Venturer

In this very hot days here (108 fahrenheit ambient temperature where i live) when i use the car with A/C on and i turn off the engine i hear like a "pssssfff" boiling sound coming from then engine 10 seconds after i shutted off it. It lasted for a very few minutes. My car is the turbo 2 S5 version.
I can't understand where it comes from. During use the car doesn't overheat and it behaves normally.
Should i worry about it?

Venturer

Anyone?

DC5Daniel

iTrader: (10)

My car has done this as long as I've owned it (7 years). The iron block of the rotary retains a lot of heat, and the coolant system runs at a relatively low pressure to protect the seals. I've been told this is normal for the boil over to happen when the car is shutdown and the water pump is no longer circulating coolant in the engine.

If someone chimes in and says this isn't normal, then I think we both might be in for some heartbreak!

Pete_89T2

iTrader: (4)

It's hard to diagnose sounds some one describes over the internet, but here's how the system is supposed to behave under such conditions (i.e., engine has been running, heat soaked & then shut down hot after driving in high ambient temps).

As soon as you shut it off, coolant stops circulating, its temperature will quickly rise and it may even start to boil in various hot spots within the engine. This boiling liquid wants to expand, so it raises the pressure in the closed system until it exceeds the pressure of your radiator cap, which is about 1 bar or 14.7psi (1 atm. at sea level) IIRC. When that happens, the coolant has an opening to vent into the coolant overflow tank if everything is set up & working as intended - that's when you'll hear a noise as the boiling coolant pushes its way into the tank, and that is probably what the OP is hearing. Should only last a few seconds after engine shutdown. If you check the level of coolant in the overflow tank at this point, it will be above the "Full" line (assuming it was at the full line when the engine was cold and radiator was full). Now as the car cools down, and the temps/pressure of the coolant in the engine/radiator drop, it will start to draw a vacuum that will pull the coolant in the overflow tank back into the radiator/engine.

Venturer

Ok thank you very much for the explanation, very clear and detailed.

So it's normal, i'm comforted.

But i have another question : why does this happen often when i use the a/c? With a/c off this doesn't happen. Only one time this happened with a/c off when the ambient temperature was 42.5 celsius (108 fahrenheit).
Do you think the load of the a/c at idle will raise the temperatures?

Thank you

scathcart

Because you mentioned that this only happens when you use air conditioning, i would think what you’re hearing is the pressure equalization across the expansion valve when the compressor is shut down. Check your system pressures.

Venturer

No Scathcart it isn't the sound of the a/c refrigerant in the a/c pipes what i hear. I restored personally the FC A/c system and charged it with R12. I can recognize the sound of expanding refrigerant. And however the a/c system works perfectly.

What i hear it's a boiling coolant sound and something like a relief pressure wasting.

Pete_89T2

iTrader: (4)

Your car will run hotter with the AC on, especially if there's any stop & go driving involved. When the AC is on, the compressor is putting more load on the engine, plus the AC condenser will be dumping its heat right into the front of the radiator, raising the ambient temps of the air entering the radiator a bit.

Though scathcart is also correct about the pressure equalization in the AC system when you shut it down. If you're still running R12 in the system, pressures on the high side will be around 120~150psi or so, while the low side is maybe around 15~30 psi. When the compressor shuts off, the high & low side pressures will equalize, eventually reaching the static pressure of the refrigerant gas, which will depend on ambient temperature. Sometimes I can hear mine for a few seconds though it's not very noticeable.

NCross

iTrader: (18)

I agree /\

If all you hear is some air percolating when you shut it off and see no coolant loss I would not worry about it. Although... I replace radiator cap as maintenance. If a cap does not hold pressure it will push coolant into the reservoir and suck air into the system when pressure equalizes.

Venturer

That's exactly what i hear! And no coolant loss at all!

8994655

My car does that when AC is on for a while.....like they said if coolant is fine you are fine.

jpiteo

I have a similar problem. 1989 RX-7 non-turbo

• High ambient temp (+90)
• A/C on
• While running, the water temp is normal

On shut off

• I get percolation
• Boiling into overflow tank
• Water flows out of overflow tank

Is this a radiator cap problem? Or...?

JWP

rlynchster

When my 1986 N/A finally went, it was because the channel between the exhaust, and water, in the block, finally corroded.

The effect was that exhaust gas pressurized the water channel and forced water out of the system. The natural place for the water to go was to the overfill tank.

Towards the end, the overfill tank would catch all the water being pushed out and fill up once every three days. . . . This of course was followed by the low water alarm.

If your water cooling system is running hot, it will push water to the overfill tank.
If your radiator cap is bad, it will allow water to fill the overfill tank when it shouldn't.
If you have a corroded block, it will pressurize the system as above.

If you are experiencing this, and the low water alarm is not going off, that is another indicator that something is going on. The sensor on top the the radiator is either non functional, or the amount of water you are losing to the overfill tank is not an issue. . . .Yet. . .