87Turbo Coolant Overflow Issue a sign of BHG?

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Thread: 87Turbo Coolant Overflow Issue a sign of BHG?

  1. #1

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    87Turbo Coolant Overflow Issue a sign of BHG?

    Hello SF.
    My 87 7MGTE has an ongoing issue with recovered coolant in overflow tank not returning to radiator when temps cool down. After driving, the overflow tank level will go from between "Low" & "High" levels to approx. 1.5" above the "High" level. When cooled down, overflow tank level stays the same and the level in radiator is down, so I have to take coolant from overflow tank to fill up radiator. When doing so I notice small coolant loss of approx. 1/4 of a quart for day's worth of driving (35 miles or so). The car runs fine and doesn't overheat (gauge shows needle in the middle when warmed up and during driving). Although idle will go down to 550RPM at times especially while driving in stop & go traffice.

    From reading other posts I've read here, there's a good chance my engine has a BHG.
    So, I did a dry compression test (twice) and I think the numbers are okay?: #1=152psi, #2=145psi, #3=142psi, #4=135psi, #5=140psi, #6=150psi.
    If I have a BHG, shouldn't I get lower compression readings?
    I'm assuming that since there are no signs of external coolant leaks, leak is internal (thru HG?) so when engine cools vacuum is not created to recover coolant from overflow tank?

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  3. #2
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    compression test will only show a bhg if its totally toast. 135 vs 152...im not convinced the numbers your posting show a healthy motor.

    with those results, I would look at 2 things

    1. leak down test. this is putting compressed air in each cylinder, one by one, at that cyl top dead center position (valves closed). put 150 lbs in and listen. it will leak into the crank past the rings, somewhat (remove oil cap and listen). most importantly remove radiator cap and listen there / look for bubbles. **should also note it can lead through intake (pop intake hose off by tb) and tailpipe...through the valves.

    I would do it with the car hot if you can. some gaskets only leak when hot (at first...)

    2. coolant analyzed for combustion gasses
    Last edited by madisonMK3; 04-30-2019 at 07:03 AM.

  4. #3
    7M Fanatic sixpack's Avatar
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    Head gaskets that are starting to go will only leak under running conditions because combustion pressure can go as high as 1000 psi.
    This is why cars driven by an old woman have lasted 200K, and then when grandson inherits the car go almost instantly.

    If you have a fresh radiator cap (another reason these engines can push coolant into the recovery tank, but not pull it back when cooled off),
    then I think that you probably do have a gasket going.

    And you better do it soon because some of the coolant will get into the oil, and start harming the bearings, leading to the next issue, failed rod bearings,
    long rumored to be the second weak point of the 7M.
    It isn't, the 7M has just as good bearings as any other car. but when running on diluted oil, they will fail, just like any other engine would.

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  6. #4

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    The previous owner had this car since 89 but barely drove it. I picked it up 5 months ago and first thing I did was retorque the heads and work on the cooling system (flush/backflush the engine block, radiator and heater core. Replace hoses, t'stat and radiator cap). Doing those things helped driveability and operating temps seem to stay in the middle unlike before where it would get two or three lines below red zone; but the overflow tank issue didn't go away.
    OK, I agree that doing HG replacement ASAP is best option since I especially do not want to risk warping the head. I've parked the car and will start getting parts ordered.
    In the meantime, I'll do a leakdown test.

    Thank you for the replies.

  7. #5
    90T 3p141592654's Avatar
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    You might double check you are using the right radiator cap. The pre-90 uses a "mini" cap (32mm ID and 16 mm deep) and the 90+ uses a micro (31mm ID 10 mm deep). The micro will fit on the mini rad outlet, but won't work at all. Most replacement rads use a mini, so it can get confusing on which cap to use. Also check your hose between the cap and the reservoir for any pinholes that will prevent it from vacuuming back the coolant into the rad.

  8. #6

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    Quote Originally Posted by 3p141592654 View Post
    You might double check you are using the right radiator cap. The pre-90 uses a "mini" cap (32mm ID and 16 mm deep) and the 90+ uses a micro (31mm ID 10 mm deep). The micro will fit on the mini rad outlet, but won't work at all. Most replacement rads use a mini, so it can get confusing on which cap to use. Also check your hose between the cap and the reservoir for any pinholes that will prevent it from vacuuming back the coolant into the rad.
    I checked the box that my new radiator cap came with and confirmed that it's a mini (Stant part no 10227).
    And yes, I also thought that there might be a small hole or crack in my overflow hose so I had temporarily replaced it but didn't make a difference. I even switched the overflow tank cap with the one off my NA but that also made no difference.
    Thanks for the heads up on radiator caps, I wasn't aware of the "micro".

  9. #7
    90T 3p141592654's Avatar
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    Well those were the easy things to check. I would do a pressure test. If you cannot suck coolant out of the reservoir you probably have a pretty good sized leak in the cooling system.

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