What causes smoke on decelleration?

[stangman67]

I did a T5 swap and a new Holley rebuilt by Jet Performance. Before the Holley I had an Edelbrock and it smoked after hard acceleration while decelerating. I thought it was just messed up jetting, but with the new carb it runs awesome! So, it wasnt an over rich problem. It certainly doesnt run like it needs a new engine. So now, when cruising along and then downshifting, so the engine revs a bit, it will push some serious bluish/white oil buring smoke. The coolant level never changes, but it does use oil. I never noticed this before with the auto. The motor is two years old with TRW forged pistons, RPM heads and all new stuff. Its hard to think it needs valve seals or rings already. Im thinking possibly PCV system. Any help would be great! Thanks.

 

[1970 slantroof]

USUALLY if it smokes on acceleration it's rings and on deceleration it's valve stem seals. You could pull the valve covers and give the seals a visual check without too much trouble.
Howard

 

[thehueypilot]

USUALLY if it smokes on acceleration it's rings and on deceleration it's valve stem seals. You could pull the valve covers and give the seals a visual check without too much trouble.
Howard

I think you got that backwards.....deceleration will cause the cylinders to pump oil which is worn rings/cylinder walls. If it smokes on intial start up (sitting overnight) and goes away it's worn valve guides or hardened valve seals. A compression check will tell you for sure which it is.

 

[1970 slantroof]

No I do not have it backwards. Coasting against a closed throttle creates a very high vacuum in the intake tract which will pull oil past the intake valve stem seals if they are worn. Smoking under acceleration is caused by the high cylinder presssure pushing oil past the rings. It has been that way since my first car I worked on in 1951.
Howard

 

[stangman67]

Hmm, that kinda stinks. I guess valve seals arent that bad, and I could do them in the car, but it doesnt smoke after idling for a while, nor after being parked all night. My volvo definatly needs valve seals, and it smokes after idling hot for a few mins and everytime i fire it up. This leads me to believe its not, esp. since the heads have 5K miles on them. But that would be better than rings.

 

[limey66]

No I do not have it backwards. Coasting against a closed throttle creates a very high vacuum in the intake tract which will pull oil past the intake valve stem seals if they are worn. Smoking under acceleration is caused by the high cylinder presssure pushing oil past the rings. It has been that way since my first car I worked on in 1951.
Howard

Mr slantroof has it exactly correct.

 

[yeloxr7]

Mr slantroof has it exactly correct.

Yup. valve stem seals and/or guides. I had the same problem with the 351W.

 

[ddonaca351]

slantroof is right...

BUT it could also be the pcv or valve cover "vent" hose is routed to manifold vacuum instead of just to the air cleaner / atmo
This can pull oil into the motor on decel / shift, but it is *usually* just a small amount. The valve guides will be MUCH more noticeable.

DD

 

[yeloxr7]

slantroof is right...

BUT it could also be the pcv or valve cover "vent" hose is routed to manifold vacuum instead of just to the air cleaner / atmo
This can pull oil into the motor on decel / shift, but it is *usually* just a small amount. The valve guides will be MUCH more noticeable.

DD

If the PCV is routed incorrectly the engine wil run terribly. This creates a massive vacuum leak.

 

[ddonaca351]

You wouldnt believe how many times, over the years, Ive seen carbs jetted AFU to make up for just such a screw up.... guess it comes partly from growing up in the country, if ya know what I mean.


DD