Sanity Check

[jeepxj3]

What valve springs did you use to get valve float at 4000 rpm?

[RenoF250]

What valve springs did you use to get valve float at 4000 rpm?

The original 4.0 valve springs. The machine shop said they were fine. They looked good to me but this cam has more lift than stock.

Not sure it is valve float, could be something else but I have no idea what. I do not intend to do it again to try to figure it out.

[RenoF250]

Was just talking to the friend that was here when we broke it in and we started wondering about the accuracy of the tach. Figured I would check it with the speedo. At 35 in 3rd it says I am turning 1,500, by my calcs that is wrong. I have 4.10 gears, 32" tires, and and AX-15 which is 1.44 in 3rd so it should be turning ~2,100. It is acting like the tach is set to 8 cyl but I checked it before I put it in and just checked it again and it is set to 6. I even moved the switch to the other 2 positions and then put it back in 6.

Anyhow, that would mean the float was at ~5,300 rpm which makes more sense. I would rather not have gone that high. Means we broke it in with too much rpm also.

I guess the good part is that means it could have been the computer shutting it down since I see the Cherokee tach redlines at 5,300. Does the PCM do that?

[jeepxj3]

What valve springs did you use to get valve float at 4000 rpm?

The original 4.0 valve springs. The machine shop said they were fine. They looked good to me but this cam has more lift than stock.

Not sure it is valve float, could be something else but I have no idea what. I do not intend to do it again to try to figure it out.

That's the one, 280/280 .448 of lift

I thought that the stock valve springs were only good to 0.440" lift?

[RenoF250]

What valve springs did you use to get valve float at 4000 rpm?

The original 4.0 valve springs. The machine shop said they were fine. They looked good to me but this cam has more lift than stock.

Not sure it is valve float, could be something else but I have no idea what. I do not intend to do it again to try to figure it out.

That's the one, 280/280 .448 of lift

I thought that the stock valve springs were only good to 0.440" lift?

Not sure, there are what was on there and they are cone shaped at the top. Not even close to binding when I turned it over but the lifters were not pumped. The stock pushrods were too long though and even with them on they were not close. They only had about 0.030" left before bottoming in the lifter so they were nearly the same as pumped.

[RenoF250]

BTW Crower also said stock springs were okay with this cam so I did check into that.

[SilverXJ]

I thought that the stock valve springs were only good to 0.440" lift?

While they may have enough room to handle the lift, they may not be strong enough to handle the ramp rate.

What happened when it hit the RPM?

[RenoF250]

I thought that the stock valve springs were only good to 0.440" lift?

While they may have enough room to handle the lift, they may not be strong enough to handle the ramp rate.

What happened when it hit the RPM?

Yeah that is why I was thinking it was floating.
It cut out fairly abruptly at the 4k indicated. Almost like you shut it off.
Like I said though, I think it was actually 5,300 making me think the computer might have shut it down.
I did not think valve float was that abrupt.

[RenoF250]

I thought that the stock valve springs were only good to 0.440" lift?

While they may have enough room to handle the lift, they may not be strong enough to handle the ramp rate.

What happened when it hit the RPM?

Yeah that is why I was thinking it was floating.
It cut out fairly abruptly at the 4k indicated. Almost like you shut it off.
Like I said though, I think it was actually 5,300 making me think the computer might have shut it down.
I did not think valve float was that abrupt.

I figured out the tach problem. The PCM only puts out 2 pulses per rotation so you have use the "4" setting. I put it on 4 and it appears to be accurate now but that means it pooped out at 6k not 4k.

I wish I would have known that about the tach before but I had not read anything about it. It was not terribly easy to find when I was looking for it.

[jeepxj3]

What year is the Jeep?
Can you plug in an OBD reader and get the rpm off it?

[akadeutsch]

As I understand Valve Float it means that your engine is turning over so fast that the valve spring can not push the valve closed quick enough for the engine to build compression. Meaning, the Valves stay open. Your engine will sound/run like s**t as both valves are open during all strokes, but it should not Die abruptly. In Jeeps we dont have to worry as much about bent valves because we use dished pistons but it is still possible to bend valves....but you would know if you did that. Floating can also become more prevalent the more you do it. Just curious as to what your machinist told you about the condition of your springs before you put them back in? I am considering changing my springs out as well. As they tested 3 pounds under spec.

[akadeutsch]

I stole this from another Forum. I think it helps explain Float.

"When valve float occurs the various parts of the spring are moving in opposite directions. This quickly causes extreme heating which reduces strength. So each time valve float occurs the spring's ability to control the valves is reduced causing valve float to occur at lower RPM."

"In the case of completely worn out springs, it will show up as a loss of power at high rpm as well as a high rpm missfire sensation. Most people miss-diagnose the problem as ignition related when it's just worn out springs."

It sounds like something else killed your engine.

[SilverXJ]

Valve float will act as if you hit a wall and the engine will not rev any further. The Jeep rev limiter & speed limiter will cut power and then return power when the speed or RPM drops. It cuts the fuel.

[RenoF250]

As I understand Valve Float it means that your engine is turning over so fast that the valve spring can not push the valve closed quick enough for the engine to build compression. Meaning, the Valves stay open. Your engine will sound/run like s**t as both valves are open during all strokes, but it should not Die abruptly. In Jeeps we dont have to worry as much about bent valves because we use dished pistons but it is still possible to bend valves....but you would know if you did that. Floating can also become more prevalent the more you do it. Just curious as to what your machinist told you about the condition of your springs before you put them back in? I am considering changing my springs out as well. As they tested 3 pounds under spec.

He is an old guy of few words, just said "yeah those springs are fine". I had taken two off myself and they were well over min length. I did not test with pressure and don't know if he did. The springs I have seen that were bad were all short and uneven lengths though. Since they made it to 6k they are fine for me.

It is a 1994 so OBD I, I do not have an OBD I reader.

It basically revved great, then pooped out, then ran fine, it never died, just hit a wall and sounded crappy. I was out of it immediately so if it was the PCM it never would have had a chance to return power.

Also, in regards to the springs, I have watched them through the oil fill oil at idle and they are not even close to binding.

[SilverXJ]

Also, in regards to the springs, I have watched them through the oil fill oil at idle and they are not even close to binding.

You can't tell anything from that due to a) a bad view and b) the hydraulic lifter collapsing. You need to either use measurements and then compare with the springs or use a couple solid lifters in place of the hydraulic lifters, mock up the build, and check it that way.