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Thread: 1998 528 overheating at idle + running hot issues

  1. #1
    Join Date
    Sep 2016
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    Southern California
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    02 325XiT, 98 740il,

    1998 528 overheating at idle + running hot issues

    TLDR; early production 98 528 overheating while idling but not driving, replaced entire cooling system (replaced water pump twice) and sensors and issue is still there. Passenger side of radiator and lower hose get warm but not hot like upper hose and driver side radiator. Runs between 101c-116c and it cools down with Higher rpms. Leak down test showed negative head gasket leak

    Any ideas or help would be great


    So my friends 1998 BMW 528 early production has been having an overheating issue that started about a month or so back

    He was driving when all of a sudden he heard a snap. Followed shorlty by his cars temperature guage starting to go into the Red.
    He pulls over and shuts off his car - fast forward to when I take a look at the car at his house after it had been towed and I found the water pump pulley snapped and laying on the under cover with the belt snapped. I figured the water pump failed which caused the pulley to lock up and break along with the belt. Pretty good assumption right? So we decided to replace the water pump and pulley, but figured if we are taking off the fan clutch that we should just go ahead and replace most of the cooling system: water pump,thermostat, upper and lower radiator hose, fan clutch. So we replace everything, bleed the system and go for a test drive. Well we noticed that it still started to overheat. After this incident we use a live data chart on a scan took to check actual temperature - it was anywhere from 101c getting up as high as 117c. Seeing as how we just replaced the item that psyically broke and more...I figured it should be running cool after that. By the way we used a Graf water pump, an oem thermostat and hose and clutch. So the parts should be quality. We start inspecting to see if the thermostat was opening up by: checking the upper and lower hoses,monitoring temperature etc..which the thermostat was opening up. But we did notice the electric fan in front of the radiator was not turning on when it was running hot - but it would turn on if you command it on with the ac button. I figured the Temp switch in the radiator must have been bad. So my friend orders Every other part in the cooling system...radiator, Temp switch, electric fan, expansion tank and we just replace it.
    So at this point...everything is new in the cooling system with quality parts.
    We even ran a leak test to see if the Head gasket was blown - test was negative.
    Recently we noticed 2 key things:
    The lower hose was getting warm...but not really hot like the upper hose
    And 2
    When we idled the car it would get hot, but with RPM (either at idle or driving) it would cool it down.

    To me that seems like a coolant flow or blockage issue . The rpm's are helping the water pump circulate and lowering temp.
    But we just replaced everything including the water pump... So we warranty replaced the pump. And same issue was happening to us the past few days

    We are lost at this point
    We don't know why there's a coolant flow issue
    And why the electric fan isn't turning on when its hot ( maybe cause that passenger side of the radiator and lower hose doesn't get too hot)
    Last edited by BimmermanJp; 05-21-2018 at 01:59 AM.

  2. #2
    Join Date
    May 2013
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    2000 BMW 528iA
    Did you bleed it properly?
    2000 528i sport
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  3. #3
    Join Date
    Feb 2003
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    Yonkers, NY and Argentina
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    1998 528i 5speed Sport
    Bleed it again. Put the heater at full heat, turn ignition to on, so the electric coolant pump circulates, open the bleeder screw and check for bubbles. Add coolant to the resevoir. Do it till only coolant comes out from the bleeder screw.

    Sent from my SM-G930V using Tapatalk
    e30 325i, the mighty 4 door granma mobile....Gone
    e39 528i, 4 door sports tank

  4. #4
    Join Date
    Feb 2014
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    ‘01 540’00 528T’03 525T
    He probably doesn’t even know about the 2nd bleeder screw....
    Jack the front end up, turn on a brain cell, bubbles will head upwards. Easiest way to bleed an I6 E39, jack it up, turn on the heat, all the way up, stratification knob showing 3 dots, car running, open the bleed screw on the thermostat housing, when it’s just oozing coolant, close it up, open the bleed screw on the expansion tank, when that starts to ooz, seal it up if the gauge is in the usual normal spot, hot air coming out of the vents, your good to go. If you don’t bleed the system right, it will overheat. Car may need a burp or two, but with the front end jacked up, first time is usually the charm. The V8’s are a lot easier to bleed, don’t even need to jack the front end up.

    - - - Updated - - -

    When the car cools down, check the expansion tanks level, top of the floater should be at cap level.
    Set the controls for the heart of the sun

  5. #5
    Join Date
    Sep 2016
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    02 325XiT, 98 740il,
    No we definitely bled it properly. Multiple times with the heater full blast and on an incline
    It's not an air pocket issue. Cause that would be a sporadic overheating. This only happens at idle which shows a coolant flow issue

  6. #6
    Join Date
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    I'm not sure it applies to E39s, but it does to earlier BMWs regarding bleeding the system. Does the heater work? Since it's relatively high in the system, if there is air trapped it might just well show it by not working if it isn't bled.

    Past that, don't assume new parts are good, even good names, particularly with so much of the production in China these days. The thermostat in particular seems suspicious, if it's opening, you should have coolant flow, especially with new parts such as the radiator which I was going to mention until I saw it was already replaced. But as you've figured out, even if it's overheating, if the coolant returning from the radiator is cool, the sensor won't tell the fan to come on. It really doesn't sound like a head gasket, but it might be worth one of those test kits that test for CO just to rule it out. You might be up for a trip of taking things apart, one by one, checking for flow or restriction and approaching it that way.
    98 328is
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    and decades of owning and driving BMWs

  7. #7
    Join Date
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    02 325XiT, 98 740il,
    The heater does work perfectly. It gets HOT. I'm positive there's no air in the system.

    My next step was to replace the thermostat - because the lower hose is just warm at idle and hot with rpms. Maybe a sticking thermostat?

    Or the water pumps we have been using are the metal impellers I'm wondering if we should switch back to a composite.

    We did a leak down test on the expansion tank and it didn't find any exhaust gases in the coolant
    We did an oil change and didn't see signs of coolant/oil mixture

  8. #8
    Join Date
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    ‘01 540’00 528T’03 525T
    If it’s not a bleeding issue, than the thermostat you got is not opening up, your water pump is fine.
    Set the controls for the heart of the sun

  9. #9
    Join Date
    Aug 2016
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    Virginia,USA
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    3,144
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    528i, 525iT, M5
    Fan in your application uses a different set up from later units. Sensor on side of rad. Check fuses as your fan is a three speed. QSilver will post the same scalematic of your system for the 27th time shortly. Meanwhile look up any "my engine fan is not working" post with the early 6 application (528).

    My internet guess is pusher fan not operating correctly. Question: does than fan cycle at idle? Will it turn on with the A/C switch in the on position?
    Last edited by StephenVA; 05-21-2018 at 05:58 AM.
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  10. #10
    Join Date
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    02 325XiT, 98 740il,
    Yeah thermostat is what I'm trying next

    Correct the fan doesn't turn on due to temperature
    But if you read the full post you'd see that I mentioned the Fan will come on by turning on the AC
    So I don't think it's a fuse. It's that the temp sensor isn't getting hot enough to turn it on.

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