318 Where's the Air Coming From?

This is my first time posting to the forum. It looks like a tremendous resource!


Help! I have a 1968 or so rebuilt Chrysler 318 in our houseboat. All are in great shape, except the engine has developed an issue with extreme air in the water discharged from the thermostat and then overboard. I made some illustrations to explain my problem. I also replaced hoses with clear tubing for testing purposes so I could see what was going on.



First, I'm showing the basic setup. River water enters through a strainer... to an intake water distribution manifold... to a two chamber raw water pump... to the engine and manifolds. The light green lines represent the manifold cooling. Purple lines represent engine cooling. And, the cross-hatched purple lines represent a hot water recirculating loop used when the engine is cold.




The thermostat at the output directs water to the manifold exhaust and out the back of the boat when the engine has reached operating temperature. At startup or when cold water is directed through the recirculating loop, from the engine to the Intake Water Distribution Manifold, back through the pump, and back to the engine.




My problem is that the engine is injecting a large quantity of air into the cooling water.
The problem is amplified when the engine is cool because the thermostat diverts water with large air bubbles through the recirculating loop and BACK through the engine. Now there's even more air.



As a test, I removed the thermostat and BLOCKED the recirculating loop hose. Water from the pump to the engine was now clear. Water exiting the thermostat chamber had some air that went to the manifolds and out the back of the boat.


Where is that quantity of air be coming from? How can I fix this? Any thoughts would be MOST appreciated!!

The only logical conclusion I can reach is a head gasket leak...if you can catch a sample from the tstat housing, combustion leak detection fluid should tell you for sure...I would also suspect there would be some water intrusion into the cylinder(s) and the crankcase...oil level risen?

Thank you makomark!

I think you’re probably right. I changed the oil a week or two ago and the oil was dark gray and the level was a little above the full line on the dipstick. So, now I start looking for parts. Thanks for your insight. I am very mechanical but this is a little out of my wheelhouse. Have the best day!

I'd make sure on the diagnosis before spending money...that said, I think you have a 50+ yr old block so its very possible, especially being raw water cooled...may be faster to get a long block or consider switching to a SBC...

Where would I get a longblock? Also I’m not familiar with what SBC is. And again, thanks for your help.

Lots of rebuilders - a www search will get you started. Not everybody offers long blocks; I'd suggest that route because of the raw water cooling. The other item that may be due for reconditioning would be the distributor - just due to age and ignorance about previous maintenance.

The SBC is smal block chevy - a 350CID or a 383CID version. more displacement = more torque in about the same footprint. big plus there is a much broader base of parts...just food for thought.

I think I'd zip all the plugs out, read them, and check compression prior to tearing into or buying parts.

Thanks, guys. I appreciate your input. I'll definitely do some testing first. I purchased a cylinder leak-down tester, hoping that can confirm some of this. I had the engine rebuilt around ten years ago and installed an oil drain hose where the drain plug was. A friend suggested using that to drain a small amount of oil into a cup to see if there is any water present. That would be another simple test. I've had impellers go on the water pump a few times, and the engine got quite hot before I realized what was happening. BTW I did replace the distributor and electronic ign module quite a few years ago. I believe it's a Chrysler 2444983. As far as I can tell, it hasn't missed a beat. I sure hope I can find parts if needed. Gaskets and replacement bolts. One step at a time. Thanks again for your willingness to help.

Me again, at this point, I am totally baffled, but I wanted to give an update. The manifold cooling circuit is working fine. There are no bubbles or air. On the other hand, the engine circuit is still problematic. The engine is getting clean water with no air present from the pump. Water circulates through the block and reaches the thermostat. At this point, there is air quite a bit of air in the water.


What I've done:
1) Compression test = all cylinders seem ok
2) Cylinder leak-down tests = no leakage
3) Installed plugs in the two exit ports on the thermostat. Installed a special plug I made on the water input hose to the block. Then, with the cooling system isolated, I used the cylinder leak-down tester to apply 30 lbs air pressure to the cooling system. It held the 30 lbs with no apparent leaks.


The engine does not overheat. I'm just worried that the water pump impellers could get damaged. There is usually water flowing through the pump but with lots of air at times. But, of course, this occurs at startup when the engine is cold and the thermostat is recirculating water (with air in it) to bring the engine up to temperature.


I think I'm losing my mind!! Where is the air coming from, and do I need to worry about it?