Coolant leak-expansion tank overflow tube

I have had a coolant leak start recently and wanted some thoughts from the experts. First a little background-'86 328. Several months ago, I replaced a number of the coolant lines, so I had the system drained. I refilled the system and successfully bled the system. The radiator fans were cycling, engine temp gauge was steady and I had heat on both the driver and passenger side. Everything was fine for a while and then I noticed coolant dripping out of the overflow tube on the expansion tank (the one at the radiator cap). When I first noticed it, there was a solid line of coolant from outside into the garage, so it was more than just a drip.

After reading on the forum, it sounded like a radiator cap was the easiest first step in troubleshooting. The original cap was still installed. I don't have the make and model handy, but it was the one recommended here on the forum. The leak continued after the installation of the new cap, but not as bad. However, after a drive last night, I had a solid line of coolant up the driveway. I have looked and the leak is coming from the overflow tube. The outside of the tube in dry and the only part that is wet is the very end.

Some other observations-the engine temp gauge is steady on the mark between 140 and 195. I think I still have heat on both sides. I have opened the bleeder valve on the top of the water pump (with the car running) and coolant instantly seeped out when it was loosened. I think the biggest clue as to what is wrong is that every time I loosen the bleeder screw on top of the radiator (with the car running and up to temp), I get air escaping. I have now done this 3-4 times. The expansion tank is not overfilled. Prior to the drive yesterday, it was about half full.

Any thoughts? One guess is that there is a loose hose clamp somewhere and as the system cools, it is sucking air into the system, causing the coolant to back-up next time the car is driven.

My plan of attack was to put a wrench or screw driver on all the hose clamps I recently touched and see if anything jumps out at me. I have not put the car on stands to inspect the hoses, but I don't think there is a leak large enough other than the overflow tube to drip on the floor. That being said, there is an oil soaked piece of cardboard under the rear of the car, so it would be hard to tell if it was a coolant drip or oil drip.

Any thoughts would be greatly appreciated. I am sort of scratching my head at the moment.

 

That is by far the easiest part to replace. It had occurred to me that the new cap might have been weak. They are cheap enough, so I might just order a new one. I'll look at the specs tonight and post that information.

What is the general operating pressure of the system in normal driving conditions?

I did pressure test the system after the hose replacement and it appeared to hold fine. I am by no means an expert here, but I think it lost maybe 1 or 2 psi over a 24 hour period starting at 15 psi. I figured that was good enough.

 

I think it is about half full, but may be lower after yesterday's leak. Something did change since it wasn't leaking and then all of a sudden coolant was leaking from the overflow tube. I had not added coolant after the system was refilled after the hose replacement and it didn't leak for a couple of months.

I do know that the coolant level has to reach the top of the expansion tank for it to leak, so the question is what is causing it to reach those levels (heat expansion or something else). I'll also check the level tonight in the tank.

I don't know if this helps, but I pulled up to the garage door last night and left the car running. I opened the bleeder screw on the radiator. At that time, I noticed the line of coolant on the driveway. Once the air bled out, I pulled into the garage and there was only a drip or two in the 20-30' I had to drive to park it. Either letting the air out allowed the coolant level to drop or the car idling did the same.

 

Thanks for the info on air in the top of the radiator. I had wondered if that was normal.

I had thought that if the expansion tank was a a little too full, it would eventually self-correct once enough of the coolant had leaked out. So far, it hasn't.

 

Here is the radiator cap that I currently have installed. It was installed about 6 weeks ago after I first noticed the leak.
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Looks like a 16 psi cap.
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I would guesstimate that the coolant level in the expansion tank is about 25% full. This level was observed last night after the car had been sitting for 24 hours and it was leaking when I pulled it into the garage. No coolant has been added or removed since I pulled it into the garage.

If I stick my middle finger down into the tank, the tip just touches the coolant. I have small hands, but I looks like the coolant is approximately 7 cm down from the cap.

Is this the correct pressure cap? Any more thoughts as to what would be causing the coolant level to rise in the expansion tank? I don't think thermal expansion explains that much of a rise. When I filled and bled the system it after the hose swap, I filled the tank to the correct level per the owner's manual. As I said before, there were no leaks for a couple of months after the refill and now it has all of a sudden started leaking.

Could I idle the car long enough to get it up to temperature with the cap off and observe what is happening in the expansion tank? Not sure exactly what this would tell me, but I could get a sense of coolant flow through the tank and if/how fast it rises. I still would think that if there was air in the system restricting coolant flow through the system, the car would be running warm. Sorry for the rambling thoughts, but I am just thinking out loud.

 

The last couple of time I have driven the car it has leaked. The ambient air temps haven't been above 80 degrees on any of the drives. Sunday, the temps were probably in the upper 60's. 3/4 of an expansion tank should be more than enough room to allow for thermal expansion on a regular drive around town.

I'll check for anything that might compromise the seal between the cap and expansion tank. I ran my finger around that lip last night and nothing jumped out at me, but I wasn't looking that closely.

I still don't understand why the coolant level would even be rising that high in the expansion tank. I think If I can answer that question, I will solve the leak issue.