Weird Coolant Issue

Im saying this because of the of large number of water pumps leaking, and the small number of thermostat gaskets I’ve seen leaking. Take a hard look at your water pump, and make sure it’s not coolant running down to the thermostat from the water pump.

kind of doesn't make sense if the coolant level is normal when it's not running, a tiny leak wont cause the level to drop like that while it's running, normally a small leak is just that a small leak and you LOOSE coolant and the level goes down from normal and stays that way.

Doubeleive said:

kind of doesn't make sense if the coolant level is normal when it's not running, a tiny leak wont cause the level to drop like that while it's running, normally a small leak is just that a small leak and you LOOSE coolant and the level goes down from normal and stays that way.

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My thoughts too. I would not be surprised if fix the leak and still has a problem/concern.

My guess is that the conditions were poorly described. I’m thinking when you are borderline low enough to trip the low coolant sensor, the lowest the coolant will be at the sensor is when you start a cold vehicle, and sustain higher rpm.

When I ran shops the most important part to start the repair was info from the owner. I asked a bunch of questions and did my very best to duplicate the problem. Sometimes here, the people asking for help are not the best at describing the problems and when they occur. Most of us here start out asking them questions because so much info is left out. So many times I see a section of the forum for each rig but during the problem description process, we don't get the exact year, model or engine/trans description.

Sorry guys. Yes I have had to add coolant intermittently. I should have noted this for sure.

I bought a pressure tester for the cooling system for my e38 740I.

It was a good thing I did as it came in very handy for the Tahoe. I put it up to about 15 PSI and it immediately started coming from the seam (not the lower radiator hose but the seam of the thermostat housing) I changed the water pump at about 180K along with the thermostat ( I have almost 200K on the vehicle ). It looks like something didn't set well when I put on the thermostat. I am going to give it about a month and see if the problem is solved.

It is kind of odd that I couldn't see this while the vehicle was running or as it cooled down. You would think i would have a small puddle under the car. Strange that it took a tool to provide the location of this leak and profusely. It was leaking pretty good too at 15 PSI. Well we will see if this was the only leak I had. Give it some time. I saved the coolant and ran it through a sieve. It seems to be running fine and not losing any coolant so far but again it was a slow leak over time.

Let me know if I missed something. These forums are super important to keep my vehicles running. I want to keep this car well past 300K miles. I think it can make it.

incidentally , the e38 did not show the leak like the Tahoe so I am still confused as to where the slow leak is on the 740i. I replaced the entire cooling system and seals/gaskets methodically. Maybe it is air in the system. Anyways thank you for your help.

tomloans said:

I saved the coolant and ran it through a sieve.

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You saved what coolant?

Let me know

wjburken said:

You saved what coolant?

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When I took the thermostat off, the coolant needs to be drained. It was still in good shape.

Just did the pressure test again while cold after running it about 20 miles yesterday. Holding pressure. This is rare to have this thermostat oring leaking but here it is. If something else comes up, I will let you guys know. Here is one of the best $45 I spent...

WINTOOLS 18pcs Auto Cooling System Radiator Cap Pressure Tester Kit Pump Gauge Adapters https://www.amazon.com/dp/B07JZ7STBG/ref=cm_sw_r_cp_apa_i_H3z0Db7RRN845