Heater hot then cold

[the boffin]

Hi Guys wonder if you can help

Just Had my bongo front heaters playing up with a really weird fault. when i go up hill or a slight gradient my heater gets hot, then going down hill it gets cold, i just took it to a local garage who tested the head gasket for leaks and thats all ok.
I was wondering if this sounds like a cracked head, air lock or blockage as my coolant level was low.

Also my temp gauge went sky high and i had bubbling in the expansion tank..

Any help would be gratefully appreciated.

[francophile1947]

Your system needs bleeding properly before you drive anymore.
Firstly you need to find out where the lost coolant went to, fix the leak and then follow one of several approved methods for bleeding the Bongo - there are 3 videos outlining the most popular method
How did the garage check the head gasket? The symptoms you mention do point to it being faulty

[Northern Bongolow]

as john says,it sounds like you have a leak.ask the garage to do a pressure test,that should show up most leaks.the 3 video's are on youtube,type in bleeding a mazda bongo,and they should pop up.they show one way to step by step bleed all the air out.
the bubbling in the tank was the superheated water escaping as steam.

[the boffin]

The garage tested the head with a syringe type pippet with a rubble bulb on top with a fluid in it which the mechanic said should change colour if the head gasket was faulty, but it stayaed the same colour apparentlt it tests for gases present in the heating system.

[Northern Bongolow]

there are different ways a head can go.
water into oil. water into exhaust.oil into exhaust.
but usually with bongos it seams to be water into inlet/exhaust, not sure if this would show up on your test. all these are usually done cold,and may only show later when the engine is hot.

[mister munkey]

The garage tested the head with a syringe type pippet with a rubble bulb on top with a fluid in it which the mechanic said should change colour if the head gasket was faulty, but it stayaed the same colour apparentlt it tests for gases present in the heating system.

That sounds like a Sniffer Test to see if exhaust gasses make it into the coolant. Mine passed that on both my previous head failures - It ain't a 100% certain test of the system.

As Francophile says, it all points to something potentionaly expensive, suggest its not driven until sorted.