My '96 Tahoe with the 5.7 Vortec 5700 SFI V-8 has me puzzled over where the coolant is going...leaking. Some times I add about 8 oz. to fill the radiator to the top, and other times I may add about 12 oz. This has been going on for a year or more. There is no external leak to be found on the engine, the radiator, or heater core. My first thought it was moving by the intake gasket into the motor oil. I drained the engine oil and taken it to a person that gets paid to examine engine oil for coolant. There is no evidence there is any coolant in the oil. So, here is the only theory where it's going...into the combustion chamber and burning with the gasoline. Is this possible?

The best way to check for coolant in the combustion chamber is to pull each spark plug and check for coolant contamination. My '98 7.4 uses coolant. I know it's going to the #7 cylinder just by looking at the plugs. I have no external leaks. It could leaking past the intake or head gasket or cracked head.

A good radiator shop can pressure check your coolant system for leaks.

Another thing is to keep your coolant recovery tank full and watch for a drop in that. If it stays full and you have to add coolant to the radiator then there is a serious leak in the system.

Are you filling rad or overflow tank/
How long does it take it to leak out?
How manny miles on it?
Has intake gasket ever been replaced?

For hard to find coolant leaks they make a dye to put in the coolant.

Pressure testing cold and warm and hot may show the leak.

Watch all hose connections and intake gasket and water pump and rad and overfow tank and hose and heater core for seapage cold/warm and hot.
Also look on under side of engine and back of engine and ac drain hole.

And as said pull plugs and check there.

Are you filling rad or overflow tank/
How long does it take it to leak out?
How manny miles on it?
Has intake gasket ever been replaced?

For hard to find coolant leaks they make a dye to put in the coolant.

Pressure testing cold and warm and hot may show the leak.

Watch all hose connections and intake gasket and water pump and rad and overfow tank and hose and heater core for seapage cold/warm and hot.
Also look on under side of engine and back of engine and ac drain hole.

And as said pull plugs and check there.


I put the coolant in the radiator, and it may take a 100 miles before it's needs some more coolant...maybe 8 oz at best. Tahoe as 112,450 miles, and the intake gasket has never been replaced. There is no leaks anywhere on this engine. Not one tiny bit of coolant that I can find. I've checked when and cold, and right after having driven it several miles.

However, I have notice that inside my exhaust pipe...rear tip is reddish/brown color. If memory serves me correctly, it's always been dark/blackest color since the day I bought it new in 1996.

Keep on looking for leak.
Always watch engine oil for coolant in it.
It should show up some place.
Try some coolant dye for the hard to spot leaks.
Does adding coolant to overflow tank keep rad full or does the coolant tank run dry before rad gets low?

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Does adding coolant to overflow tank keep rad full or does the coolant tank run dry before rad gets low?

The reserve reservoir tank hasn't held coolant in years...at lest six or seven.
If I add coolant to the reservoir tank, it's gone on in days. I've never considered those reservoir tanks to be a reliable source for accuracy.

The reserve reservoir tank hasn't held coolant in years...at lest six or seven.
If I add coolant to the reservoir tank, it's gone on in days. I've never considered those reservoir tanks to be a reliable source for accuracy.

That may be your problem.:rofl:

The reservoir tank has to be working and coolant in it to keep the rad full.

Repair the tank and make sure the rad cap and hose between rad and tank is good.

let us knw how it goes after you fix that tank.