My 1998 528 bmw was running great. It has been really hot outside the last few days. When I was parking the car, the coolant went into the red. I turn the car off, left it cool down&check coolant. It's only slightly below the line. I took it around the block a few times later, and not in the red (so far). What do you think happened?

The A/C causes the engine to work harder (that's why fuel economy suffers when it is run).

If your car has an electric fan - make sure it runs when the engine is hot. If it does not - you may have trouble w/o the A/C running if is gets warmer outside. Look for fail fuses, disconnected plugs, etc.

You may also have flaky thermostat that is not opening fully when needed to exchange hot water in the engine block with cooler water in the radiator.

First, check the coolant level. You should find a semi-transparent reservoir / tank located under the hood that has a cap marked "Coolant". The side of the tank should have two indicator lines - one for HOT and one for COLD. If the engine has been run up to normal temperature (so that it can provide heat in the car), you should see the coolant level at or slightly above the HOT line. If the engine is cold - first thing in the morning or after sitting for several hours - the coolant should be at or slightly above the COLD line. View these levels while the engine is off and the car is parked on a flat, level surface. If there is not enough coolant - you will need to add the CORRECT type for your car.

Hot Engine
(1) With the engine hot, add enough coolant to the tank to bring the level to the HOT line. You may have to check the tank several times after driving if the coolant level in the radiator is down a great deal. NEVER open the radiator cap when the engine is warm or hot!

or:
Cold Engine
(2) With the engine cold, add enough coolant to the tank to bring the level to reach COLD line. Additionally - with the engine cold - you can add coolant directly to the radiator by removing the cap. Start the engine, set heat in car to HOT and add coolant mixture to radiator until level comes up to the bottom of the filler neck. Once the engine is HOT (getting heat in the car) check and top off radiator as needed and put cap back on radiator.

Check the levels in the TANK several times over the next few days when cold. If the tank ever empties -you'll need to have this looked at by a mechanic.

1. Check coolant level and then do an actual temp check of the coolant once the thermostat has opened your checking to see if it is opening).
(if coolant level was really low, monitor it every day for a week of driving -- need confirm not loosing it inside the engine)
2. Replace fuel filter.
3. Verify fuel pump is working properly.
4. Ensure fuel lines are not touch hot engine/exhaust parts (this percolates the fuel).
5. Run fuel injector cleaner (concentrated for 2 tanks) Use premium gas for one tank.
6. replace spark plugs (first if you have not for some time)
(check spark wires (no more than 50 ohms resistance) Replace if greater)

it is hot and the low coolant light is on, two clues that your coolant is disappearing. three places for it to go. 1) into combustion chamber will cause rough running, white exhaust, and coolant smell at exhaust pipe, 2) into lubrication system, pull off your oil fill cap and look for foamy **** in hole or on bottom of oil cap or on dipstick, 3) outside engine and you will find coolant puddles where you park. coolant is generally green or orange/brown these days. also generally will be stains on the engine where the coolant leaks down and off.

first two mean a bad gasket or crack and will be expensive. last one means someone has to find the leak and stop it. if your interior stinks and your passenger's feet sit in a slippery wet spot, then the leak is in the heater core, that will not be cheap.

This could be leaking fuel injectors, a stuck Fuel Pump relay, and/or faulty fuel pressure regulator. Have you had any of these items checked and/or replaced? If not, it may be time to replace them. Also, check your fuel filter. If it hasn't been replaced in the last year, replace it.

I’m happy to help further over the phone at https://www.6ya.com/expert/jeremy_69f3cc28d95bf514

bad thermostat and coolant maybe low. when a engine gets to hot at certain boiling point it evaporates

overheating is most likely a bad thermostat, but the coolant loss is a second big concern. Does the exhaust look like steam? You could have a cracked block and the coolant is leaking into the engine. Also check the oil for contamination.

It sounds like just a coincidence. I dont know what a GRR noise is supposed to mean. It either Cranks, No Crank, or it Cranks but does not Run. Just tow it to a qualified shop and find out what the problem is.

You can try this.
Turn on the key and listen for the fuel pump to prime.
It will sound like a gentle hum. when it stops, turn the key off. wait a few seconds and do it again. Do this three or four times, but don't try to start it. after the third or fourth time you turn it off, try to start it. It may not be priming the fuel injectors fast enough.

Hello...

I would start, since it is cold outside by checking to make sure the battery is in good shape. If it is not, it will run but when the cold comes it will eventually not turn over. Note: Turning on the lights to see if they are bright is not a good check for the batteries abilty to crank the car. If you have a Checker or Autozone or other auto parts store see if they can test your battery for you.

With the sudden stop starting I would also suspect possibly the fuel pump or the pump relay.

Good Luck...

Mike