A friend's '94 4 Runner

[Young Bull]

Howdy,
A couple I know built a 94 4Runner and are having issues with their heat. After the rig warms-up when they turn on the heater it blows hot air for about 10 seconds and then gets cold. According to the owners the rig never over heats but it does blow white smoke until it warms up.
Where do I start to look at to give them a hand? I know nothing about these rigs, so do they have a hose lose? Is the heater core failing or failed? Do these engines have head gasket issues like the LC's?
Any input would be appreciated, they don't have a lot of money and if I can help them fix the problem it would truly help them out.
Thanks.

 

[2ndGenToyotaFan]

Have them do this: Once warm, turn the heat on till it is blowing cold, then turn the fan off (leave the temp on hot) and wait a minute. Turn the fan back on, if it's warm again for 10 seconds then goes away like the first time, then I would say that they have a heater core issue. Not a lot of fun to change those.... Probably $150 for a heater core, and 4-6 hours to change it out.... (the whole dash has to come out...)

Yes, the 88-95 V6's had headgasket issues. But this doesn't sound like that problem to me...

 

[Young Bull]

Thanks for the input, I'll try it tomorrow.