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Thread: Coolant Leak. How to replace rear coolant hoses?

  1. #1
    Team Z8 Z8DinanS2's Avatar
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    Nov 2013

    Coolant Leak. How to replace rear coolant hoses?

    I noticed a water leak and read on forums how when you can't see it it is likely under the plenum. Sure enough, i removed the plenum and found a pool of water.

    Problem is I can't figure out how to get to the rear coolant hoses. The wiring harness is inside a plastic housing on both sides of the engine and these hoses are under there somewhere. Anyone know how to remove those plastic housings?

    Once removed will I be able to access these water hoses?

    These cars are reaching an age where coolant hoses will fail so I want to replace them all while I am in there.

    Any help would be greatly appreciated!

  2. #2
    I suggest you do a search of the E39/Z8 section of the With over 20,000 E39 M5's built for the global market and most of them having much higher mileage than the typical Z8, many of the issues likely to confront Z8 owners have been discussed at length.

  3. #3
    Z8 Millennial Monster Z-acht's Avatar
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    Sep 2012
    It must somewhere within these articles
    1 of the 71 original Dutch sold cars (jetblack/red)

  4. #4
    Team Z8 Z8DinanS2's Avatar
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    Nov 2013

    Found the leak....we will all have this problem sooner or later.

    I think I have resolved the issue. There's less on the net than one might expect. After removing the plenum, velocity stacks, lower plenum which requires disconnecting several hoses, disconnecting the intake hoses, undoing dozens of nuts and bolts, etc., I discovered there are two hoses at the rear of each cylinder. Each is around 2" long. I don't know if this is the only point of leakage as I did not, but should have, pressurized the system with the plenum off. But these two were definitely leaking. I have confirmed on M5 forums this tends to be the issue. (The other issue is o ring failure typically after an incorrect thermostat fitment. EVERYONE with a Z8 should go to BMW and order these two little houses. Less than $20 bucks. It is not a question of if they will fail but when. If you notice your coolant is low and you top it up and needs to be topped up again a week or so later you likely have a leak from these hoses. The leaks start as seepage from what I can tell. Changing the rear coolant hoses for the heater, etc. are a PITA! Not much room and in my case the original hose was so tight I had to cut the plastic housing to free it. (So much for a quick release fitting!)

  5. #5
    Z8 Ate My Homework! 2112's Avatar
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    Oct 2005
    Sounds like some skin will be torn off the knuckles. 😕

    and yet I hate the thought of someone else laying on my fenders.
    2000 Red over black
    heavily modified for performance. Although, not to the level of GM's car

  6. #6
    Team Z8 Z8DinanS2's Avatar
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    Nov 2013
    Yes, actually, I did lose some skin and some bruising from squeezing my hands between the engine and firewall. First time doing one of these jobs, particularly without a manual, is always the worst. I could probably cut the time in half next time around.

    That's why I wore cotton sweat pants (no pockets, no zipper, etc.) and a soft cotton ensure no abrasion marks on the fenders. Climbed onto the engine using the performance package brace as a support to reach some area more easily

  7. #7
    Team Z8 Z8DinanS2's Avatar
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    Nov 2013

    Diagram of Coolant Hoses

    Here's the image of the coolant components where you can see the two small hoses that typically fail first. BTW, also noticed the fuel hoses in the V are rubber hoses. Given the age of these cars probably a worthwhile part to replace next time the plenum is off or sooner perhaps. #18 left and right are the small hoses needing replacement.

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