understanding lifter pump-up induced valve float

[acannell]

is this correct?

at high rpm, the springs arent completely pulling the the valve closed (what springs, why, and what rpm, is a separate topic)

meaning that for some amount of time, the cam lobe ceases to touch the roller on the follower, presumably immediately following the peak of the lobe

in that moment, the lifter rises, closing the gap

but what that now means is that when the cam is at that same position, the valve will be open further than it should be, since the lifter has now extended beyond the point required to keep the lash at zero.

the phrase "pump up" suggests that this occurs incrementally over some number of cam revolutions..not important really...

question: is this lifter over-extending a guaranteed outcome of spring induced float on any engine with hydraulic lifters? Or is there a separate phenomenon of float where the lifter does not over extend? (solid lifters?)

Let me ask that a different way:

Solid lifters would not solve this problem, because the springs are still not closing the valve soon enough, so there would still be float. Right?

And a third way:

So how does the situation differ assuming everything is the same but one has solid and the other hydraulic?

I am guessing the that hydraulic lifters extend the amount of time the valves remain in float. I.e. even after the rpm drops to where the springs can keep the valve closed, they are kept artificially open until the lifters bleed out (doesnt that require engine-off?) So there is "spring only float" which happens at high rpm when the springs cant do their job, before the lifters have pumped up. "Combined float" which is at that same rpm after the lifters have pumped up, and the springs are still/would be floating. And "lifter only float" when the engine is back at a lower rpm where the springs could do their job, but the lifters are now over extended, causing the valves to stay open further than they should. (how long does that last?)

Okay and another question...what springs/cam/rpm are known to experience this, and why are the springs too weak? Is that something that can be identified from the cam profile + spring characteristics?

[Dr. Johny Dodge]

it's not that the springs aren't pulling the valve shut but they're not strong enough to keep the valve from bounceing back away from the seat when it does close

rpm isn't really revellant as valve float will occour at a slightly lower rpm everytime - if valve float is an issue in a particular motor

Idono if lifter over height / pump up would really be a big part of the issue

I do know the gaps can be large during float as my buddy's 440 chucked some of the lash caps off some of the valves when it floated the last time

[shackwrrr]

I ran into exactly this on my van. Previous owner installed the wrong valve springs that only had 60 lb seat pressure. At 3700 rpm if would just stop accelerating. When you returned back to idle the engine would die and have zero compression until the lifters bled down.

[4 l-bodies]

I ran into exactly this on my van. Previous owner installed the wrong valve springs that only had 60 lb seat pressure. At 3700 rpm if would just stop accelerating. When you returned back to idle the engine would die and have zero compression until the lifters bled down.

I bought a GLHT close to 20 years ago that had almost the same exact problem. Vehicle was a real pooch. In my case it was a bit after 4000 RPM. These were conical springs that also had 60lbs on the seat. So about half of what they were supposed to have!
Todd

[thedon809]

I think there are 2 different types of valve float. One is where the valve bounces on the seat after it closes, the other is where the valve and spring simply can't keep up with the cam profile. I want to say hydraulic lifters can only pump up so far. All newer mopar engines have beehive springs. I believe you can run a lighter spring pressure with a beehive and not have to worry about the valve bouncing. Lighter spring frees up power and efficiency.

[acannell]

the valve bounces??

thats news to me

so in that case theres got to be at least three kinds of valve float

valves bouncing
valves staying open because the cam is rotating so fast (profile dependent as well) that the spring is no longer able to accelerate the mass of the valve+follower to follow the lobe in time
valve staying open because the lifter is pumped up and creating an offset until it bleeds down (engine shut off)

[zin]

While it's not really a case of float, another issue related to the valve train is lash adjusters collapsing due to aerated oil, etc.

As for valve bounce, check YouTube for some interesting videos. ..

Mike

[Dr. Johny Dodge]

I'm not really sure what happened to the 2.5 in my old 87 shelby z


it ran like hell for a week - I knew I was chewing it to hell continueing to drive it but didn't have much choise at the time

it had the 2.2 computer - ran it that way for 2 years

when I took the head off years later I found the #3 piston had a chunk missing from the top of the piston

ok , I've heard all about needing to mod the cal or change the injectors to avoid it going lean or detionating but I think I had something much different happen

see, the chunk missing from the piston was directly below the exhaust valve

the piece from the piston was wedged between the exhaust valve and seat - big piece

roller follower was sitting on end between the springs at #3

and the tip of the lifter looked to be extended kinda high

I parked the car the begining of 98 and only took it all apart a couple of years ago

the lifter was still all the way up and rock hard

so hard (stuck) that I could place it in the vise and wack the tip as hard as I could with my eastwing frameing hammer without the tip moveing at all

it's been my thinking since I found all this that the lifter failed first, there was not enough room for the follower so it got chucked by the cam - in the process, the follower caused the valve to drop to low and that caused it to hit the pistion busting the piece out

the piece stuck in the valve is just karma saying "are ya lookin for this?"

[Ondonti]

I heard about "pump up" issues with the neon 16v heads and I wonder if that is really just an issue of using the wrong valve springs? I never researched this. Using a restrictor instead of valve springs