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Old 08-31-2019, 01:28 PM   #1
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Running Hot, Adjust the Coolant Mix?

Hi Everyone!

I recently discovered some great mountain roads near me, and have been enjoying some top down driving this summer! But I am having some cooling system issues. I flushed the coolant system when I installed a third radiator, new water pump and low temp thermostat about 10k miles ago, and used OEM coolant in a 50/50 mix. The coolant tank and cap are fine, no leaks, radiators fans all work and are clean.

I have been running at the middle tick between 180 and 250 this summer in temps over 70 F, even with my high radiator fans manually triggered. (The temps climb normally to 180, then if it is hotter than 70 F it starts climbing again after a few minutes.) Yesterday, I got caught in traffic and my coolant temp started to pass the middle tick, I pulled over and let it cool down. The coolant level was good, oil was fine. I started it back up to get to where I was going, and the temp kept climbing. I pulled over again and got a red coolant light as I stopped. At that point the coolant temp was 122 C (250 F), and the engine fan triggered. I was also having issues with my idle at this point. When I came to a stop instead of dropping to 900rpm, it was fluctuating, and I had at least one stall from the rpm getting too low. Once I got back on the freeway, no issues, it cooled back down once I got air into the radiators at 70mph+.

I am dealing with 90-110 F temperatures at about 5,000' feet this summer, but I also deal with snow in the winter, as low as -10 F. I am thinking I should adjust the coolant mix to have more distilled water than anti-freeze/coolant so that it has a higher capacity for cooling.

I can't think of anything else to change/try/test, any advice? Thanks!
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Old 08-31-2019, 02:42 PM   #2
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At 250 your oil is probably toast.

At that altitude you may need a higher pressure cap or you may want to consider running Evan's waterless coolant that doesn't need pressure.
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Old 08-31-2019, 02:43 PM   #3
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When the coolant flush was done how did you purge any air in the system ?
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Old 08-31-2019, 03:42 PM   #4
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Chi-Town, I'm going to be changing the oil this week, thanks for the heads up, I will take a look at that coolant suggestion. What would a higher pressure cap do?

rfuerst911sc, I used a Uview to refill the coolant system, and I didn't have any issues for two years until yesterday.
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Old 08-31-2019, 04:58 PM   #5
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I'm at 4000', very dry, desert climate. This week as I've had the car back together finally, it's been in the 95 to 100 degree range.

This motor has a stock thermostat (as far as I know. It's used) and unknown mileage on the water pump. And it's rock steady at just barely past the 180 mark. Even in stop and go traffic.

I think you've got another problem somewhere.
You sure your water pump is good? What brand pump is it?

With the 180d thermostat in the last motor, I could run at the track all day (3700') without heating issues. And I'm not going slow.

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Old 08-31-2019, 04:59 PM   #6
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Water at higher altitude, where atmospheric pressure is lower, boils at a lower temperature. Conversely, water under pressure boils at a higher temperature. The pressure rating of the cap will have no effect on the coolant temperature but will help keep air pockets from forming.
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Old 08-31-2019, 05:11 PM   #7
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I installed a new OEM Pierberg water pump.

The only other thing I can think of is that there is an issue with the actual radiators, but I have never had a WP impeller fail, so I don't think there is any blockage. The car warms up normally to 180 and stays there for several minutes, but then it just seems like the cooling system cannot keep up with the outside temperature and the temps start rising unless I'm going 50mph+ without stopping.

Everything is perfect in cold weather, the coolant temp sits a tick below 180 with the 160F thermostat.
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Old 08-31-2019, 06:58 PM   #8
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Process of elimination. These are things that have caused 986 overheating in the past. Check off the list and see what you find:

* Totally clogged airflow between the rads and A/C condensers.
* Air bubble in the coolant causing intermittent overheating.
* T-stat stuck closed.
* Water pump impeller chunks in the head coolant passages.
* Reversed send/return coolant lines after changing the WP.
* WP impeller chunks blocking radiator passages.
* Failed rad fans

Good luck!
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Old 09-02-2019, 09:08 AM   #9
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Thank you for all of the responses! I should have been more thorough before I said the fans where fine, the relays click over but when I pulled all 4 relays this morning and jumpered pin 30 & 87, the driver's side fan is completely dead. I hadn't been able to tell just by listening. I'm going to start by replacing the fan since it doesn't require removing the entire bumper/radiator assembly, and see what my operating temps look like.
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Old 09-14-2019, 05:20 PM   #10
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I replaced the fan last week. I bought the entire Bosch (OEM) assembly for $175 + shipping. It took three hours accessing the fan through the wheel well, using the Bentley Manual and Pedro's Garage guides. It's not a complicated repair, the hardest part is getting the bumper support on and off.

I was waiting for the temps in my area to rise again, it was 86 F today, so I did a test drive. I drove for about an hour, and the fan was the cause of all my problems. Without using my manual high speed fan switch, I was running between the 8 and 0 in 180. With the high speed fans it dropped to 180. Still not 100 F outside like when it originally overheated, but I am confident I have resolved the issue. I am amazed that losing one fan caused my coolant temps spike so much.
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Old 09-14-2019, 09:55 PM   #11
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Quote:
Originally Posted by rick3000 View Post
I replaced the fan last week. I bought the entire Bosch (OEM) assembly for $175 + shipping. It took three hours accessing the fan through the wheel well, using the Bentley Manual and Pedro's Garage guides. It's not a complicated repair, the hardest part is getting the bumper support on and off.

I was waiting for the temps in my area to rise again, it was 86 F today, so I did a test drive. I drove for about an hour, and the fan was the cause of all my problems. Without using my manual high speed fan switch, I was running between the 8 and 0 in 180. With the high speed fans it dropped to 180. Still not 100 F outside like when it originally overheated, but I am confident I have resolved the issue. I am amazed that losing one fan caused my coolant temps spike so much.
Excellent news! Well done.

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