smoke
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Junior User
Joined: May 2006
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smoke
i have a 70 f250 with a 390. when i am driving down the interstate at around 65-70 mph and floor it then let off it lets out a little smoke. it does not have kick down linkage. it has the stock fomo 2 barrel. i seen it ine day in my rear view and did it a couple times to see if i kept doing it and it did. could it be in the heads? everything on this engine is original except tune up parts and headers. any advice?
Logistics Pro
Joined: Jan 2002
Posts: 4,907
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From: suburban atlanta
Most carb'd engines will put out a cloud of black smoke when you floor the gas pedal. Two reasons for this are: 1)power valve opens up and 2)accelerator pump squirt, both add more fuel to the mix. If you are crusing, and take your foot off the gas totally, and then wait a few seconds and then put your foot back on the gas, you have essentially used engine vacuum to increase the flow of oil (vacuum assisted) down the valve stems. That may cause a puff of blue smoke, and is acceptable to have a very slight puff if the engine is otherwise ok. Thirty-plus year old engines tend to burn more fuel and oil than newer engines.
tom
tom
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,562
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From: Long Island USA
Sounds like a bad intake gasket, but a few things to check first.
Check the PCV valve (if it has one) and make sure it rattles when you shake it up and down. Hand-in-hand with the PCV is whether or not you're pooling oil up in the heads and when you let off, it's sucking the oil down the intake valve guides - or sucking it through the PCV.
Drive the thing on the highway like you did to get it to smoke, pull over, do not shutdown the motor, pull the PCV valve out and see if the valve cover is full of oil.
And by "full of oil" I mean enough oil to come to about 1/2" above the bottom of the valve springs.
You could also pull over, shutdown, and quickly pull the dipstick, wipe, insert, remove, and check the level. If it's 2qts or more low, I'd think that would have to mean it's all in the valve covers.
Check the PCV valve (if it has one) and make sure it rattles when you shake it up and down. Hand-in-hand with the PCV is whether or not you're pooling oil up in the heads and when you let off, it's sucking the oil down the intake valve guides - or sucking it through the PCV.
Drive the thing on the highway like you did to get it to smoke, pull over, do not shutdown the motor, pull the PCV valve out and see if the valve cover is full of oil.
And by "full of oil" I mean enough oil to come to about 1/2" above the bottom of the valve springs.
You could also pull over, shutdown, and quickly pull the dipstick, wipe, insert, remove, and check the level. If it's 2qts or more low, I'd think that would have to mean it's all in the valve covers.
