Agree with the venting idea - And that could be an early sign of a head gasket leak. If you have or can get a combustion gas leak test (it's a kit), check the coolant for combustion gas.

We have it parked. The wife stays at home so it won't be driving anywhere.

Could you please you elaborate more on why the pressure is building and where it's being vented to?

It vents out the expansion tank cap, there is a pressure relief valve of 2 bar built into the cap. The venting is a byproduct of some other problem, under normal system pressures and operation it should not be building up 2 bar of pressure to vent - so that is why you need to pressure test the system and see where it is leaking from.

When you leak out coolant you have too much air in the system which builds up pressure with the temperature, thus it vents. So find out why you are leaking, have it filled/bled etc to correct coolant level, and then you shouldn't be hearing any more "pop" and spraying coolant, which is coming from your expansion tank cap.

I also suggest getting a 1.4 bar cap from an E30 BMW. Makes it more likely that the cap will be the pop, and not some hose. You can search previous discussions on the matter.

You could have popped something just by getting the mixture wrong.

Thanks for all the insight guys, it's very helpful. I'll pull the skid plate off this weekend and pressure test. Hopefully it's not something horrific

Keep posting I have two of these cars and need to have my fixit solutions at the ready.


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Will do!

Please save time and money and buy a pressure tester kit
Radiator Pressure Tester Kit

What you do is make sure the engine if full of antifreeze/water. Engine cold remove cap and install the pressure tester pump...pump it up to the cap pressure 10psi? you don't need much. What happens is the air pressure will force the coolant out where it is leaking and in less than 5 mins you will see where the exact leak is no matter how small or large. If the gauge stays at the pressure you stopped at and does not move for 10mins than you have no leak..in that case I would let sit overnight with pressure gauge at 10 PSI.

You have replace the common parts and that's fine but these X5's anything can leak after that. I have an 02 4.4 and I've replace and I mean EVERY coolant hose-pipe fitting radiator-water pump on the car. I have a 06 3.0 I just replaced the water pump and upper and lower hose and rest is fine.

BMW used some really shit plastic and rubber in these X5's and the deteriorate big time or time. To use my 02 as an example, 14 years all rubber and plastic goes....already hanged my cracked plastic timing chain guides.

So save money and time, buy a pressure tester.

Not really.

So our system based on the rating on expansion tank is 2 BAR. 10 psi would be roughly .7 BAR. You need to pump that sucker up to about 29 PSI to reach the limit of the cooling system. Yea, I know, that is insane psi for any car. And our cooling systems really don't reach that limit normally.

I suggest you go higher than 10 psi to test. Like 20.

And as stated prior consider switching that 2 bar cap to something like 1.4 e30 BMW cap or the 1.2 cap. At least give yourself a fighting chance before blowing hoses off on a 10 year old car when over some issue causes over pressure.

There are a lot of bad info on message boards and the internet. Take it from a ex tech. Only pump your system up to the cap pressure limit. Any higher and you can do damage to your system especially blowing the seal on the water pump or damaging a worn radiator, remember these cars are old.

Most coolant leaks will start to leak at just 5 psi.