Recently, on my 04 disco, the coolant tank will be full to correct fill level at start but after driving, expansion tank looks empty. Engine temps stay perfectly normal. No visible leaks or smell of coolant, no white smoke. When I open the full cap on the expansion tank, fluid then fills from the lower hose on the tank - like it’s under a vacuum.
It feels like I have air trapped in the lines but when I open up the bleeder valve, I get coolant coming out immediately.
Any ideas what’s going on?

 

A possible explanation is (weak?) coolant hoses expand during hot pressurized operation containing more coolant. When engine stops and pressure fades, they deflate returning to smaller size expelling the extra volume into the reservoir.
Engine temperature "stays perfectly normal" means nothing if you are looking at the dash thermometer which is nothing else than a useless adorn.

 

Temps are monitored with Scan Gauge. Hoses are the same I’ve had on for past two years with no issues.

 

Run it with the cap one turn loose and see if it does the same thing.

 

Is the cap supposed to let air through to balance the back pressure ? Or does it seal completely?

 

The cap is supposed to vent at 15 psi. Running it one turn loose does not let the system build pressure and thereby does not cause the hoses to swell (and visually lower the coolant level). If you have a small coolant leak in to the engine it will also reduce the leak/flow/drinking rate due to there being no pressure to force it in to the cylinder. I permanently run all of mine 1/2 - 1 turn loose with no adverse affect other than the boiling point of the coolant is lowered from 250 to 230. We all know if it ever gets to 230 you have bigger problems anyway.

 

What rate does your coolant drop?

 

Pretty slowly. But, as soon as I open the reservoir cap, the ET fills right back up. I’ve bleed the system repeatedly, thinking there’s air in it and that’s creating a vacuum, but still nothing.

 

I asked because I had a similar situation. It turned out to be a very small hg failure on the block exterior at the back of the engine on cylinder 7. There were no misfires, high temps, white smoke or anything like that

 

Yeah...I’m really hoping it’s not head gasket issue.
Anyone have another proven method of bleeding the coolant system? I tried Disco Mikes method - maybe just wasn’t patient enough with the bleeding.