Ok, so I recently had a new converter installed along with a new tune. Since the install, my car was randomly bouncing off the rev limiter. I also have 4.10 gears and one theory presented was that the gears were what was causing the bouncing. I was pretty sure that wasn't the reason because driving the car hard for a few runs at the track seemed to improve the performance considerably. Especially when my tires stopped spinning during launches. So it really seemed like it had to be a problem with A/F of some sort and the ECU's learning.

The reason why I was pretty sure it was a learning problem was because when the car was driven softly, it would go back to the same strange problem of bouncing off the limiter. The behavior is that I launch hard, the car will randomly hesitate, then shift, then hit the limiter, then seem to lose all power for 2 seconds or more. Sometimes it is the 1st to 2nd shift, sometimes the 2nd to 3rd shift.

Well today I decided to try something. I removed the breather tube from the CAI. It's the breather tube on the driver's side going from the valve cover to the CAI. I kept the hose on valve cover, but pointed it straight at the coolant fan. From my understanding, this is an open positive ventilation going into the crankcase to help with negative pressure in the crankcase from the PCV system.

Well pretty much instantly, after a few hard revs to get the ECU to learn, my car is a completely different animal. There is zero behavior I was seeing before. My car NEVER hits the rev limiter now and shifts 100% perfectly. I tested it my local track today. The numbers are worthless because it's an odd size track. It's 1000 ft so it's not even 1/8th. But not once out of 8 runs did it bounce off the limiter and the car ran perfect. My numbers were about 2 tenths better at this track compared to last year, so the car ran absolutely fantastic.

So maybe it's something with an issue for air getting to the valve cover through the CAI? And why would that matter so much? I would think at WOT that the PCV valve should be closed anyways. So shouldn't there be a neutual situation anyways? But maybe the PCV hasn't fully shut during that initial WOT in 1st especially and the vacuum system just wasn't working correctly until I allowed that breather tube to vent straight to fresh air?

Any thoughts from anybody? Anyways it was a relief to pin something at least down with this.

 

I'm confused by the first part. PCV allows positive pressure in the crankcase to escape. The pressure is generated by imperfection between the cylinder walls and the pistons. As such, I would assume that there would be higher positive crankcase pressure under load.

Is the PCV side you disconnected before the MAF? If it's after the MAF, it's unmetered air, and should cause problems. I'm not really sure how pointing it at the fan would cause any kind of meaningful airflow into the tube.

The power loss could be caused by the ECU trying to stop you from bouncing off of the limiter.

It sounds like the primary issue is a bad tune.

 

I'm probably not doing a very good job explaining this. Just to be clear, I didn't touch the PCV side which is the passenger's side going into the valve cover. And yes, I do understand that the PCV is a positive pressure system designed to allow pressure to escape from the crankcase into the intake and not vice versa.

I'm referring to the driver's side going into the valve cover. Which if I'm understanding correctly is a wide open breather to replace any needed make up air not being supplied by the intake. But I'm finding a ton of conflicting information on this too.

I did watch my A/F's and they did seem very good. Ultimately though this may be a tune issue as you've stated and this air probably does need to be metered too. I've also read conflicting info, some saying this dangerous to do, others saying it's not a big deal. So I'm going to try another revised tune. I'm also thinking of replacing the CAI I have. I've read about some having issues with this particular CAI. So I'm considering that might be contributing as well.

 

Who is doing your tuning? Sounds kind of scary in it's current state.

Is your traction control off?

 

My understanding is that they're both positive pressure vents. Fordparts lists one as an "elbow" and the other as a valve, so I could be wrong.

I don't believe that it's supposed to make up for air. It's downwind of the MAF, so any air it brings in is unmetered, which I take to mean that it's not a significant amount.

 

I'm not sure what's happening here. I would reattach the hose and see if the problem returns. And, if you left the port in the CAI open, crap could get sucked into your engine...which is bad. If you plugged it, I wouldn't expect any change in the tune/shifting as you've described.

What made you think to pull that hose in the first place?

 

I have a new tune I just loaded tonight and I'm going to try a different CAI. I haven't had a chance yet to test anything more yet. I did leave the port open. I pulled the hose because I was trying to identify if maybe there was a vacuum leak some place. And the funny thing is when I created the vacuum leak the car instantly was running perfectly.

 

Yes, traction control was definitely off.

The way I understand how the PCV system works, (which could be completely wrong too), is that air flows in through the breather hose into the crankcase and out the PCV hose. Under most circumstances, the breather vent tube is used to flow air into the valve cover which creates that positive pressure for the PVC valve to work.

Man this is confusing...Now this is where I keep reading conflicting info. When at WOT, the pressure between both vents in the valve cover neutralizes, and that's where both sides them become positive. But others say that you will always still have that vacuum with unequal pressures between the vents albeit at WOT the pressure differences are much much less. Either way, it seems like my issue occurs basically at this "equilibrium" point for whatever reason. As mentioned before, I'm not sure I can blame the tune just yet. It could be the PVC valve, or it could be the CAI. Maybe when I have the breather hose connected to the CAI, when this pressure equilibrium occurs, there isn't enough vacuum and the engine becomes choked of air. I'm of course completely speculating. And when disconnected somehow this "vacuum leak" prevents that situation.

 

excerpt from https://en.wikipedia.org/wiki/Crankc...ilation_system

Looks like you were correct. I just don't understand how it's able to get flow through the crank case when both sides of the engine are ultimately connected to a vacuum.

 

Yeah..I was having a hard time wrapping my head around it too. The only thing I can think of is that the vacuum must be larger on the PCV side since it goes straight into the intake manifold and therefore the opposite side goes negative pressure. Kind of like playing tug of war where both sides are "equal" until one side gets momentum and can actually reverse the other's side momentum and start pulling them towards you. So basically, you could probably reverse the vents and they would work the same. Put the PCV on the driver's side and going into the IM and you'll get the exact same functionality.

 

Problem solved...I loaded a new tune and replaced my CAI and the car is driving freak'n awesome now with all vacuum hoses reattached. No issues at all that I saw with a couple of really hard test pulls today. I don't know ultimately what was the real culprit...it could have been a few things but I'm happy with this combo I have now. I'll be testing at the track again this weekend if the weather allows me to.

 

With everything running solid now, this was my best time so far at the strip last Sunday:

60' 1.666
330' 4.865
1/8th 7.521
MPH 92.97
1/4 11.732
MPH 118.07

And this car still has more in it. I'm still launching at a conservative 2000 RPM's. I'm extremely happy with how the car is running now. It is spot on. I did about 10 passes at the strip and not one single pass had any issues. It shifted perfectly and didn't hit the limiter one time, nor any of the strange drops of power near peak RPM. I also had 2 consecutive runs at one point there were identical all the way to the thousandth. I'm a pretty happy camper right now