Low coolant light on, Air in reservoir

[oldmansax]

Low coolant light on, Air in reservoir

New Problem!! When I stop & let engine idle low coolant light comes on. After I drive away or raise RPMs the light goes off ( 30 seconds to 2 minutes). No coolant loss. Every thing checks full. Also, there seems to be a small amount of air (bubbles) in the coolant reservoir. Engine runs fine & not hot even under load (lowboy trailer with 8000lb tractor). Anybody else had this problem?

[Mainous]

Are you saying the truck is not low on coolant when the light comes on? Everything else you describe would point to a leaking head gasket.

[oldmansax]

There is still coolant in the reservoir, but very little. Assuming the coolant sensor is the device in the front of the reservoir with 2 wires coming from it, the coolant level is below that. But, coolant is back to normal when I start it cold in the morning and I guess it comes back up while driving because the light goes off. The sequence is: light is off upon cold startup in morning. Stays off as long as I am driving (RPM up). Comes on when I stop for stop light or sign or any other stop after about 30 to 120 seconds at idle. Light goes off after returning to highway speed after about 3 to 5 minuites. I was thinking head gasket too except it's not a lot of air & I haven't lost any coolant. I have been driving it for a week now & the coolant just keeps leaving the resevoir & coming back. Is there any special way to make sure the system is completely full? Thanks--

[Mainous]

There is an airbleed on top of the thermostat housing that you can open. It's the highest point on the engine. I would get the truck warm (thermostats open), top it off and then open the bleed. Then check to see if it needs topped off again. If you run low on coolant again, you have a head/gasket problem.

[oldmansax]

Thanks! I'll try that & see what happens

[ucdavis]

I have a similar condition, though not at idle.
After ascending a good size hill and beginning descent (i.e. engine is about as hot as it gets, maybe 220-230) the light comes on for about 30 seconds then goes out. Never use any coolant.
I assume this is simply the engine sucking in coolant as engine cools off rapidly to 190 or so.

[MartyB]

You are a describing a similar problem that I had and have a thread going, over the weekend. I may have some air bubbles causing the light to go on. I also have had it go on when going over a bump bigger than expected like RR tracks.

Maybe the sensor is haywire now that the truck is older, and more sensitive to air etc.

[oldmansax]

I took it over to my mechanic yesterday & he is not convinced it is a head gasket mainly because it hasn't gotten any worse over the last week. I've put almost 1000 miles on it since I first noticed the problem last weekend. He is checking with the regional Detroit Deisel distributer to see if this is something they have seen before. I'll post here whatever they say

[toyboxrv]

I've had intermittant problems with the low coolant light coming on when it's not low. Usually on the same hill or the same turn each time, but it doesn't go off until cooling down overnight. Hasn't done it for a while and I never lost any coolant. A weak head gasket may let compression pressure past, but not lose coolant though, at least until it fails completely.

[ZZ]

My truck's low coolant light comes on daily. I can come home and ease the cap off and the radiator will be full to the neck with the light on. I've put two new sensors in it and they all do the same thing. I haven't noticed any air bubbles and there is no coolant loss.

[oldmansax]

I finally pulled the engine down last week, (put it back together Tuesday) & found it did have a blown head gasket. I'm glad I don't have to do that for a living. After repairs, it runs fine. I drove it to Ohio over the weekend & brought back a 9000 # trailer. It didn