Strange engine overheating and bubbling in overflow problem

Doomslayer

Doomslayer

Active Member
So before you all say head gasket listen for a second.

Car overheated at track day due to a broken rad. I fixed it. Thought it was all good and moved on.

I've been doing mountain runs recently and my car has been fine until it stops. When stopped at idle it dumped about 3L of coolant out of the overflow bottle. We refilled and it drove fine

Did a mountain cruise and a run a few days after keeping an eye on the gauge and it was fine.
Did one the next day (busy week lol) and same issue as the first time but I caught it earlier and we only lost about 1L all from the overflow bottle.

We diagnosed to maybe a broke thermostat. As we tested with the cap off for a head gasket and it was fine.

I've just taken the car to my uncle who's quite a knowledgeable mechanic and he's tested it. We believe the overheating issue was due to an insufficiently filled overflow bottle but we tested again for the head gasket and the STRANGEST THING happened.

In the overflow bottle at idle. There are heaps of bubbles. When at 2000rpm, no bubbles at all.

Idle: Bubbling ALOT from the overflow and slowly filling the bottle.
Around 1500rpm and upwards: No bubbles running fine.

It was bizarre.


My question is: Anyone else had similar issues before? What could it be? What can I test? I'm already going to replace the radiator cap and thermostat just to be sure. Hoping it's not a head gasket.

Thanks
 
did you bleed the cooling system after replacing the bad radiator? if not, maybe there’s air in there?
 
When you said tested for the head gasket with cap off, was that just visual or exhaust gas test, other? I would not discount this entirely, overheating can be caused by a few things:
  • Coolant isn't circulating - pump, thermostat or other blockage
  • Insufficient coolant. Leaks, getting burned or air pockets.
  • System isn't holding pressure (cap or a leak)
  • Fans not working correctly. (Easiest to check)
Some other possibilities are:
  1. Radiator cap, as you mentioned, which keeps the system from being properly pressurized, any milky coloured residue under cap or discolored coolant?
  2. Fan isn't running. This doesn't have much of an effect other than when going very slow or parked. Could be a bad temp sensor, wiring to the fan or the fan motor. Fans rarely fail, but temp sensors are a maintenance item.
  3. Leaks. Radiator, overflow tank, hoses, etc.
  4. Water pump and/or thermostat. These will cause coolant not to circulate correctly. So, it just sits in place in the engine and gets hot.
  5. Head gasket(s). Coolant can leak into the cylinder and get vaporized, leaving you with insufficient coolant. The car may or may not run rough if this happens, depending on the severity of the breach in the gasket. You'd typically see white smoke (steam) coming out of the tail pipe, but it could be too faint to notice if the leak isn't large.
  6. Air pocket in the system, as a result of a leak or coolant system service.
I'd start by pressure-testing the system, basically a bicycle pump with a pressure gauge, adapter for your radiator or reservoir. You pressurize the system and look for leaks: either visible leakage, or a drop in pressure on the gauge.

If that doesn't show a leak, do a compression test, after that while plugs are out, disconnect ignition circuit pressurize radiator and get someone to turn it over while looking for moisture at spark plug holes (make sure no one is smoking under bonnet !) if you want to be doubly safe, cut the fuel as well.

Test the coolant for exhaust gases using correct test kit, which would reveal a problem with the head gasket.
It has to be one of the above surely ?

Good luck!
 
wagonmaster said:
When you said tested for the head gasket with cap off, was that just visual or exhaust gas test, other? I would not discount this entirely, overheating can be caused by a few things:
  • Coolant isn't circulating - pump, thermostat or other blockage
  • Insufficient coolant. Leaks, getting burned or air pockets.
  • System isn't holding pressure (cap or a leak)
  • Fans not working correctly. (Easiest to check)
Some other possibilities are:
  1. Radiator cap, as you mentioned, which keeps the system from being properly pressurized, any milky coloured residue under cap or discolored coolant?
  2. Fan isn't running. This doesn't have much of an effect other than when going very slow or parked. Could be a bad temp sensor, wiring to the fan or the fan motor. Fans rarely fail, but temp sensors are a maintenance item.
  3. Leaks. Radiator, overflow tank, hoses, etc.
  4. Water pump and/or thermostat. These will cause coolant not to circulate correctly. So, it just sits in place in the engine and gets hot.
  5. Head gasket(s). Coolant can leak into the cylinder and get vaporized, leaving you with insufficient coolant. The car may or may not run rough if this happens, depending on the severity of the breach in the gasket. You'd typically see white smoke (steam) coming out of the tail pipe, but it could be too faint to notice if the leak isn't large.
  6. Air pocket in the system, as a result of a leak or coolant system service.
I'd start by pressure-testing the system, basically a bicycle pump with a pressure gauge, adapter for your radiator or reservoir. You pressurize the system and look for leaks: either visible leakage, or a drop in pressure on the gauge.

If that doesn't show a leak, do a compression test, after that while plugs are out, disconnect ignition circuit pressurize radiator and get someone to turn it over while looking for moisture at spark plug holes (make sure no one is smoking under bonnet !) if you want to be doubly safe, cut the fuel as well.

Test the coolant for exhaust gases using correct test kit, which would reveal a problem with the head gasket.
It has to be one of the above surely ?

Good luck!
Click to expand...


You are a champion Wagonmaster.

just visual test on the rad
1. No milky colours on the radiator cap but I'll probably still replace it
2. Fans run all good we tested those
3. No leaks in the engine bay or where I park AT ALL. only leaks from the overflow hose when it overheats
4. I'm going to replace the thermostat
5. Haven't seen any smoke from my exhaust but I'll let my mates know so they can tell me if I'm driving in front of them. I have been burning a fair bit of oil but that may just be due to the overheating
6. mmm I thought maybe this was it from in insufficiently filled overflow bottle

Fix the thermostat and cap. Then I'll pressure test :) Thanks mate!
 
The big thing you can take away is that by it cooling down when revving, is that it is a flow related issue. The pump is the only thing that directly changes with rpm. Personally I would re-fill the cooling system with a vacuum bleeder. This way you should have no possible air pockets. Hopefully the issue goes with that. Get the thermostat done too still, as it's a possible symptom too. Good thing is, is that it's mechanical, so no possibilities of weird electronic junk causing havoc.
 
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