Loud hiss and white smoke from under the truck!

[TASummers]

Hey guys,
So yesterday, after a cold start, put the truck in drive, and several times, a short, loud hiss - almost like a pressure relief valve going off - would come from under the truck, somewhere behind the drivers seat.
It happened several times throughout the day. The last time, however, the hiss happened for several seconds, and white smoke - maybe a gas - came out from somewhere behind the driver seat. It was almost like a burst of freon, but behind the driver seat?
I do not have controllable rear a/c.

Any suggestions?

 

[Wooluf1952]

Welcome

In case you haven't heard:
http://gmtnation.com...eres-the-story/

Your info says you're driving a 1974 Trailblazer LS? Probably a miss-type.
Does the white smoke have a sweet smell, like coolant?

 

[Blckshdw]

Well the gas tank is on the driver's side of the truck, and occupies that area. As @Wooluf1952 asked were there any smells or odors present?

 

[mrrsm]

Have you performed any service or refreshment of the AC System lately with R-134A? Could you sense whether or not the White Smoke was a cold spray of Freon ...or perhaps just a blast of cold air escaping from under the dash panel and appearing like frosty cold air that was just condensing atmospheric moisture inside the truck cabin?

A Low Charge of R-134A can cause the Evaporator under the dash to form over into a block of ice from freezing condensation onto it over time. This will issue out as clouds of 'White Smoke' looking air when the stuff melts enough to finally allow the blower motor to push the ambient air across the block finned coils inside there. That same cold moisture laden air can travel through to the back seat air vents and give the same effect as being White Smoke.

 

[TASummers]

Thanks for the replys so far guys. This white cloud came from under the truck - not inside. The smoke did not have a smell that I could discern, even though the driver window was open. I'm positive I would've smelled it if it was fuel.

I did service the a/c with a can of r134 yesterday am. The white smoke/gas did make me instantly think of freon - the white cloud dissipated almost immediately.

The wind was blowing from behind the truck. When the hissing started, I immediately looked out the window, and witnessed the white cloud coming from behind my seat. Again, with no rear a/c, freon coming from under the rear seat (outside of the truck) makes no sense.

Is my LS plumbed for rear a/c??

 

[Tiggerr]

Only the long wheelbase version (3rd row seat) has rear AC... only thing the SWB has is the vents in the console.. at least my Rainier has those idk if that came on a trailblazer..

Wonder if the SAIS could be the culprit? It's in that area under the truck under the drivers seat...
Are you getting a CEL at all?

 

[budwich]

"I did service the a/c with a can of r134 yesterday am" .... I don't think that "service" was well advised. Have you checked your pressures lately? Most AC systems DO NOT require "a can of r134" as it doesn't get used up in operation. So the question would be what's the history on the "service event"?

 

[TASummers]

I am not throwing any codes. What is SAIS?

I use the term "service" loosely. I have a slow freon leak. About 1 can last summer topped it off. I added another can this year.

 

[Tiggerr]

SAIS is the secondary air injection system.., emissions crap... not a likely source... only other thing I could think of in that area... other than abs unit and fuel tank/filter

Odd that it happened after you were into the AC but there's nothing AC related in that area of the truck

 

[mrrsm]

If you don't mind... when the engine is off and it and the radiator are both cooled down... remove the radiator cap and inspect inside for what looks like a brown pudding emulsion. If you find anything like this... check your coolant reservoir for the presence of this same brown discoloration... it should only appear orange in color. If it is brown and darkening... then more troubles are at hand.

The reason I suggest this is that if your engine suffered from a head gasket failure and coolant leaked into the cylinders... upon combustion... it produces a pure white cloud of exhaust smoke that gives ample evidence of the gasket failure. If your exhaust adjacent the underside of the vehicle was leaking ...the white smoke could escape from such places in this manner and with the wind blowing it about and sending it out under the driver's side of the vehicle.

Unfortunately, a cracked lower radiator hose, a failed thermostat gasket or loosened retention bolts, a fracture or separation in the heater hose and its aluminum line ...or even a crack in the engine block on the drivers side could also evince leaking coolant steam from time to time depending upon varying thermal conditions whenever the motor is either heating up or cooling down. Very low levels of coolant in the radiator might bear out one or the other of these problems if you inspect the areas for any 'steam cleaned' appearances and coolant residues.... or find this damned nuisance brown pudding mud inside of the crankcase.

If you see no evidence of this brown pudding like oil and coolant mixture under the cap or inside the top of the radiator... then there should be no worries about internal engine problems and you should continue seeking the answers to the source of the white smoke as your next course of action.

If you do happen see this goopy brown stuff inside there, then not running the engine before performing a complete oil and oil filter change with an engine flush additive would be the only way to preserve and protect the babbitt surfaces of the bearings inside the engine from being corroded by the coolant and causing the bearings to seize up in the engine if you continue to drive with this junk in there. The engine should not be even be allowed to idle under these conditions.

What happens is that if corroded babbitt surfaces begin to stick to and seize upon the rotating engine components... it will ruin the motor. This brown pudding mixture will have completely destroyed the capacity of the engine oil to lubricate the internal engine parts and will 'gel up' to a consistency that will prevent it from being suctioned into the oil pick up tube in the crankcase and may cause the gerotor oil pump to cavitate and lose its prime.

The chemistry of the coolant dissolving the internal engine bearings takes place regardless of whether the engine is running or not for as long as the coolant is mixed up with the engine oil inside of the oil passages. It should it be flushed from the motor as soon as this action may be done. If you pull the dipstick out and find this stuff on the end and higher up past the fill mark... What follows will be the complications of having to pull the engine head, changing the head gasket and flushing the engine clean of all this mess with multiple oil and filter changes as necessary to salvaging the motor. If God Forbid this problem prevails... many here would recommend performing an engine swap over everything else I mentioned above for the R&R of the engine head.

 

[carshinebob]

a short, loud hiss - almost like a pressure relief valve going off - would come from under the truck, somewhere behind the drivers seat.

Adding refrigerant and getting some sort of pressure valve type of release plus the ac compressor is on the drivers side. I think I have an idea. Over pressure in your ac system could be venting (or spraying) from the back of your compressor and coming out from under your Trail blazer. What do you all think? ~BOB

 

[TASummers]

Adding refrigerant and getting some sort of pressure valve type of release plus the ac compressor is on the drivers side. I think I have an idea. Over pressure in your ac system could be venting (or spraying) from the back of your compressor and coming out from under your Trail blazer. What do you all think? ~BOB

The only thing that makes me question this is that I witnessed the cloud coming from somewhere under the rear seat area...

 

[Mooseman]

A/C refrigerant systems are sealed. There is no vent. If high side pressure goes too high, the high pressure switch cuts out and stops the compressor.

Now, what I'm thinking is that even if you said that the vapour and hissing is coming from the driver side that it's the condensate drain tube that's dripping onto the exhaust pipe. Mine did that and would notice it when I turned off the truck.

 

[Blckshdw]

Hmm, from the initial post, he said it happened on a cold start, but after putting the truck in gear. I guess we need a time frame there. Was it right after pulling off, or was there a bit of driving first, before the phenomenon?

 

[TASummers]

Hmm, from the initial post, he said it happened on a cold start, but after putting the truck in gear. I guess we need a time frame there. Was it right after pulling off, or was there a bit of driving first, before the phenomenon?

Let's call it a cold or a warm start. The first time, the truck was truly cold. Each additional time it happened, the truck had been off for approximately an hour. Once the truck was up to operating temp, it would not repeat this hiss or cloud.

If you don't mind... when the engine is off and it and the radiator are both cooled down... remove the radiator cap and inspect inside for what looks like a brown pudding emulsion. If you find anything like this... check your coolant reservoir for the presence of this same brown discoloration... it should only appear orange in color. If it is brown and darkening... then more troubles are at hand.

The reason I suggest this is that if your engine suffered from a head gasket failure and coolant leaked into the cylinders... upon combustion... it produces a pure white cloud of exhaust smoke that gives ample evidence of the gasket failure. If your exhaust adjacent the underside of the vehicle was leaking ...the white smoke could escape from such places in this manner and with the wind blowing it about and sending it out under the driver's side of the vehicle.

Unfortunately, a cracked lower radiator hose, a failed thermostat gasket or loosened retention bolts, a fracture or separation in the heater hose and its aluminum line ...or even a crack in the engine block on the drivers side could also evince leaking coolant steam from time to time depending upon varying thermal conditions whenever the motor is either heating up or cooling down. Very low levels of coolant in the radiator might bear out one or the other of these problems if you inspect the areas for any 'steam cleaned' appearances and coolant residues.... or find this damned nuisance brown pudding mud inside of the crankcase.

If you see no evidence of this brown pudding like oil and coolant mixture under the cap or inside the top of the radiator... then there should be no worries about internal engine problems and you should continue seeking the answers to the source of the white smoke as your next course of action.

If you do happen see this goopy brown stuff inside there, then not running the engine before performing a complete oil and oil filter change with an engine flush additive would be the only way to preserve and protect the babbitt surfaces of the bearings inside the engine from being corroded by the coolant and causing the bearings to seize up in the engine if you continue to drive with this junk in there. The engine should not be even be allowed to idle under these conditions.

What happens is that if corroded babbitt surfaces begin to stick to and seize upon the rotating engine components... it will ruin the motor. This brown pudding mixture will have completely destroyed the capacity of the engine oil to lubricate the internal engine parts and will 'gel up' to a consistency that will prevent it from being suctioned into the oil pick up tube in the crankcase and may cause the gerotor oil pump to cavitate and lose its prime.

The chemistry of the coolant dissolving the internal engine bearings takes place regardless of whether the engine is running or not for as long as the coolant is mixed up with the engine oil inside of the oil passages. It should it be flushed from the motor as soon as this action may be done. If you pull the dipstick out and find this stuff on the end and higher up past the fill mark... What follows will be the complications of having to pull the engine head, changing the head gasket and flushing the engine clean of all this mess with multiple oil and filter changes as necessary to salvaging the motor. If God Forbid this problem prevails... many here would recommend performing an engine swap over everything else I mentioned above for the R&R of the engine head.

Thanks so much for taking the time for such a thorough explanation. I'll check for this sludge this afternoon and report back.

I will say, I do have a coolant leak that started a week ago. I have been unable (yet) to locate the origin. I was going to make another post regarding leak, but as it may be related, will share the details here.

It is coming from the drivers side of the engine, and appears to be coming from fairly high. I was thinking maybe freeze plug? What else could it be?

The water pump is dry, as is the thermostat housing. I am seeing fresh coolant "higher" on the block, beginning toward the front of the engine, but have not been able to locate the specific area of the leak. I think I may have to pull off the inner fender liner to get a better idea.

Mod edit: Posts merged. Please use the +Quote or the Reply buttons of the posts you wish to quote. Thanks.

 

[TASummers]

Quick update: i checked the radiator and overflow for sludge. There was none. However, I did realize that I'm using that orange Dex-cool antifreeze, and that green stuff I thought was antifreeze on the drivers side of the block is oil... Guess I'll figure out where that's coming from.

The truck has not produced a loud hiss or a mystery cloud for days. Go figure...