Pushing coolant in my Zetec powered Seven at higher revs. Why?

I've had an issue lately where with higher revs and longer WOT pulls, the expansion tank seems to foam/aerate to the point where it starts to push coolant past the cap. Once I use more sensible throttle the level settles back out. It seems to be aerating via the top bypass/bleeder hose. I've tried repeated bleeds and burps. Rad did some have a small leak I had professionally repaired. I've taken some coolant out of the level to try and prevent this, picture show is level at full cold. I may just replace the header tank and cap just to be safe. Zetecs have a thermostat that sends coolant through a bypass until they come up to temp, bypassing the rad. Also anyone know what expansion tank this is? Couldn't find a part number. No heater in this car. Silvertop Zetec with ITBs if matters Any ideas?

You may be able to test the cap with compressed air in the coolant system. Be careful, use a regulator. If the pressure for opening checks out, you probably have a hg issue or are lifting the head.

What belt setup are you running? 
Does it have a reverse rotation water pump? 

head gasket leaks at high speed?  That's how my Alfa showed it needed replacing.

test the rad cap to make sure its holding pressure.  if the cap is leaking, you could be boiling coolant in the block and pushing out the cap/overflow.  if the cap is good, pressure test the system and make sure it holds over several hours.  if it doesn't hold, you either have a leak somewhere else which should be obvious, or you can begin to suspect the head gasket.

In reply to Saron81 :

I have to take pic and find the part number. Also not sure what water rail setup I have. I know it uses the stock thermostat housing as opposed to the Raceline. I know my belt setup has no idler to reverse rotation. 

The other weird thing is, in neutral, if I rev up to say 5k, I can see the tank get foamy, and then will settle out normally. If I'm getting combustion gasses in the coolant, wouldn't the pressure continue to rise or hold? Maybe I'm just being optimistic.

When the vapor load is low it can vent the pressure once it's higher it pushes water.

NA or forced induction? Head bolts all tight? Possibly a broken bolt? Bad head gasket? Bad T -stat or radiator cap?

Funny I've been driving a zx2 for couple years and never knew there was a silver top.. And  My 99 doesn't have a pressurized tank, just old school overflow bottle. 
 

Anyways, sounds like water pump issues. Sounds like your pump is causing cavitation which would explain the  bubbles. Just my .02cents

 

I'm thinking it's some sort of head gasket leak, either one that's just slightly leaky and only causes a problem at high revs & load, or head lifting. Cavitation doesn't cause frothing in the overflow bottle, in fact if your entire coolant system were made of transparent materials you'd only be able to *see* cavitation right around the water pump impeller.

In reply to Subscriber-unavailabile :

An under drive pulley might help.   I ran one on my Z2SR,  lots of high revs.   Never lost coolant.

As I recall, the ZX 2 had a reverse rotation water pump.

An  under drive pulley might help with high revs.

Might be time to test for combustion gases in the coolant?

I had exactly this experience for 6 months in a Zetec engined seven before losing both the crank and cam oilseals on the same downshift.

After complete disassembly of the engine there was a visible combustion to coolant leak on the head gasket and a set of perfectly aligned rings on cylinder 3.

The coolant surge was an early indicator of cylinder head lift. (I had new stock ford cylinder head bolts on at the time.)

I had already tried new coolant cap and it did not resolve the issue.

There is a coolant gas tester you can get to check for gasses in your coolant. (Samcrac has a video on it testing his I8).

I'd like to be wrong about this, but my experience is that it's a head gasket. 

"Might be time to test for combustion gases in the coolant?"

I agree that it's time to run the dreaded 'bringer of bad news' test or tests (i.e. leak-down or combustion gases in coolant test).