Help: Engine overheating and lots of bubbles in coolant...

[redrocket328]

Hey all,
   Hoping you can throw some ideas at me for this issue. I have a 1993 BMW 318is.

   I purchased the car with the exact issue it has today, I originally diagnosed the car as having a bad head gasket, or cracked head. After removing head gasket, visual inspection looked good, so I assumed it was a cracked head since apparently this is pretty common. After taking the head to the machine shop and having it pressure tested, they said that it was NOT cracked. So knowing that, maybe it was a small leak in head gasket, so I had them rebuild the head while it was off.
   I put the rebuilt head back on with complete new head gasket kit and it is having the exact same issue as before. When the car is cold, I don't notice any real bubbles in the coolant but as soon as it warms up, there were violant bubbles coming through the coolant system.

   1)I am thinking that it is just a hairline crack in the head that seems to seal when cold but as it heats up the crack expands and opens up.
   2)Other thought, is that it may possible be a cracked block.
   3)Are there any other things that could cause this?

Thanks.

[deansweet]

improperly bled coolant system(air pockets)?
coolant level too low?
bad thermostat?
bad coolant pump?
Bad fan clutch?
radiator clogged?

[DesktopDave]

I'm thinking you're right (but hoping you're wrong) about the cracked head.  I'd check all the other stuff first, to be sure.  

Like Dean says, it's possible that you're getting a boil-over in the system. The M42 operates at a higher coolant pressure than most cars.  If the system can't hold 30psi it'll boil over.  Another annoyance is that bleeding these cars is a PITA, especially with that mickey-mouse throttle pre-heater disaster under the intake manifold.  Also check for coolant at the plastic manifold on the driver's side of the block - they tend to crack & leak a bit as they age.  Don't take it off unless you have an OEM BMW spare as they always dissolve into at least three pieces.

Alternatively, can you get the exhaust gas tested for coolant when it's good and hot?  Or the coolant tested for combustion gases?

[redrocket328]

Thanks Dave. Can you explain more about the coolant boil over? What do you mean if the system cant hold 30 PSI, it will boil over? Why wouldnt it be able to hold 30 PSI?

Can I purchase one of those testers at autozone that checks for exhaust gases in coolant?

I'm thinking you're right (but hoping you're wrong) about the cracked head.  I'd check all the other stuff first, to be sure.  

Like Dean says, it's possible that you're getting a boil-over in the system. The M42 operates at a higher coolant pressure than most cars.  If the system can't hold 30psi it'll boil over.  Another annoyance is that bleeding these cars is a PITA, especially with that mickey-mouse throttle pre-heater disaster under the intake manifold.  Also check for coolant at the plastic manifold on the driver's side of the block - they tend to crack & leak a bit as they age.  Don't take it off unless you have an OEM BMW spare as they always dissolve into at least three pieces.

Alternatively, can you get the exhaust gas tested for coolant when it's good and hot?  Or the coolant tested for combustion gases?

[DesktopDave]

The M42 cooling systems must hold >30psi pressure to function properly.  When any part of the system develops a pressure leak, the cooling system might be able to hold 30psi when it's cool but likely won't keep holding at operating temp - 190degF or so.  If the coolant boils in the head, you get steam pockets and the pressure rises until it finds the weakest point of the system to escape.  That's what I'm hoping your car's problem is...not a cracked head.  I always hope for the cheapest resolution to any problem...

  Most cars use a slightly lower pressure, but BMW decided that all their newer motors would have high-pressure cooling systems...and it's caused a huge amount of grief over the years.  The older motors - M10, M20 & M30 all ran at 15psi and could tolerate a fair bit of overheating.  The problem that they tended to have was insufficient radiator area.  So BMW solved the problem with another problem.   Doubling the pressure makes a cooling system substantially more efficient.   OTOH, the expansion stresses are larger and the plastic/alu components eventually fail from structural fatigue.  That's why newer BMWs consider radiators & water pumps as consumables.  They're only designed to run for about 100K miles from what I've seen.

[irish44j]

cracked heater core, perhaps?