Blowing smoke in 1970 Chevelle

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Originally Posted by wag123
Originally Posted by ragtoplvr
I think the 396 should have a TH400, no vacuum modulator.
Quadrajet on ethanol fuel is famous for float getting fuel logged and sinking.
Used to be so common I had a scale. I have not seen it in years.

Rod
1. Big blocks came with the TH400. Yes, both the TH400 and TH350 DO have a vacuum modulator. It is a part that commonly fails and will cause smoking like the OP describes, and it can cause all of the transmission fluid to be sucked out of the transmission. I have replaced MANY of them.
2. The OP has a Edlebrock Thunder 1806 series carb (basically a Carter AVS), not the OEM Quadrajet. If the car was running so rich that it was blowing a cloud of smoke, it would not idle.

007, the PCV valve might have gotten temporarily stuck closed (allowing the crankcase to pressurize), and then eventually unstuck itself. If it were my car I would replace both it and the transmission vacuum modulator. They are both cheap.



THIS RIGHT HERE ^^^^^^^^^
 
Car only sat a day before the issue. All fluid levels were ok when checked. I will change the PCV just in case. How does one check the transmission vacuum modulator and is it any easy swap or something that needs to be adjusted after install?
 
a modulator is an easy swap, some of them are adjustable. Should be on the back (or maybe the side near the cooler lines on a TH400) of the trans just above the pan rail.

You could check it by pulling the vac hose and seeing if it has trans fluid in it.
 
Originally Posted by 007
Car only sat a day before the issue. All fluid levels were ok when checked. I will change the PCV just in case. How does one check the transmission vacuum modulator and is it any easy swap or something that needs to be adjusted after install?
The way that I check them is to attach a hand vacuum pump with gauge to the vacuum line going down to the transmission at the engine and try to pull a vacuum. You can also pull the line off on the bottom at the regulator and see if transmission fluid drips out (it should be completely dry).
Replacing the vacuum modulator is a very easy DIY job. No adjustments are necessary, just replace it and go. Some of the companies that sell performance parts offer an adjustable regulator, but the ones that came from the factory are not adjustable. You are going to want the adjustable one if you have a lot of cam and low vacuum, and/or if you have a higher RPM than stock torque converter. The adjustment is made with a small allen wrench through the vacuum nipple.
 
Originally Posted by wag123
Originally Posted by ragtoplvr
I think the 396 should have a TH400, no vacuum modulator.
Quadrajet on ethanol fuel is famous for float getting fuel logged and sinking.
Used to be so common I had a scale. I have not seen it in years.

Rod
1. Big blocks came with the TH400. Yes, both the TH400 and TH350 DO have a vacuum modulator. It is a part that commonly fails and will cause smoking like the OP describes, and it can cause all of the transmission fluid to be sucked out of the transmission. I have replaced MANY of them.
2. The OP has a Edlebrock Thunder 1806 series carb (basically a Carter AVS), not the OEM Quadrajet. If the car was running so rich that it was blowing a cloud of smoke, it would not idle.

007, the PCV valve might have gotten temporarily stuck closed (allowing the crankcase to pressurize), and then eventually unstuck itself. If it were my car I would replace both it and the transmission vacuum modulator. They are both cheap.

Shift quality would also be affected too as the fluid is consumed. Pull vacuum line of modulator and take a look see. Have a drain pan handy.
 
Originally Posted by Lubener
Originally Posted by wag123
Originally Posted by ragtoplvr
I think the 396 should have a TH400, no vacuum modulator.
Quadrajet on ethanol fuel is famous for float getting fuel logged and sinking.
Used to be so common I had a scale. I have not seen it in years.

Rod
1. Big blocks came with the TH400. Yes, both the TH400 and TH350 DO have a vacuum modulator. It is a part that commonly fails and will cause smoking like the OP describes, and it can cause all of the transmission fluid to be sucked out of the transmission. I have replaced MANY of them.
2. The OP has a Edlebrock Thunder 1806 series carb (basically a Carter AVS), not the OEM Quadrajet. If the car was running so rich that it was blowing a cloud of smoke, it would not idle.

007, the PCV valve might have gotten temporarily stuck closed (allowing the crankcase to pressurize), and then eventually unstuck itself. If it were my car I would replace both it and the transmission vacuum modulator. They are both cheap.

Shift quality would also be affected too as the fluid is consumed. Pull vacuum line of modulator and take a look see. Have a drain pan handy.



Will do. Thanks for the input!
 
Originally Posted by wag123
BTW 007, what did the guys on the Chevelle site say?


Similar ideas.
 
Just started the car up after I changed the PCV valve. Still no smoke, NOTHING! Bought a new transmission modulator which I will install shortly. I am stumped!
 
Check for any residue of transmission fluid in the vacuum hose to the modulator.
 
If all the fluid levels are good and no leaks... I would guess the smoke is from rich fuel mixture, maybe a float got stuck and overfilled the bowls? Now it's unstuck?
 
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