Thread: Low Coolant light during Highway driving only

Hi, new to this forum.

Saw a similar thread topic to this but it seems to have gotten a bit off focus.

About 18 months ago I had the UIM replaced on my 2002 GP 3.8l Naturally Aspirated v6.

I was getting a low coolant light after driving on the highway for 5 min or so. The minute you slowed it went off.

The problem is back, and I am thinking the UIM was fine.. Oh well now.

So I have done some more playing. This time oddly after I refilled the overflow the light still is coming on at highway driving.

So today on the way to work, when the light came on I shifted to neutral. Light immediately went off. Put back in drive and it took another 3 to 5 minutes for the light to come back on. Once the light came on I again put in neutral but kept the RPM's up light stayed on. Took foot off the gas light went off. Back into drive no light about 3 to 5 minutes later light came on as I was exiting the freeway - coasting rpms were low below 1500 I shifted to N (little to no change in RPM) light immediately went off.

When I got to work the parking lot was to dark to see what level the coolant was at in the overflow bottle.

This behavior has been very consistent so I don't think it is a bad sensor.

A few things I don't know... Where is the low coolant sensor in this system.
Is it possible with engine roll (which is for-aft right) some line is getting pinched... eventually enough coolant is pumped at the restriction (increasing pressure) that the level seen at the sensor drops, once the engine roll comes off the line opens a bit and lets the coolant flow? Of course during coast down the engine roll would be in the opposite direction so not sure why the coast down part happened....

Your coolant level sensor is in the passenger side of the radiator about 1/3 way down from the cap. It's the square sensor under the top trans. line. Can't miss it.

Have you checked the coolant level cold? Is it full? If so, then you may have some air in the system causing air pockets to get trapped.

My 00 GT did this exact thing when the coolant was low. Light on during hwy speeds, let off just for a moment and light goes out. Return to normal speeds and light comes on. Mine was due to a cracked rad.

What I would do is bleed the cooling system real good, top it off and take it for a few drives. IF the light comes back on then you need to look for leaks. You say you had the UIM replaced already, but have you done the LIM gaskets and coolant bypass elbows? If not I'd be willing to bet that those will be what's next to replace seeing as how you prob. have at least 130K on the car by now, just a guess.

I think the elbows were done with the UIM, car has about 80,000 miles on it. When this started the coolant was low, but I have filled it now and it still does it.

Then I would try bleeding the air from the system and go from there. If you aren't familiar with the procedure, there is a small bleed screw on top of the thermostat housing. With the car running and hot, open the bleeder screw and watch for the coolant to stop spitting out and have a steady stream. Close the valve and drive it a bit. Make sure your overflow is full and then let it cool down. Check the rad level once cool and see where you are at. Top off as necessary. These cars can be hard to bleed sometimes. I had to do my 00 GT twice to get all the air out.