Where is the Coolant Going
 

I'm losing coolant at the rate of about 2 oz per 100 miles for about the last 2,500 miles. I've taken it to two BMW indies.

The first could find no leak and said he was stumped. Pressure tested the car overnight. The second said he found a radiator and a thermostat leak via pressure test and he replaced both.

After the radiator and thermostat were replaced, the low coolant light came back on after 400 miles. The indy who replaced the radiator/thermostat told me there were no external leaks after a second pressure test but that they car smoked on start up after the pressure test and told me I need a new motor because the engine is consuming coolant somehow. Compression of all cylinders was over 200 except for #1 at 175.

I've owned the car since new. 2001 3.0 207,000 miles. The car has never overheated. The engine has no power issues, never smokes on regular start up, idles smoothly.

The second mechanic told me that it cannot be the expansion tank, which was replaced 35,000 miles ago, because it did not leak under pressure. The water pump was replaced 65,000 miles ago. They said they checked the coolant pipes going in to the engine and they had no leaks.

The first mechanic told me to keep driving the car until the source of the coolant loss revealed itself, and he doubted there was a crack in the head or block, or a head gasket failure because there would be other symptoms besides a slow coolant loss.

It's tough because I feel nervous driving the car for extended distances -- don't want to get stranded by the problem ultimately getting suddenly much worse.

Tempted to replace the rest of the cooling system --- hoses, expansion tank, and water pump and see if it stops the problem.

Advice appreciated.

Is it possible that you have a small hole or a worn connection on the hose (small dia.) that goes from the expansion tank to the pump?

I only bring it up as I had mine (4.8iS) pop off while driving and didn't lose very much coolant eventhough I had driven 15km in 44c heat. I'm unfamiliar with the 3.0 so I'm not sure how your line are set-up.

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I had a similar problem with a truck. Three years and I could not find a coolant leak. One day, boom, head gasket failure. Bore scope the #1 cylinder. Does the piston top look cleaner than others? Does the plug look different from others? Even is the leak is small, steam cleaning will occur over time.

I had a pinhole leak where the belt ate through the coolant hose on the bottom left of the radiator. I had the system pressure tested several times no leaks. When we were replacing the clutch fan I spotted a few drops, kinking the tube revealed a nice stream.

I once had a tiny, hidden leak on my 2001 3.0i.

It was at the front end of the heater pipe (hard plastic, not hose) that runs along the left (driver's) side of the head / upper engine block.

It was so slow, and being hidden under a bunch of other stuff, and with a long way to drip and evaporate before it reached a visible area, it took a long time to find it.

Part number: 11-53-1-705-210

In my case, it was a new part I had put in there, which had defective o-rings. Replaced with Genuine BMW, as I should have used from the start.

I had slow leaks in the front end of the hard pipes as well. So slow as not to show on the driveway but I got a Harbor Freight Pressure tester and pumped the system up to 13 psi I believe and came back a bit later to find it had lost a pound or two of pressure. I kept topping off the reservoir until I changed the hoses and hard pipes while doing a larger repair and the problem became obvious when I looked at the ends of the hard pipes.

You need to rule out, or in, the idea that your head gasket is leaking coolant into a cylinder.

Here is how you do it...

1. Run the car to full operating temp one afternoon or early evening...

2. Let it sit overnight,

3. In the morning, before start-up, pull spark plugs and swab each cylinder looking for coolant.

I used a 1 foot piece of flex tube and (securely) taped 1/2 a q-tip to the end of it.

In my case, I found coolant in cylinder 6.

If you're getting that much coolant ingress, you should be able to scope it instead.

Assuming OP has a scope

$10 will get you one off Amazon by tomorrow.

The #6 cylinder was very slowly leaking coolant through a worn head gasket. So, the top end of the engine is being rebuilt, along with new timing chains and guides.

Two of the spark plugs were contaminated with oil and the valves were leaking, so that is likely a significant component of the oil consumption (1 qt per 1,000 miles).

The piston rings are not going to be replaced. Not cost-effective. It will be interesting to see what the new oil consumption rate will be after the work is done because theoretically it will all be coming from worn rings.

How was the leak found? Do you have any pics?

Piston rings are not likely worn… it’s far more likely that the bottom oil ring is clogged with carbon as the original oil drain holes were too small. Clogged rings can lead to oil consumption.

You can clean them with a proper BG Engine flush.

I bet via an endoscope peeking the cylinders with spark plugs removed or via Effduration's suggestion (https://xoutpost.com/1221915-post7.html)

How's the BG engine flush used? I'm about to change the oil pan gasket. I can do a quick flush before draining it. After buying the X5, I did a Liquid Moly flush. The oil change was nasty black. :wow: Might as well do another good flush before the oil pan comes off.

https://www.e46fanatics.com/threads/...ption.1259657/

See post# 4

It uses BG EPR #109 in the crankcase and among other steps, lets the car idle for several hours.

I have not done this myself, but would do it if I had a bothersome oil consumption issue.

MrMCar (author) is a long-time BMW Tech.

I don't have pictures. I'm not mechanically inclined so I took the vehicle to a trusted mechanic and he did the borescope that was suggested here.

Of note, prior to the coolant loss becoming a major issue, I used Seafoam in the crank case prior to the last oil change to try and mitigate the oil consumption. Consumption went from a quart every ~700 miles to a quart every 1,000 miles. And the engine ran with less vibration at idle as well.

Not sure how Seafoam compares to the BG product.

For me...

I've found/had small leaks from the o-rings on the transmission cooler, the smaller diameter hose from expansion tank to water pump, leaking drain petcock on radiator tank and a hairline crack in the radiator plastic tank that masu have seeped only occasionally until it detonated the other day.

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I JUMPED IN WITHOUT READING ALL OF THE POSTS....

I had an E36 with the M50 engine, and I missed the part where you say which motor you have, so my comments might not help you in your quest for information.

My M50 developed a cracked head that gave me absolutely in symptoms at all, except for the minor (like yours) consumption of coolant that could not be explained. You should Google, BMW M50 CRACKED HEAD, then look at the Images. I'm pretty sure the M54 will have a similar, if not identical, problem.

The Backstory....
I had just bought my E36 and was driving it over a mountain road to commute to the office. I was coming home one night and smelled coolant, not realizing it was mine I passed a couple of places where I could pull over and wait for the tow truck, except that cell service on this road was spotty at best 20 years ago. By the time I realized that the coolant was mine, I stopped and hosed down the front of my car and attempted a roadside repair that held to the summit of the road I was on, then I coasted without the engine to the bottom of the hill and called AAA. They brought the car to my house, I bought a bunch of stuff and put it on, and this cured the immediate problem, and brings us to why this is relevant to you. After I fixed the catastrophe, I found that I was consuming coolant similarly to your consumption. Then, one random Monday morning several months later, my engine hydrolocked upon Engine Start in my garage. I pulled the plugs and turned the engine by hand and the contents of Cylinder 3 came out. Coolant.

My car was consuming coolant very slowly, and there was a crack between the #3 Exhaust Valve Seat and the adjacent water galley. The coolant was leaving so slowly, there was no perceptible steam. Temp control (by the needle remaining at midscale) was good. My mechanic could not explain the hydrolock, except to say that the head gasket was toast. When he pulled the head, he discovered that the head gasket was not toast, and we needed to find a different problem. That's when I found out about BMW Cracked Heads.