air in cooling system

befor you write off a leak, try pressurizing the system cold, and then again hot.....just to see what happens. One of my d1's had an intake manifold leak in the back corner where it's hard to see....would leave a puddle under the intake manifold ...on top of the valley pan gasket...but never made it to the ground when parked....just a thought

If you're getting combustion pressure into the cooling system (i.e. head gasket) the cooling system hoses will get rock hard when running. An inexpensive chemical block tester can check for the presence of exhaust gases in the cooling system. Since you had a recent service to the cooling system it's likely it just needs to be bled. Might help to raise the front of the vehicle and just run it for a bit with the cap off. I had a chevy blazer with the plastic tanks on radiator. It would only leak a small amount (at the core to tank seal area) during start up after sitting overnight or for longer periods. I have a gravel driveway so I never spotted anything on the ground. Over several weeks I would get the air noise in heater core. I finally spotted the leak when we had a little snow and I noticed a few green drips right at start up. As soon as it had a little pressure it quit leaking.

Most likely since the system was recently open that it wasn't fully bled of air.

Assuming of course you have tried the normal method of raising the overflow tank and filling until coolant comes out the open bleed screw...

Then as follow up, a few tricks: When you are up to temp, run the truck with the heat on high to circulate coolant through the heater core to get any bubbles to move out of there.

When the block is stone cold, first thing in the morning, open the bleeder screw and see if there is any air... being the highest point any bubbles should collect there... if any, replace that air with coolant, top off through the bleeder hole using a small funnel, etc.

You may have to repeat a few times to get it all out.

Dave03S said:

Most likely since the system was recently open that it wasn't fully bled of air.

Assuming of course you have tried the normal method of raising the overflow tank and filling until coolant comes out the open bleed screw...

Then as follow up, a few tricks: When you are up to temp, run the truck with the heat on high to circulate coolant through the heater core to get any bubbles to move out of there.

When the block is stone cold, first thing in the morning, open the bleeder screw and see if there is any air... being the highest point any bubbles should collect there... if any, replace that air with coolant, top off through the bleeder hole using a small funnel, etc.

You may have to repeat a few times to get it all out.

Click to expand...

Walt has a D1. So, no bleeder screw. And in the D2, I believe, the heater core always has fluid going through it regardless of whether the heat is on.

For what it's worth, my '96 did this a while back. I bled the system got a few bubbles out and never had an issue again.

Yep; before you start pressure testing, let's bleed it a few more times.

Occam's Razor, people.

I have encountered probably 2 or 3 DIIs that seemingly had air in the cooling system that would never bleed. One of them I did a head gasket on which didn't fix it and it ended up getting a motor under warranty. That one specifically would be able to go 1-2k miles without ever overheating, then randomly it would overheat.

One of the other ones would act like the thermostat wasn't opening. It would overheat but the thermostat would still be closed. I wedged the thermostat open for the guy and it was fine for a long time.

Make sure your coolant does't get low and just keep driving it. If it isn't overheating, there likely isn't a large air pocket.

One other thing, block test kits are extremely unreliable. If it passes, that doesn't mean anything. They pretty much only work when you have a really bad problem and don't need the block test kit to tell you i.e. clouds of steam coming out of the exhaust.

I've found driving around the block for a few minutes with the cap off (from cold up to full temp) has been a effective way to bleed any remaining portion of air out. Obviously be cautious of the hot coolant bubbling in the expansion tank