E34 Coolant Leak - Leak location detective help

[LA Touring]

Hello-

I have a '91 Touring that has an intermittent coolant leak that has me baffled. I have very nominal knowledge of the cooling system so I'm sure I'm missing something. It's been going on for a few years and finally decided to solve it. My old mechanic had replaced the radiator and it still had a leak which he couldn't locate - hence my "old mechanic". I'd love to crack this myself otherwise I'll bring it to my guy.... Here's what i know:

- There's no standing leak. I.e. if the car's just sitting unused it won't leak.

- After 3-4 days of running to store, errands in LA weather 75-85 degrees, I will find it's about 6-8oz low on coolant. Meaning about 3-4" below fill line.

- Some days after longer/ hotter drives, I'll notice a small puddle of coolant in driveway. This seems to leak AFTER I've returned to my house. Maybe it a coincidence or maybe has something to so with it sitting hot without air? Anywa, when I check it's lost up to 8-12oz at once.

What does this tell you? Why would I lose more when the car runs hotter? And usually all at once?

Thanks for help in advance!

John

[moroza]

Rent a cooling system pressure tester (free plus a ~$300 deposit at AutoZone). Use it in a quiet place. Report back.

[dannyzabolotny]

As Moroza said, get a pressure tester. You can rent one or you can buy one at Harbor Freight, it’s like $80 there. That’ll let you pressurize the cooling system to operating pressure but without the engine running, so it’ll enable you to see what’s leaking. Common leak points on a 525i are the hoses going to the heater core, the heater core itself, and the 4-way hose underneath the intake manifold. Those can leak without being too obvious.

[locknload]

If that doesn't reveal the leak, do it with the engine warmed up and running. See if it holds pressure. Things can change when they heat up.

[fo3]

A large quantity/concentration off proper coolant helps. You can easily see and smell it if it's strongly coloured and concentrated.
I've got a 90 525i as well and I've had to deal with some coolant loss. Usual spots for me was:
coolant expansion tank and it's o ring seals
The coolant level float below that
Probably old hoses that have never been changed, ie throttle body to air box t'stat and back to head
heater valves, pump and lines to heater

If it's the heater core you'll smell the coolant when you put the heater on.

I spotted blue coolant under the rad to find out about the coolant level float sensor leak- having a clean engine bay (and a white engine bay) helped noticing the BMW blue coolant pooling down there just after a hot drive like you mentioned. There's a U beam bracket under the rad that holds most of it, so hardly any leaks on the ground, it's mostly held up in the bar where it evaporates. Replacement sensors suck, just get new o rings unless it's cracked.

Also the crystalised coolant additives leave signs even if they are never wet - such as due to heat evaporation very very quickly, such as the air box t/stat hose to the cyl head which left a 'salt' trail down the block.

That's where mine was leaking and I suggest that's where you start.

[LA Touring]

great idea- cheers

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thanks- noted!

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fantastic detailed assessment - cheers!

[LA Touring]

hello...do u happen to know the max pressure for e34 radiator cap? i can't find a confirmed number doing some web research? i'm getting 15-20. then I see the some that are higher?

[LA Touring]

So I found the recommended pressure and found the leak. Any suggestions on what's failing here? hard to see if it's a hose because it's sort of dripping off the lip of the cover there. https://youtu.be/esS3ZNEQ49M

[fo3]

Is it any of those hoses I told you about? Seems closer to the engine block than the throttle body hoses would be so I'm not sure. You'll just have to get in there rather than guess or speculate.
hoses.PNG

#14 is throttle body to air box t/stat
#6 is throttle body to "octopus" heater hoses/rad return

On mine it was #13, cyl head to air box t/stat hose leaking. But your leak doesn't look like that hose (replace it anyway), but it could be the "octopus" hose on yours. I got #13 and #14 hoses from rock auto pretty cheap (I had to search hard because I think 90/91 M50NV is different to the M50b25TU 92+ that most places only stock.

[LA Touring]

it looks like #15. seems to be coming from that area very distinctly. i watched a vid on getting into the intake manifold and looks like a lot. I could probably figure it out if I had enough time- say if there were a global pandemic and I was just hanging at home all day. actually if then I think I'll just take to a mechanic. i'll try rock auto though. cheers