BimmerFest BMW Forum banner

Coolant leak

427 Views 19 Replies 3 Participants Last post by  bri123
There's a coolant leak that's coming from either the side or the rear of the motor. Does anyone have any suggestions on what it could be? I plan on lifting the car tonight to see if I can find the source.
1 - 20 of 20 Posts
Pure speculation, but look at all the hoses on coolant pumps (I think your car will have 2 or 3 electric ones and the one on the engine), along with all coolant related solenoid valves (changeover valve) and "T" connections.

Inspect the coolant connections for the turbo charger also. Your car has enough miles on it they may an issue.
There's a coolant leak that's coming from either the side or the rear of the motor. Does anyone have any suggestions on what it could be? I plan on lifting the car tonight to see if I can find the source.
Thank you for the suggestion.

I got her a car and look but really didn't see anything stick out. The crazy thing is once I add coolant to the reservoir, it immediately leaks out from wherever the issue is. It looks like it's leaking from above the oil drain pan, but since I can't really see it, I can't tell for sure.
Pure speculation, but look at all the hoses on coolant pumps (I think your car will have 2 or 3 electric ones and the one on the engine), along with all coolant related solenoid valves (changeover valve) and "T" connections.

Inspect the coolant connections for the turbo charger also. Your car has enough miles on it they may an issue.
You may want to pull the engine shroud off the top (ISTA has instructions, it is not difficult) and look from above. That will let you see more. You could try a UV flashlight, but I am not sure if the blue BMW LC-87 (Zerex G48) coolant glows or not (assuming that is what you have in the system). I have some in the garage, so I will try that later to see. If you have the more recent BMW LC-18 (Valvoline HT-12) green stuff it (should) glow green (that has only been used since 2020 or so).

If it drains out that fast it could be the coolant expansion tank is cracked, or the hose that attaches to it is cracked.

Which tank are you adding coolant to? The low temp (bigger tank) or high temperature (smaller tank)?
Thank you for the suggestion.

I got her a car and look but really didn't see anything stick out. The crazy thing is once I add coolant to the reservoir, it immediately leaks out from wherever the issue is. It looks like it's leaking from above the oil drain pan, but since I can't really see it, I can't tell for sure.
It's not from the expansion tank, unless there's another one in the rear of the engine. I did take off the engine cover, but still couldn't see anything. The oil pan has a formed cover around it and it blocked my view from underneath. So basically I couldn't see any on the rear coolant connections.
Can you take some pictures of where you think it is coming from? You say it is from the rear of the engine? Which tank are you adding coolant to?

You say it is draining down and into the acoustic cover for the oil pan? It sounds like the leak is tight to the engine block. I am not sure how easy that cover is to remove or if there is room to remove it. - https://www.realoem.com/bmw/enUS/showparts?id=7E23-USA-09-2015-G12-BMW-740i&diagId=11_6357

On a 2016 740i it looks like there are a few connections toward the rear of the engine- https://www.realoem.com/bmw/enUS/showparts?id=7E23-USA-09-2015-G12-BMW-740i&diagId=17_0990

I just tested a 50/50 mix of the blue coolant and distilled water with 365nm UV light- it does not glow, so that is no help. But because you know you have a leak and know you may need to replace something (which means potentially draining coolant), you might consider using a 50/50 mix of a cheap fluorescent green coolant so you can use a UV light to help see where the leak is coming from. Later you can drain and fill with a proper BMW coolant.
It's not from the expansion tank, unless there's another one in the rear of the engine. I did take off the engine cover, but still couldn't see anything. The oil pan has a formed cover around it and it blocked my view from underneath. So basically I couldn't see any on the rear coolant connections.
I'm adding coolant to the larger expansion tank (radiator expansion tank). The smaller expansion tank still had visible fluid

I'll try and get pictures tomorrow.
Ah, in that case, I would look closely at the area where hose numbered "22" attaches to the engine block. - https://www.realoem.com/bmw/enUS/showparts?id=7E23-USA-09-2015-G12-BMW-740i&diagId=17_0990

Does it leak when the car is off, or only when the engine is running?

There is also a transmission oil cooler (heat exchanger) at the rear of the engine where coolant goes. - https://www.realoem.com/bmw/enUS/showparts?id=7E23-USA-09-2015-G12-BMW-740i&diagId=17_0994

If you can find that, look closely at it's coolant lines (shown circled in red here) -


Map Font Slope Auto part Parallel
I'm adding coolant to the larger expansion tank (radiator expansion tank). The smaller expansion tank still had visible fluid

I'll try and get pictures tomorrow.
Yes, it leaks as soon as I pour in the fluid with the engine off.

Do you think this hoses you listed are easier to spot from the top or underneath?
22 should be easier to trace from the top, because it comes from the top driver side corner of the radiator and goes back toward the rear of the engine. The transmission oil cooler is probably easier to spot from underneath, since it is really far back and under everything.
Yes, it leaks as soon as I pour in the fluid with the engine off.

Do you think this hoses you listed are easier to spot from the top or underneath?
Was the Turbo coolant lines TSB/SIB performed on this car? If so, the very likely culprit is a leak from a failed clamp on the hose marked #2. Very easy to check by simply taking the engine cosmetic shield off (pops out) and either running the engine or pressuring coolant tank with a pump. Mine included, there have been several reports of this failure at the same exact spot so its worth a check. Due to the location, it is possible to feel that the leak is from the rear underside of the engine even though coolant is running off the top and through the V valley of the engine.

This thread has some pics for orientation.
I have time today and can investigate more.

One thing I noticed is when I added the coolant directly to the larger expansion tank it would leak out immediately towards the back side of the car. The smaller expansion tank may not have loss any fluid, or as much fluid. How are those two tanks related?

And if I have to replace any of these lines are on the back side of the motor, is this achieved by top access or underneath the car access?
Was the Turbo coolant lines TSB/SIB performed on this car? If so, the very likely culprit is a leak from a failed clamp on the hose marked #2. Very easy to check by simply taking the engine cosmetic shield off (pops out) and either running the engine or pressuring coolant tank with a pump. Mine included, there have been several reports of this failure at the same exact spot so its worth a check. Due to the location, it is possible to feel that the leak is from the rear underside of the engine even though coolant is running off the top and through the V valley of the engine.

This thread has some pics for orientation.
I think you will need to pinpoint the leak first. Some would need to be replaced from the bottom (like the transmission oil cooler). But once you figure out where the leak is, you can look it up in ISTA to see the recommended procedure for replacing it.

Also, from what I can tell, the small (high temperature) tank cooling circuit and large (low temperature) cooling circuit are separate cooling loops. As I understand it - the high temp circuit cools the cylinder head. There may be times when certain valves are opened and the coolant mixes between the two, but I am not sure.
I have time today and can investigate more.

One thing I noticed is when I added the coolant directly to the larger expansion tank it would leak out immediately towards the back side of the car. The smaller expansion tank may not have loss any fluid, or as much fluid. How are those two tanks related?

And if I have to replace any of these lines are on the back side of the motor, is this achieved by top access or underneath the car access?
Just for clarification, is this what your refer to the larger (low temperature) tank is #1 => https://www.realoem.com/bmw/enUS/showparts?id=7E23-USA-09-2015-G12-BMW-740i&diagId=17_0988
I think you will need to pinpoint the leak first. Some would need to be replaced from the bottom (like the transmission oil cooler). But once you figure out where the leak is, you can look it up in ISTA to see the recommended procedure for replacing it.

Also, from what I can tell, the small (high temperature) tank cooling circuit and large (low temperature) cooling circuit are separate cooling loops. As I understand it - the high temp circuit cools the cylinder head. There may be times when certain valves are opened and the coolant mixes between the two, but I am not sure.
Yes, that is it.

I just thought of something- If you have the plastic panels on the bottom of the car under the engine bay, it could be the leak is further forward, and is running on top of one of those (that) panel(s) toward the rear, since those panels have a natural slope toward the back even when the car is parked on a flat surface.
Just for clarification, is this what your refer to the larger (low temperature) tank is #1 => https://www.realoem.com/bmw/enUS/showparts?id=7E23-USA-09-2015-G12-BMW-740i&diagId=17_0988
ISTA states I need to deactivate the high voltage vehicle electrical system. . Is this for gas powered cars or EVs.. or both? I only have 1 battery.
That is only for the hybrids like mine (740e and 745e). Don't worry about it.
ISTA states I need to deactivate the high voltage vehicle electrical system. . Is this for gas powered cars or EVs.. or both? I only have 1 battery.
Thank you.
That is only for the hybrids like mine (740e and 745e). Don't worry about it.
Here are a few updates and questions:

1. there are no visible leaks from the transmission cooler. This would be the easiest fix, but oh well.

2. There is a visible leak I can see from above and underneath the car (see image), however, I do not believe this is the main leak I'm seeing. Mainly because the leak bigger leak is running over the engine oil pan. On the image, there are 2 items marked if anyone can tell me what they are. This will need to be fixed as well.

Hood Automotive tire Vehicle Motor vehicle Automotive mirror






3. I need to remove the wipers in order to free up room. ISTA states I need a special tool (see pic) or I risk breaking the wiper console at a predetermined breaking point. Any advise on removing the wipers without the tool?

Hood Automotive lighting Automotive tire Motor vehicle Guitar accessory




4. How is this clip opened? I messed around with it a bit, but couldn't figure it out.

Automotive tire Automotive fuel system Motor vehicle Automotive exhaust Automotive design





5. If I remove the air intake header (see pic) will that open up that area more so I can see the hoses that are more on the rear of the motor?

Yellow Font Line Motor vehicle Pattern




6. As I stated above, the leak comes from above the oil pan. I was not able to see the leak under the car. The hose diagrams are not that great, but what are the possible hoses that are back that far? Under the air intake header, there is an auxiliary coolant pump, I'm wondering if the leak is somewhere there.

I'm sure I'm forgetting something. Any help would be much appriciated.
#1 - One less thing to worry about I guess.

Do you use green coolant in the car in either coolant loop? Maybe the previous owner did?

#2- I don't really have enough of a view of where that is to get a feel for what those could be, but the larger yellow arrow (closer to the bottom of the picture) looks like the engine oil heat exchanger on the oil filter assembly shown as #4 here - https://www.realoem.com/bmw/enUS/showparts?id=7E43-USA-08-2015-G12-BMW-740iX&diagId=11_6422 That has coolant connections to it (not through hoses, but through the oil filter housing), and the shape and color look correct, but I am not sure. That is at the very back of the engine (on my engine it is, not sure about the B58), so it makes sense. Here is a picture- BMW Heat Exchanger - Genuine BMW 11428583901 If there are leaks around it, the seals would need to be replaced under it (no need to replace the exchanger itself). Definitely look that up in ISTA if you replace those seals (oil and coolant mess otherwise).

#3, you must use a wiper arm puller of some kind. It does not need to be the BMW version though. Here is the one I use (also works for battery terminals)- https://www.amazon.com/OTUAYAUTO-Battery-Terminal-Wiper-Puller/dp/B076PY8785/ref=sr_1_3?crid=1I30S5NA67N4H&keywords=Wiper+arm+puller&qid=1675128708&sprefix=wiper+arm+puller,aps,113&sr=8-3 A local Autozone should have something similar.

#4 I honestly don't know about that one. I would have to find the same thing on my car and try to figure it out. Can you take a picture of the real thing (with a wider view to see where it is in the engine bay)? I may not have that same thing in mine though (totally different power-train in mine).

#5 That may help, but I don't have the same engine to tell for sure. But that part is also a cooler for intake air, and has coolant lines to it - #23, #12 (and #4 to the smaller coolant tank) shown here - RealOEM.com - Online BMW Parts Catalog
Here are a few updates and questions:

1. there are no visible leaks from the transmission cooler. This would be the easiest fix, but oh well.

2. There is a visible leak I can see from above and underneath the car (see image), however, I do not believe this is the main leak I'm seeing. Mainly because the leak bigger leak is running over the engine oil pan. On the image, there are 2 items marked if anyone can tell me what they are. This will need to be fixed as well.

View attachment 1081845





3. I need to remove the wipers in order to free up room. ISTA states I need a special tool (see pic) or I risk breaking the wiper console at a predetermined breaking point. Any advise on removing the wipers without the tool?

View attachment 1081846



4. How is this clip opened? I messed around with it a bit, but couldn't figure it out.

View attachment 1081847




5. If I remove the air intake header (see pic) will that open up that area more so I can see the hoses that are more on the rear of the motor?

View attachment 1081848



6. As I stated above, the leak comes from above the oil pan. I was not able to see the leak under the car. The hose diagrams are not that great, but what are the possible hoses that are back that far? Under the air intake header, there is an auxiliary coolant pump, I'm wondering if the leak is somewhere there.

I'm sure I'm forgetting something. Any help would be much appriciated.
1 - 20 of 20 Posts
Top