Coolant leaking through reservoir cap, need new cap?
Hello Alfisti. I am a new member, and I am having some trouble with the cooling system on my Milano.
The problem only seems to happen after the car has been run for a while at temperature. The easiest way to recreate the problem is to run the car on the highway for a while, then pull off and idle (usually I'm stopped in traffic). There will be a temperature spike, although it won't reach the red line. Then after I get the car moving again, it slowly cools down, but it's now leaking coolant through the cap. Usually by the time I reach my destination (another 10 minutes or so, mostly uphill) the float sensor on the coolant reservoir has tripped.
I have a few original Alfa coolant caps, but I don't trust any of them. I'm told I should use 0.7 bar cap, but I don't know where to get one. Also, I don't have the ability to pressure test the caps -- my mechanic has unfortunately lost his adapter. I've done a fair amount of reading on these boards and I know that the new made caps can be questionable.
Does any one know where I can get a trustworthy reservoir cap, and/or an adapter to pressure test these caps? I am worried the problem might be more serious, but I would like to eliminate the cap as an issue first.
1987 Alfa Romeo Milano, 2.5L V6 (Silver). My father bought it new, and I've been driving it for many years now. It has over 240,000 miles on it. It is a daily driver. When the engine needed an overhaul a few years ago, the mechanic instead replaced the engine with another one (still 2.5L) which had fewer miles. I've also had the transmission rebuilt. Next on my list of major overhauls is the exhaust, as it has lots of little leaks. However, this cooling problem has forced me to delay that.
The problem was first noticed a couple of weeks ago on a short trip out of town. It lost coolant through the cap after some hard driving. I initially thought the coolant reservoir had gone bad -- a problem I have seen before -- but after replacing it with an original tank I realized the coolant was escaping through the cap and not a leak in the tank. Around the same time my job changed office locations, and I now had a longer drive. So I was encountering this problem everyday on the drive home. At least once the car ran very hot (at the red line but not over it), and the coolant tank was empty by the time I made it home.
I arranged to take the car to the mechanic to have him do a thorough check up of the cooling system. He discovered that the thermostat was stuck open (which would explain why the car took so long to warm up), and replaced it with a 164 thermostat. He also replaced the radiator! However, he used a radiator from a three liter Verde. Now the car warms up in a reasonable time, and runs at a much better temperature than it ever did. However, the next day, after some hard driving it again bled coolant through the cap. *sigh* Now believing it was the cap, I rummaged around the garage and managed to dig up an old 0.7 bar cap. We couldn't properly test it, but it appeared to be the best of the lot. The next day it made the afternoon commute alright so I figured I could take it on a short road trip.
I had to travel about 150 miles, mostly along an interstate, to get to my destination. Before starting I checked the coolant levels and made sure the tank was filled to the seam, or just slightly over. The car was fine on the outward trip. The temperature was perfect -- it used to never drive this consistently, I am really liking this new radiator -- and it didn't appear to have leaked any coolant. The car then sat for about 7 hours, cooled down completely, and the coolant level still looked good. On the return trip, the temperature was fine on the highway. But the moment I got off the highway, and idled at a light, the temperature spiked. Once I got it going again, it cooled down, but sure enough, as I arrived at my destination the coolant level warning light came on, and it had leaked down through the cap.
The "other" cap:
When I went searching for that old 0.7 bar cap, I also found a Saab cap that my mechanic had given me a few years ago. It fits the Milano coolant tank, but I remembered it was of a higher pressure. I did some researching online and I think the cap should be 15 psi (slightly more than 1 bar). It's in almost unused condition, and I know that Alfa did make 1 bar caps for the Milano, so I decided to try it. Actually, I first tried it a couple of days ago. It didn't leak, but the increased pressure caused the small hose that runs from the top corner of radiator to the front of the coolant tank (the radiator overflow hose?) to leak at the radiator. Loosening the cap and relieving some pressure stopped the leak, but I decided to replace the hose anyway. However, the next morning I took the car to mechanic to have the AC charged, and he thought the 0.7 bar cap would be better.
After yesterday, I decided to try the Saab cap again. So I filled up the reservoir, put the cap on and let the engine warm up for 15-20 minutes. I then ran some errands around town, which gave me the opportunity to stop and check the cap frequently. The cap never leaked -- after about an hour the coolant tank looked like it was starting to bulge! To its credit, the tank didn't leak either. Eventually the lower radiator hose started to slowly leak at the t-junction, so I very carefully loosened the cap just a little to relieve the pressure, then made the short trip home. I strongly suspect that the Saab cap holds more than 1 bar of pressure! I'm *not* going to use it again.
1. The problem takes a long time to present, which makes it difficult for the mechanic to see it.
2. At first everything appears normal. As the engine warms up, the coolant level in the tank rises, the thermostat opens, the fan turns on. However, eventually, as it continues to cycle, the coolant level in the tank appears to drop. I haven't really studied it before, but is that expected?
3. The radiator overflow hose(?) stays cool for a long time, even after the thermostat has opened. However, the other small hose at the top of the coolant tank -- the one that runs to the t-junction behind the engine, and links up with the heater -- gets hot quickly.
I apologize for the long post, I just wanted to be as complete as possible. If anybody has any ideas, it would be greatly appreciated.
Last edited by cplMilano; 06-16-2019 at 03:50 PM.