I’ve replaced the radiator, cap, coolant lines & the thermostat thus far but I’ve not solved this problem:
When the vehicle is driven about town at under 60-65 MPH, it never (months) runs out of fluid but take it on the freeway at 65+ MPH and it runs out of radiator fluid in 2-3 hours and after short cooling time, can run out very quickly if I return to freeway speeds right away.
No specific site of leakage obvious when I stop, overflow bottle is usually still full. The correlation to speed seems consistent but could be a chimera for heat from the transmission (automatic) or something.
This vehicle has the little 4 (I still think in CID, so I can never keep track of the engines measured only in Liters), no power steering, A/C (that hasn’t worked in a couple of decades), Automatic
A simple and inexpensive cooling system pressure test might be in order, some parts stores will loan you this tester for free…Otherwise, any repair shop can do it. This will make any external leak show itself…I assume there is overheating associated with this…? After 20 years of service, radiators can become plugged up with deposits or the thin cooling fins placed between the core tubes get eaten up by road salt and the radiator simply stops working…The faster you go, the hotter the engine gets…If the coolant recovery system does not work, this is a sign of a blown head gasket…Engine gasses blow the coolant out of the cooling system and keep the coolant from returning to the radiator…
There are a few tests I’d recommend.
You can start with a simple pressure test of the cooling system. That just might disclose a leak.
You can also add a UV sensitive leak tracing dye. You add it to the coolant, take your drive, and turn on the blacklight. The leak path will glow. The kits are available everywhere.
Since it loses fluid a lot more rapidly on the highway, it’s very possible that the seal on the water pump impellar shaft is failing, so be sure to check under the car at the front of the engine. There’s a weephole there where a water pump seal leak would leak out of.
If you still cannot find it, you may want to consider the possibility that the coolant is being drawn into the cylinder(s) via a breech in the headgasket. You might be able to determine this with a simple test that checks the coolant for the presence of hydrocarbons, a cylinder leakdown test, or even just opening the radiator cap (while cold) and running the engine… if bubbles come out the fill hole, you probably have a headgasket breech. You’ll want to confirm with one of the other tests.
Kits for all of these simple tests are available at any parts store. Post back with your results.
Why did you replace the coolant lines? Is your transmission fluid milky, or you engine oil? If neither are milky I would suspect a clogged radiator or a blown head gasket. If it is the radiator, replace it, if it is the head gasket replace the car. Unless you have always wanted to learn how to replace a head gasket.
Radiator had deteriorated around the seal area for the cap, so it was the first thing I replaced. I’ll run a pressure test – really hoping it isn’t the head gasket as this is already turning into the most expensive ‘free’ vehicle I’ve ever been given!
Thanks to all for the advice – really hoping the coolant pump impeller is the answer - no bubbles in the fluid at any engine RPM (and I replaced coolant lines because they didn’t come apart so well on a '94 Michigan vehicle and started leaking (pin holes weaping) after I put in the new radiator!)
Whoops…I missed the comma between radiator and cap…
Here is a little trick…Remove the feed (hot) heater hose from the firewall fitting and plug the fitting. With the radiator cap off, place the open end of the heater hose in the radiator filler neck. Start the engine. Observe the flow from the hose at various RPM’s Once above 2000 RPM’s or so the coolant discharge from the hose should be heavy and have considerable force to it…If the flow is weak, then suspect the water pump or the block and head cooling passages are badly corroded…
Affected by engine load and no visible leak? hmmm … I also am thinking head gasket. I’d rule that out before thinking it is something else. An easy thing for an experienced mechanic to check.