Folks, I really need your opinions. I have a 99 Montana, same as the venture. Last year, I had the engine replaced (not new) because the original engine had the typical intake, head gasket leaks, and two cracked heads. Found a good used engine and swapped them out. The van has run like new until a couple of weeks ago when I had to replace the water pump. Now, the van will run for a while at normal temperature, and then over heat very quickly. When it does this, I notice all the water/coolant is in the overflow tank.

I thought that maybe I had air in the system, so I drained the system, opened the bleeder screws, and refilled. This seemed to do the trick, and I drove the van that evening with no issues. Next morning I began having issues again and had to add more water. It cooled right down and I drove it back home. I checked it out after work yesterday, and the overflow tank is almost full and overflowing. I don't think that I am losing water. What in the world could be happening? I checked it this morning, and the overflow tank is still almost full, and the radiator is full up to the bottom of the neck.

It's almost like it's building up pressure...one evening it overheated, so I cracked the radiator cap a little and it cooled right back down. I'm so frustrated that I can't diagnose this!

 

I am not a mechanic. But if the problems started when you replaced the water pump, you might want to start there. It may be an odd problem like the pump is turning backwards, If the pump has a plastic pulley it may of somehow cracked and is not circulating the coolant properly...If you are getting unusually high pressure it could also be a head gasket ????

 

I'm pretty sure it's not the water pump because it's all metal, and I checked it out carefully before I put it on.

The head gasket thing worries me. I wasn't sure if a blown HG would cause these symptoms or not. I fear that may be the problem.

 

The HG can cause this, but I would rule out any leftover air in the system first. Try letting the van sit and idle with the nose up a steep incline. I do this whenever I change coolant on all my vehicles and I watch for bubbles in the overflow bottle. Then I take it for a boot around the block and repeat the process a couple of times till the temp guage settles.

To check the HG get someone to do a leakdown test for you. That will pinpoint if and where the leak is.

Hindsight is always 20/20, but for anyone looking to do the same as you and replace the engine with a good used one, I would have the HG's replaced with the updated graphite ones and the intake manifold gasket with the Felpro kit during the swap.

 

I had identical symptoms on a different vehicle. It was a leaking head gasket. I used a leak down test to confirm it. Cylinder pressure was forcing air (and other gasses) into the cooling system. This was displacing fluid back into the reservoir bottle. When the engine cooled, it would not pull the fluid back into the engine. With a leak in the head gasket, the vacuum seal is broken.

I made my own leak down tester and installed it into a heater hose (PVC Pipe and a glued on valve stem off of a bicycle tube). With no coolant in the engine you pressurize the system to 15 psi. If the pressure drops, you have a leak. You should be able to hear the leak in either the exhaust or the intake. If you rotate the engine a little and the sound stays in the intake or stays in the exhaust, it's a crack in the head. If it changes from intake to exhaust as the valves close and open, it's a head gasket. Remove the spark plugs and listen to find which cylinder it is leaking into (I know the back ones are a bear).

Good luck!

 

Dgwest...what did you ever find out?