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BLACK Coolant 700 1991

posted by  Koos on  Mon Sep 20 16:12 CST 2010
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last visit: Sun Oct 31 09:37 CST 2010
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Hi all,
I'm helping out my friend with his ~ '91 740 N/A car.

It's got a few problems:
Way too much Crankcase pressure. There's a lot of air that comes out of the oil filler hole when the cap is off. So far the seals are ok but the Oil pressure sender is leaking and we'll swap that out.

I already have a Oil Separator box to replace the old one which I would guess is plugged (the flame trap is fine...)

The real problem is that his coolant turns BLACK (not brown). About a month or two ago had had his coolant replaced/flushed by a shop (they were of the opinion that 'the only way that coolant can be black is if someone put oil in the coolant reservoir...' (doubt that) and for a while it seemed ok. now it's starting to turn black again...is the excess crankcase pressure somehow forcing oil into a coolant passage? The HG seems fine- the car runs well and has run on black coolant for a while (1+ year) with no real problems...except that it's black)

What might cause the coolant to do this? I read in my googling that degrading rubber hoses can do this, or adding dextron rather than old fashioned coolant...?

I'm worried that if we lower the crankcase pressure by restoring the PCV system, the pressure that is (possibly) forcing oil into the coolant will be gone and allow coolant to mix with the oil (the oil is fine- no milk shake), same with tranny fluid, although it's been a month since I checked it)- is this a valid concern? where can I look to attack this?

Sorry for the wordy post- thanks for any help!
Koos







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    BLACK Coolant 700 1991

    posted by  Will740turbo on  Thu Sep 23 13:04 CST 2010
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    last visit: Wed Oct 22 16:09 CST 2014
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    Any news? Get the compression tested?
    --
    1990 740 Turbo, on its way to stock specs, maybe beyond








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      BLACK Coolant 700 1991

      posted by  Koos on  Thu Sep 23 14:03 CST 2010
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      last visit: Sun Oct 31 09:37 CST 2010
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      No news yet- haven't done any further testing (daily driver...) What's appearing in the reservoir bottle is definitely oil-like and has not mixed with the coolant yet. Black, slimy (but so is coolant), no real smell to it. Oil is fine, (just changed- but was fine before) Trans fluid is light brown but that's just old fluid from the looks of it.

      However, this problem has been around for 1.5 years, so while I want to solve it, I don't think its gonna blow soon. None of the 'shops' he's had the car in have suspected the HG...we'll see what comp testing brings up.

      We did however put in a nice seat bottom from a junkyard find- mmm leather.

      Thanks for all the tips!
      Koos







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    BLACK Coolant 700 1991

    posted by  Will740turbo on  Tue Sep 21 16:16 CST 2010
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    last visit: Wed Oct 22 16:09 CST 2014
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    You have my attention.

    If you strain some coolant through a paper towel, is it black particulates in the coolant, or does the black color travel along with the liquid?

    This is an interesting article.
    http://www.penray.com/images/99.014.pdf


    So is this:
    http://forum.miata.net/vb/archive/index.php/t-381140.html

    Sounds like it's a mystery issue and not a blown head gasket. Do a compression test to be sure.

    A quick and dirty test is to fire up the engine without the coolant cap on. Immediately place your hand over the coolant bottle's opening. If you feel pressure building up within 30 seconds to a few minutes, you're looking at a block head gasket.

    Happy Bricking and post back. I'm curious, and getting an analysis of the coolant (like you can have your oil evaluated for $25) would be very cool, for lack of a better word.



    --
    1990 740 Turbo, on its way to stock specs, maybe beyond








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      BLACK Coolant 700 1991

      posted by  Koos on  Wed Sep 22 02:21 CST 2010
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      last visit: Sun Oct 31 09:37 CST 2010
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      Thanks for the links Will740Turbo!
      I need to find a compression tester kit and do that- crossed fingers.

      It is a mystery issue, we'll see where it goes.

      Koos








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    BLACK Coolant 700 1991

    posted by  dinomartino on  Tue Sep 21 01:48 CST 2010
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    last visit: Fri Oct 24 04:43 CST 2014
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    high crankcase pressure + crap in the coolant likely = blown head gasket. Certainly did on my car.

    You never know when the final stage (crankcase milkshake) will happen. On mine, elevated crankcase pressure had been present for 3 years (at least, based on the prior owner's service tickets), the sludge in the oil showed up rights around the time I bought the car, and the milkshake arrived a month after purchase (following a coolant flush and 4 qts of Mobil-1, naturally). And mine ran BEAUTIFULLY with a bad HG, so don't take "runs fine" as an all-clear on that count.

    That said, my coolant sludge was "chocolate milk", not black. How dirty is the oil in the car? Pretty carbony and sludgy?

    Is this your friend's daily driver? I'd have a leakdown test performed ASAP. If it rules out the headgasket, then he can safely continue to drive the car while sorting out these issues. If the headgasket really is failing/failed, then driving the car as-is is a bad idea.

    John








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    BLACK Coolant 700 1991

    posted by  Steve on  Mon Sep 20 22:03 CST 2010
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    last visit: Fri Oct 24 07:16 CST 2014
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    Look for a failing head gasket by doing a cooling system pressure test along with a test for the presence of exhaust gases in the coolant via a tester. Coolant should definitely NOT turn black. If oil is entering the coolant, it will separate out in the reservoir. I suspect exhaust gases.
    --
    See the 700/900 FAQ at the drop-down menu above right.







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