I have a 1995 Deville with the 4.9L engine and have had cooling system issues with this car since I purchased it. First, there was a plastic T that was broken and leaking so I replaced that. Then, the radiator was leaking so I replaced that. Since the radiator replacement, I put a 1/2 gallon of coolant in the radiator reservoir. I came back a week later to have to put another 1/2 gallon in the radiator reservoir, twice. I do not see any physical leaks on the ground...

Any thoughts short of pulling the plugs and doing a fluid test?

The only thing I can suggest is to follow the cooling system an scan for any moisture. The water pump area is a known weak point. Look at the heater hoses, thermostat housing, upper and lower rad hoses, water inlet. Also check the pipe that runs under the harmonic balancer, sometimes those can corrode and leak if you are in the rust belt.

EDIT: I saw you are a fellow Milwaukeeian. where-abouts are you?

Well said, drewsdeville.

I dropped a wrench last week on that lower pipe under the harmonic balancer and it spung a leak.:bomb: I fixed it by getting a longer legnth of hose to replace the small length, lubed it up, and slid it over the hole and used a bunch of clamps! There is another small heater hose 1" in lenght, behind the front exhaust manifold above the A/C Comp. I also had a leak sometime ago on the metal neck of the lower rad. hose connection to the block. Had to do the same with a piece of cut hose.

Quickest way to find the leak is using a Pressure Tester. Many Auto Part Stores like Advance, AutoZone, ect. will loan it to you. You should probably take the thermostat out when doing this and look for bubbles in the tank. Do the test both hot and cold to see if you can smell it as it maybe evaporating when running onto something hot, or when cold, you may find the drip. Check the exhaust when running and look for vapor.......also, when hot, open the throttle butterflys and smell for antifreeze when the system is presurized.

A fluid test is most definitive for oil, and antifreeze. Do a google for any Diesel repair shops in your area or oil anaylsis ect........Catapillar, Detroit Diesel, Cummings as they do inhouse fluid testing with a spectrometer.

Try this : http://www.on-siteanalysis.com/pages/locations.cfm

Are you using the G.M. Rad. Tabs. or other?

I've used the bars leak full system sealer with great results on many cars. Its the one you have to fully flush the system with a flush and water. Also KW makes the same. After a flush, add it and let idle or drive around for a while even though it says not to, then drain and let dry overnight. There is also a 1 step product by Bars and others that is new which replaces the multi-step process. Also, stay away from the Blue Devil head sealer $40. I've heard mixed reviews so use it as a last resort.

You may have a leaking heater core and the fluid is going out the Evap Drain!

Are you using a new Rad Cap with the correct pressure rating? Also, do you have a Anti-freeze tester? Check the mix. Water will always evaporate especially in the resevoir so make sure you do a 75% water/25% Anitfreeze at least.

Also maybe try a lower pressure cap if there is a large leak somewhere.

Regards,
Chris

If the plastic T was broken, my money is on the heater core.

i had a leak on my 4.5 thought since the water pump went out that it was a problem with the seal . well come to find out what happened is when the water pump went two bolts in the water pump inlet striped out. the inlet and the timing cover you have to order from a dealer when you do it its a good time to replace the timing chain

Try by-passing the heater core. But you should smell it if it's leaking and there would be vapor or residue on the windsheild when on defrost most of the time.

I've had that problem with my 4.1, adding coolant after doing work that required draining the system. It seems to take some driving etc to get all the air out of the system. I looked around also for leaks, but it settled down OK after adding coolant a few times.