I was driving my 86 down the highway this evening and my LOW COOLANT LAMP came on. Then as suddenly as it came on it goes out. It would come on under heavy accelleration it would come on. I pulled over, popped the hood and made a visual inspection. Overflow tank has coolant, Temp guage reads 178 degrees. Oil pressure at 39lbs. Did a sniff test and I don't smell anything. nothing on the ground. Where is the sending unit located and how can I test it.

 

You simply have an air bubble. You need to "pack" your radiator.

It's done like this. Start the car cold, let the car idle and periodically squeeze the upper radiator hose with a rag on your hand. When the hose does not feel hard, it means the thermostat has opened and coolant is now circulating. At this time have somebody raise the engine RPM to about 1500 and hold it there. Raising the RPM's drops the coolant level in the radiator. With RPMs raised, remove your radiator cap and top off your radiator with antifreeze. Put the cap back on before letting the RPM's down again. This "packs" the radiator.

 

Hmmm. I've got the same symptom since replacing the radiator. Peridoic "low coolant" indicator. I'm a little uneasy about removing the cap once the thermostat has opened. Wouldn't the system be pressurized at that point? Sounds like it could hurt...

Am I missing something in this explanation? Thanks in advance.

 

Common C4/L98 problem that can easily be overrided. Simply disconnect the wiring plug from the radiator sensor and attach it to ground. Viola!

Of course this deactivates a critical warning system so I have fabricated a special grounding wire from the radiator to the chassis. This seems to take of the problem yet the warning will still illuminate "out of the clear blue".

Yes I have packed the rad and have drilled a bypass hole in the thermostat. Just one of those crazy problems that comes and goes.

 

I have an 86, and I get that every once in a while, usually after changing antifreeze. Then it comes and goes, and when it's cold, I remove the cap and "pack " the radiator.

When doing the "packing", the cap is off from cold to finish. Do not turn the car off after starting it, have the antifreeze bottle open and sittin on the coolant reservoir, rev the engine up to about 1200 RPM, and add the antifreeze till full, then reinstall the cap, THEN let it idle.

Two things I think you are fighting here is the 5/16 hose that goes from the radiator to the coolant reservoir gets a crack in it, and although the engine gets hot and relieves the water/coolant mixture into the overflow bottle, the cracks in that small hose prevent the water from returning to the radiator.
The second thing you need to do is change the radiator cap.
Change that hose, and put a new cap on it and I think you will be back in bizness.
One way of determining if these items are faulty is to remove the coolant dip stick, dry it off, and make a scratch on the dipstick with a knife or razor blade or use white out to mark the current water level. If it keeps going up, either of those two things is the culprit.
If you haven't changed the radiator cap in 3 years, just go buy one.
Chris

 

Coupguy: Thanks for your explanation. I did replace the cap with the new radiator. The hose could be old and decrepid. Good tip.

I'll try the "pack" procedure with the cap off and the coolant jug nearby. Since I replaced the radiator with a higher capacity radiator (dual core) and the weather has gotten colder, I have not really seen the temp go above the thermostat temp. It seems I need to run it to hot and let it "burp". Thanks!

 

On my 88. Coolant sensor was 2 in or so below cap. When thermostat popped open it would draw and level would drop. I tried the burp (let air out) method that worked for me.
Loosen upper Rad hose at Engine/Thermostat slip it off some to let air out its higher than Rad cap. Or even hold hose up and pour some Coolant into the hose and reinstall.