Phantom Coolant Leak?
I am having an issue with my 1998 M3 with about 132K miles that I bought in May 2000 with 23K miles. I can't honestly say that it's been maintained strictly according to the Old School Maintenance Schedule, but it's been well looked after and has not been abused or neglected or had required repairs indefinitely deferred. When I first bought it, I took it to a couple of HPDEs, but it was so much faster than my old 325i that it was too scary to be fun. So it has seen very little track use. Now it is no longer even my daily driver. It has accumulated less than 7K miles in the last 2 years.
At 89K miles it got a complete cooling system overhaul and has always been filled with BMW OEM coolant mixed 50/50 with distilled water. About a year after the cooling overhaul, the replacement water pump (purchased along with all the other parts from a well-known BMW parts supplier in the northeast) began to leak. It was replaced and all was good. Then, about 6 months ago the expansion tank developed a crack and began to leak. It, along with the cap and bleeder screw, was replaced with new parts purchased from a BMW dealer. Shortly after that, the upper radiator hose blew out while I was on a business trip 200 miles from home. I bought a US-made replacement hose at an auto parts store that was still open at 8:30pm and swapped it in a restaurant parking lot. I had gotten into the habit of carrying a bottle of coolant with me, so I was able to use the proper fluid to refill the system. Despite all the cooling system problems, at no time during my ownership has the engine been allowed to overheat or run hot.
At roughly the same time as the expansion tank cracked, I had began to notice a loss of coolant. I assumed it was due to the cracked tank. But even after replacing the tank, there has been a steady loss of coolant. I have bled and re-bled the system to make sure there wasn't any trapped air causing the appearance of low coolant. Finally, I added some UV dye to the coolant to try to locate an apparent leak. This stuff is amazingly bright. Even though it came with an LED blacklight, it's not needed. Even a tiny drop is clearly visible. I've driven the car a few hundred miles with the dye in the system and that should have been more than enough time for it to leak enough to be visible. If fact, I've had to continually top off the system during that time. I would think a leak that big would be easy to spot. But I have crawled over every inch of the engine bay, both in daylight and in the dark with the blacklight and have not seen a trace of the dye.
If I top off the coolant level to be even with the "COLD" mark in the expansion tank and then drive 100 miles, it will take 6 ounces of coolant to bring the level back up to the COLD mark (yes, I measured it).
I have not performed a compression and leak down check, or pressure tested the cooling system. There are no signs of coolant in the oil, or oil in the coolant. Given the known history for the last 110K miles, lack of abuse/neglect, and no history of overheating, can anyone theorize about what is going on? I'm at a loss and am very frustrated with the car. Twice I have started a Craigslist ad, but haven't gone through with it yet. Yet! :mad:
For what it's worth, I'd really like to be able to diagnose this myself if possible. The nearest dealer is well known for being a rip-off and has a very bad reputation in the area. There are no BIMRS affiliated shops within 150 miles. There are a couple of local shops that do specialize in imports and do decent work, but they charge nearly as much as the dealer! But I can admit when I'm in over my head and will call a pro if needed.
Put a fluorescent die in the coolant and use a black light at night to find where it came out.
I have added coolant twice over the last 2 years to my M3 vert. I had to do this every once in a while to some E36's I owed but not all of them. Your use is higher than I have buy a huge margin.
I found it almost impossible to detect a leak. I added dye as well to my 325 that was loosing coolant. I did find it and of all places it was a very small leak coming from my expansion tank overflow hose that goes to the expansion tank from the radiator, the small one. I also found a leak on another 328 I owned that was also impossible to find but found the phenolic to aluminum alloy seem was barely leaking causing the issue.
My advice is to put something that can catch anything that falls in your garage so that even a trace would show up. I hope for your sake that there is something because otherwise this is sounding like a head gasket leak.
There are times when you don't see oil in the coolant or coolant in the oil because the breach is leaking into the block or head and getting burned at a very slow rate.
Last thing to try is to pressure test the coolant system and let it sit for a while like an hour. If the pressure goes down maybe you can find the leak or it is leaking into the engine...
Thanks for the suggestions!
I already have UV dye in the system. So far, I haven't found an external leak. And it's not leaving anything on the ground when parked. I keep it in the garage and it will sit for a week without leaving any drips.
I think a pressure test is my next move.
Check the neck of your radiator where the top hose connects. The neck of the radiator frequently cracks. I wonder if the fluid could be evaporating through a crack at the neck of the radiator or escaping through the crack and evaporating so you don't see any evidence of it. A long shot I'm sure but may be worth checking.
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