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Thread: Coolant temps with AC on?

  1. #1
    Join Date
    Jul 2012
    Location
    Herndon, VA
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    397
    My Cars
    98 M3/4/5, RIP 528e

    Coolant temps with AC on?

    Hi guys,

    I rally-x my 98 M3/4/5. I put in a fresher motor last summer after the original had low compression and was chugging oil. During the swap I transferred all my new coolant system parts to the newer motor. I've got a Stewart water pump, Z3M radiator, new hoses, BMW coolant, OEM BMW thermostat (90*C/195*F IIRC), etc. There are no bubbles or anything in the system at this point. It also has a nice electric fan and corresponding sensor in the radiator. I ran a bottle of Evaporust Thermocure through the system to clean out all of the gunk, and it worked wonderfully. No more gummy build up anywhere!

    I've gotten into the habit of monitoring my coolant temps, and I noticed something that worried me a bit coming home from the first race of the season. At the race, temps in grid stayed around 203* (monitored via Torque app on phone). Ambient temp was about 80* with average humidity. When cruising at 80 mph on the highway, temps stayed around 200-205*. On the way home, I turned on the AC for the first time in a long time. Obviously, my coolant temps started rising when the heat from the condenser started getting pushed into the radiator. I started getting worried when the temps rose to 210*. I was going up a very long and pretty steep hill, so there was a significant load on the motor. Once coolant temp reached 210* I shut off the AC and waited to turn it back on when I started going downhill again. I took longer than I expected for the temps to drop back down to their normal range. This happened a few times during the drive.

    I know 210* isn't that much higher than 205*, what worried me was how it seemed to struggled to drop back down, given that I was going 80 mph and had plenty of airflow through everything. I didn't want to leave AC on and see how much higher it was going to go. In my other car where I can monitor coolant temps, they NEVER go above 205*, even with AC on on a hot day. I had never really thought about it, but there seemed to be a hard limit of how high temps could go.

    So my question to you is this: What do your coolant temps look like when you turn on AC? When parked? When on the highway? How high are you comfortable letting your coolant go before taking some action?

    The only thing I can think of as a contributing factor is that there was a significant amount of dust in the condenser and radiator from the race. I took my leaf blower to them the following day and cleaned them up real good. Aside from that, I'm drawing a blank. Am I just freaking out over a few degrees or is something funny going on here?

    Thanks!

  2. #2
    Join Date
    Nov 2015
    Location
    Clarksville, TN
    Posts
    85
    My Cars
    1995 BMW 325i
    running in the dirt clouds like your picture. i would use coil cleaner, alot. spray watch the foam eat the dirt. while leaf blower would be first step cleaner would get inside the fins.
    clean the coil for your home ac same way. if dirty will greatly reduce its cooling ability. dirt can act like blanket.

  3. #3
    Join Date
    Jul 2012
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    Herndon, VA
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    397
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    98 M3/4/5, RIP 528e
    Quote Originally Posted by poweredbyg60 View Post
    running in the dirt clouds like your picture. i would use coil cleaner, alot. spray watch the foam eat the dirt. while leaf blower would be first step cleaner would get inside the fins.
    clean the coil for your home ac same way. if dirty will greatly reduce its cooling ability. dirt can act like blanket.
    Funny you mention that, I actually did use a can of coil cleaner spray on the condenser when I was doing the motor swap last year. It got some stuff out but it wasn't as dramatic as I had hoped. The radiator is relatively new and didn't need heavy cleaning then. After I blew all the dust out with a leaf blower I also hit the front of the car with the power washer, so between the two I think everything should be clean now! Maybe I'll have to bring the leaf blower to the next event and clean the car off before getting on the highway. The dust was so bad that one car's air filter clogged and started to cave in! I sure am glad I run a pre-filter on my air filter!

  4. #4
    Join Date
    Nov 2005
    Location
    Wakefield, MA
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    2,659
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    '97 M3, '17 GSW 4Motion
    Your highway cruising temps are a little high. What temp thermostat are you running? I'm running a stock 88C (190F) tstat and on the highway it sits right at 190F the whole time. If the system can't cool down to the tstat point cruising on the highway i don't see how it can on track.

    Going up 5-10F with the AC on is probably normal.
    Last edited by TXBDan; 04-18-2018 at 09:37 AM.
    COMSCC T70 E36 M3

  5. #5
    Join Date
    Nov 2015
    Location
    Clarksville, TN
    Posts
    85
    My Cars
    1995 BMW 325i
    hopefully you tried the foaming coil cleaner.
    i would blow it after every session, dust plus moisture will dry and not remove easy. pressure washing your car doesn't clean as good as soap and brush ? damaging/bending the fins would be easy with high pressure water.
    i saved many not cooling ac units back in the day with bleach water and toilet brush!
    in my time abroad in lovely countries, it was a constant battle for high humid and dust for car filter, good times.
    you guys run gravel? or just dirt? i would may consider a pre screen on your front end, like screen window material to catch some dust easy to clean between sessions?

  6. #6
    Join Date
    Jul 2012
    Location
    Herndon, VA
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    98 M3/4/5, RIP 528e
    Quote Originally Posted by TXBDan View Post
    Your highway cruising temps are a little high. What temp thermostat are you running? I'm running a stock 88C (190F) tstat and on the highway it sits right at 190F the whole time. If the system can't cool down to the tstat point cruising on the highway i don't see how it can on track.

    Going up 5-10F with the AC on is probably normal.
    As mentioned in the original post, I'm using the BMW OEM thermostat, which I believe is 90*C/195*F IIRC.

    - - - Updated - - -

    Quote Originally Posted by poweredbyg60 View Post
    hopefully you tried the foaming coil cleaner.
    i would blow it after every session, dust plus moisture will dry and not remove easy. pressure washing your car doesn't clean as good as soap and brush ? damaging/bending the fins would be easy with high pressure water.
    i saved many not cooling ac units back in the day with bleach water and toilet brush!
    in my time abroad in lovely countries, it was a constant battle for high humid and dust for car filter, good times.
    you guys run gravel? or just dirt? i would may consider a pre screen on your front end, like screen window material to catch some dust easy to clean between sessions?
    Yes, it was the foaming coil cleaner. My power washer is relatively low pressure and shouldn't be damaging anything from the distance I'm using it at.

    We have run on various surfaces over the years. Dirt, clay, gravel, now back to dirt. I run a pre-filter over my K&N cone air filter.

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