Yet another coolant leak thread -- but different (maybe)

New member here and a very new owner of an '03 C4s. The tech that did the PPI on the car told me it looked to have once had a coolant leak, but appeared dry w/ just remnance remaining... not the case. PPI also revealed a sporadic misfire due to coils. Replaced those this weekend and discovered dried coolant on cylinder 1 coil. And that looked due to the overflow tube from the coolant tank was aimed right at it. Given the PO had just had the coolant flushed, i chalked it up to an overfill and moved on. Last night i realized there's a very small coolant leak that remains based on a dime size puddle of coolant below the driver's side exhaust. I started poking around online and it looks like this is common issue that is started by a poorly designed cap that when replaced w/ the updated cap it causes the tank to leak at the seam. HERE'S THE THING: the coolant tank and cap are both the updated parts and look fairly new. Blue cap and the tank is white, no yellowing. The MAX/MIN lettering on the side does show it's not brand new, but couldn't be more than a couple years old. SO dearest Renlisters, as I continue to poke around in hopes that I pinpoint the leak, I'm hoping you could help by letting me know what other leak prone areas on the same side as the tank might be? I'm probably going to replace the tank just so i know it's done and right (a tax required for buying a car without a thorough maintenance history), but I don't want to order a new tank/cap only to realize there are four other tubes/hoses/joints that i should have ordered at the same time. Yes, yes, i know, i should hop in drive it around a bit and then come back to hopefully pinpoint the leak. And that is my plan. But remnants from Erma are preventing a drive for the next day or two and given I'm a redneck in TN and there are no quick/easy methods of getting parts to my house, I'm hoping to fast track the diagnosis with your help so that hopefully i can get some parts ordered and here before the weekend. Thanks in advance.

It could be the expansion tank or one of the fittings adjacent to it, given the location on the driver's side of the engine bay. Just an idea, gl

Can you use a coolant device like a uview airlift to lightly pressurize the tank and observe leaks? They aren't costly.

If it's the cap, you'll see coolant around the top of the tank when the car gets up to full temperature. I had to replace my cap after roughly two years - so age is not necessarily an indicator (it was also the updated "04" cap). A split at the tank seam is difficult to see, but have a look when the car is up to temp. and you should be able to see if it's dripping. Good luck

Given you can't drive the car, it's difficult to get the engine hot. The proper way is to use a pressure tester to pressurize the tank to ~18PSIG for at least 20min and use an inspection mirror to check the underside and the hidden side of the coolant tank and all fittings on the tank. The CAP can also be pressured tested with an adaptor.

Definitely do not want to be a fear monger BUT........I hope he/she was not hiding MILKSHAKE NTL, you have been very solid tips/advice and wish you best as you get this sorted out

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His engine wouldn't be running strong if it had intermix. Besides, it's not an mk1 so intermix would most likely be due to a compromised cylinder that would raise a very different profile of issues... but you get points for trying to scare a newbie

If its getting difficult to trace you could try a UV light, the coolant might be fluorescent, if not add a little UV dye. In the dark that stuff will glow like a christmas tree you'll be able to trace it. UV lights are cheap from amazon

Appreciate all the info. Based on no one posting up two or three obvious things i should be looking at, I'll take it that I've done my research well and chalk it up to the tank. I'm gonna go ahead and order a new tank/cap along with a thermostat and coolant level sensor as those look to be in the same area and set back to work this weekend. If my further exploration over the coming days shows it's something different, at least I'll have some extra parts laying around for when the tank DOES go bad. It's becoming a trend, here's a couple snaps from last weekend when changing out coils. I realize much of the disassembly wasn't necessary, but one of the bumper cover mounts was a bit wonky from an episode with a light post (or something) at some point and the access removing the extra bits made changing the coils a snap. Really like how these cars come apart and go back together. Oh and thanks for the concern, but the car is solid. Beyond the fact that I have total faith in the previous owner, I had a thorough PPI performed and I've picked the car from head to toe upon getting it home (I've been wrenching on BMWs for many years). And that's after a very fun drive from St. Louis to Nashville.

You might want to check your water pump. Mine would drop less than an egg cup full as the engine was cooling down in the garage. Just a thought as the water pump and coolant tank are in the same area.

Happy to report back that it was indeed the tank. As per my original post, i was apprehensive to think the tank was the culprit given it wasn't old/yellow/brittle, but apparently the coolant expansion tanks in these cars are a wear item, not a defective part that was revised/fixed. One thing of note to anyone that may find this thread down the road, do yourself a BIG favor and buy a pair of hose clamp pliers before you tackle this job. It took me about 3hrs from start to finish, but would have been half that if i had a set of these . And that's coming from someone with no less than 30 pairs of assorted sizes/shapes of pliers in my roller.

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