Yesterday I was hotdogging it on the way home and found after a full throttle acceleration and coasting from there that there was a cloud of grey smoke as viewed from my rear view miror. When i gave it gas again it instantly stopped. Once off the freeway at a stoplight it did not smoke at idle and another full throttle blip up my street did not produce smoke. At home at idle or while reving the engine I could not make it smoke. It only smokes when after driving it 20 minutes and full throttle to the floor for extended period and then say coming up real quick on cars ahead you take your foot off the pedal to coast or hit the brakes then it smokes for a few seconds. I think it's valve float but not sure. One thing for sure she goes like a bat out of hell since LARS tuned her. Suggestions?

 

I ain't no expurt here but I will try my darndest.

Valve float is a complex series of events that happens at high valvetrain speeds. Many people think it has to do with the lifter flying off the nose of the cam lobe because the engine was reving so high that the valve spring could not push the lifter back down fast enough to keep up with the back side of the cam lobe. This is probably not the most common form of valve float. The more likely event is that the valves will bounce off the seats on closing from spring surge which would also create a little extra valvetrain clearance momentarily. The extra clearance is instantly taken up by the hydraulic lifter filling up with oil which causes the valves to hang open slightly when the lifter is down on the base circle. Obviously this means the intake valves would be off the seats during the compression stroke and when the spark plug fires so the engine will make lots of bad noises. The springs can break or just fatigue quickly when this happens. The lifters will bleed back down in a few seconds once the normal valve train motion is achieved.

Based on what you have described, I doubt you are experiencing valve float. Maybe valve seals, maybe intake manifold gasket, maybe carb issues. Maybe this helps.

-Mark.

 

Hey David - I'm guessin' that your issue is the intake manifold gasket sucking oil into the cylinders. Common.

Valve seals are the second most common but I know you've had the intake off.

It's not 'valve float.' Float is a misfire situation and has nothing to do with an engine burning oil. The engine misfires at high RPM because the springs are worn, and too weak to close the valves tightly enough at extremely high speeds. Your engine may suffer from valve float, but it's not making it smoke.

 

The smoke is most likely due to the high vacuum when you close the throttle plates at high RPM. This vacuum is sucking oil from either the manifold gasket or down the intake valve guides.
If you had been running the engine hard there would have been a fair amount of oil in the top end making it easier to suck it past the valve seals. This also makes it harder to replicate the symptoms when stationary and just winging the throttle.
If it was the manifold gaskets the motor would be running rough all the time from a lean mixture at idle as well.
My .02.

 

more work......

 

Gray smoke huh? Almost sounds like carbon blow'n out of the motor to me.

BigBlockk

Later.....

 

Well I took the car on 2 high speed passes last night and no smoke. It seems I can only duplicate this on the highway after extended hi-speed operation. I suppsoe it's a leak between the oil cavity under the intake and one or two of the runners that opens up with heat expansion and allows, as you mentioned, oil at hi vacuum to slip past the intake gasket. Sucks becuase she's timed in so well I really don't look forward to redoing the gaskets. Plus I did the gaskets perfectly, or so I thought, verbatum with the instructions.

 

If you can replicate it after a high speed run there may be a lot of oil up in the top end at that time. When you close the throttle quickly, you get a lot of vacuum in a hurry that can suck the oil down the intake guides or from the underside of the intake port gasket. If you pull the intake off and the ports are oily, you know it was just the gaskets. If the back side of the intake valves are oily but the ports look dry, it is time to either quick fix the valve seals or rebuild the heads/guides. You might get by with adding a cheap set of rubber umbrella seals untill you have the time and money to do the valve job. Or, just drive it as is.

Sorry for the goofy half-arsd reply earlier. I was pretty much asleep when I wrote it.

-Mark.

 

Mark, I took the Vette to church yesterday and drove the **** out of her and was unable to duplicate any condition mentioned above. No grey smoke, no oil consumption.......nothing. I ran the car in the garage for 3 minutes at 4,000 RPM and quickly let go of the throttle while my SO held a white paper towel by the ehaust, it ran clean and no oil residue on the towel. I took your notes and will keep the valve guides and seals as an option but I think Paul may be right as I just replaced the intake. If no oil smoke appears and if the engine uses no oil then I'll just keep an eye on it but do nothing. If there ever again an oil smoke or increased oil consumption then I'll pull the intake and check for oily passeges and also replace the valve seals. Again how can I rotate each cylinder to TDC while replacing the seals without worry of dropping the valves. I always used a air compressor but don't have one anymore. I obviously know #1TDC but TDC on the rest?? Sorry forstupid question but I just woke up.

David

 

Pull the plugs out, then bump the motor over until you feel a whoosh of air out of the spark plug hole.

Oil smoke is bluish in color, but you said gray. Sort of a whitish gray?

 

If you don't have an air compressor you can still replace valve seals by going old school. Push nylon rope into the cylinder then rotate the crank by hand to raise the piston and compress the rope. Make sure you leave some of the rope hanging out of the plug hole to remove it when done, then move on to the next cylinder.

 

If you can't replicate it, it probably was just your engine clearing some of the unspent fuel in the exhaust path after having it at WOT. I think your engine is OK.

 

Hope your right we will see.

 

good idea.

 

I just put each cylinder at TDC in turn then swapped the seals. I did push a little rope in as insurance but with the piston at TDC, there is no way the valve can fall far enough down to get lost in the cylinder. The new seals provided enough friction to hold the valve up when reinstalling the springs.

 

Thanks Steve.

 

The rope trick is what I would use when swapping seals but I would lay my money down hard on the intake gaskets considering you just replaced them. Quite a common problem.

Mr Gasket sells a trick little intake valley pan baffle that would prevent oil splash from the bottom end getting up to the underside of the intake manifold. I put one on a 357 a while back and it came in handy for oil splash and for another reason - It heped keep the lifters in place when the short block was tipped over accidentaly. It keeps the lifters in place during a valvetrain failure too.

-Mark.

 

I went out for a cruise and no smoke, believe me I tried like hell short of redlining the block and coud not get it to smoke no matter how I tried. Lets hope it was carbon. That is all.

 

Thats great news!

If you want to try cleaning out ALL the carbon in there... go to the GM dealer and get some "GM Top engine cleaner." You pour it down the carb a little at a time... cleans out EVERYTHING! Heck... I've unstuck piston rings with it...

 

Is this stuff safe?