Smoke coming out of vent when defoster is on 99 chevy blazer

99 Blazer driving hum,noise - Blazer Forum - Chevy Blazer Forums

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Dec 31, 2007 - 99 Blazer 4x4 has constant noise-humming that occurs non-stop and increases and ... Sounds like it is coming from rearend but not 100% sure. 99 LT 4x4 Rear End Whine
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i would guess that u have a burn exhaust vavle

If you overheated the engine the engine will shut down

check vacuum leak on hose on pass side of engine that supplies vacuum to HVAC unit

often that smoke is actually misty antifreeze from a leaking heater core. wipe a little of it up and sniff. sickly sweet smell it is heater core time.

check the exhaust manifold possible gasket,check flange gasket or check exhaust system for damage --cat converter smells like rotten eggs or if plugs up stalls vehicle and wont let start-----

You may have vapour lock in your heater lines. Start engine, put heat lever on high, disconnect one of the lines on your heater core, the one on the flow out side, & see if you have any air coming out. Once antifreese comes out stick line back on & retighten clamp. Make sure your cooling system is topped off. Also your water pump may not be functionihg to it's maximum. Take rad cap off cold & view flow rate. If inadequate you may have to replace the water pump. Good luck!

the most common problem is the plastic gaskets under the intake felpro makes a steel/rubber replacment

The original GM design has a fair amount of thermal blanket material on the lower surface of the heater case, in the engine compartment to protect the plastic from the exhaust components. Check to see if this material is still in place after you had done the exhaust system upgrade.

I believe the problem you have is a leaking heatercore, I have not done one on a Blazer- Jimmy but will get to do it soon to mine ( 1998 Jimmy )it looks to me as though you have to access this from under the dash and inside the truck. If your more of a large type, like me, you might want to concider having your local mechanic do it as working under the dash and upsidedown could be a little uncomfortable. Hope this helps. Cornbread.