drip coolant, none on the pavement or elsewhere. I changed the radiator cap (the old one was cracked), I drained the coolant and changed it - nothing strange at the petcock. I do notice some scale on the upper radiator hose connection but it's been there a while.
I changed the O2 sensors (sensors went bad) and didn't notice any signs of coolant on them. It was my understanding that a leaky head gasket will ruin an O2 sensor quickly.

Where the heck is it going? If it ain't the pump, head gasket, radiator cap, or hoses, where's it leaking out?

is my best bet that upper radiator hose?
Well now that I'm thinking about it, might as well clean the hose of scale and see what happens.

 

Not seeing an external leak sign, having the O2 sensors go out, may be clues that you have an internal coolant leak.

The 3.0L has a history of head cracking problems, especially if it's been overheated.

Do you see any signs of milkshake deposits in the oil, on the dipstick, around the oil fill cap, or in the PCV valve, or it's hoses????

Have you had an oil analisis done, to determine if coolant is showing up????

 

Oil analysis?

I don't see any evidence that there's coolant in the oil, nor does the truck behave like one that has a cracked head. My sister's Grand Prix cracked the head and it was obvious - overheated like mad, ran poorly, billowed steam from the exhaust, oil turned into a gloopy mess. Nothing like that on my truck.

I was just assuming it was evaporating from the overflow tank or leaking past that upper hose.

Assuming I want to test my oil, how does one go about that?

 

At this point imo an oil analisis would be worth the money. Look in the forums list of supporters for Blackstone Labs & request a no cost oil analisis sampling kit that has instructions for properly pulling the sample & how to mail a sample in for analisis. The scan is around $20 & at this point, with the amount of useage you have, is probably worth doing if it's an internal problem.

If it's not getting into the oil & no visable signs are on the ground or externally on the engine, it could be an exhaust manafold problem, as that could evaporate, but pass coolant onto the O2 sensors, giving them a whiff & taking them out. If thats so, but the leak is so small as not to cause steam & a white cloud or show up in an oil sample, you should still be able to smell it in the exhaust, as a sort of sweet coolant aroma.

If you think it's an external leak, there is a dye kit with a UV/black light, available at most autoparts stores. You add the dye to the coolant, drive around to mix & circulate it & have the leak carry the dye to the leak point, then at night search for the yellow/green glow of the dye, with the UV light.

If you can't find an external leak suspect, pull the spark plugs & look for dye signs on the spark plug internal ceramic insulator when using the UV light, they'll show/glow as a bright yellow/green color.

You know it's using/loosing coolant, so the dye should be able to find the leak source if it's external & maybe confirm about where it may be internally in the engine, if it's there.

 

Another thought crossed my mind, if you suspect your coolant recovery tank, or it's hoses may have a leak, put some of the dye in it & that'll point to the leak spot.

 

How old are the radiator hoses? How many miles on the truck? If the hoses are original equipment, they may be in need of replacing. Since you are suspecting the upper radiator hose, it may be worth changing it. The replacement doesn't cost that much and will allow you to cross that source of the problem off your list.

 

That's a deal, add some dye and eliminate the cheap solutions first.

Thanks, I'll give it a whirl

As for your advice Michigan66, yes they are all the original hoses, so the idea of replacing them as a general maintenance precaution is a good one.

This HAS to be one of the toughest trucks I've ever owned - the hoses and thermostat are all original and none of them are showing any signs of being worn out (well, maybe the upper radiator hose - but that's why we're having this conversation), heck even the headlines, tail lights, blinkers are all original.

 

Knock on wood.

 

You may also have a leaking radiator up at the top seam. Try releasing the cap and thuse removing the pressure. My radiator leaks there, but as long as I keep the system unpressurized I don't use but very little coolant. YMMV

 

Check out: What is Oil Analysis

I'm with pawpaw on this one. All things considered, $25 isn't all that much to fork over in order to get a real good look as to what is going on deep inside the engine. You'll probably spend close to that much on a radiator hose and/or a dye kit.

 

If coolant is getting out thru the combustion chamber, pulling the plugs after an overnight cold soak and examining them with a magnifying glass might reveal small droplets of liquid on the plug(s). The coolant pressure remaining after the engine is shut off will force it out any cracks as the engine cools. I've discovered a cracked head that way, even when the engine was running good. Not as reliable as an oil analysis, but it doesn't cost anything.