Odd kind of missing or stuttering at high rpm

[LXSRT8]

1. U7135
2. 9r05
3. 2007 Dodge Charger SRT

My car has 15,000 miles on it with the Mopar CAI and I only use 93 Octane gas.
When there is a sticker on the pump , I make a note of it and the most I have seen is 10% alchohol in the gas.

I did the 93 with CAI performance tune last week and it really made my car kick butt. The 0-60 timer (I always do it at the same 'safe' place near my house) was always around 5.3-5.5 secs

After the tune, I did 4 runs and 3 of then were around 4.94 secs. I screwed up my launch so the >6 second time got tossed.

These are all with the traction control on because I have never been able to figure out how to do a launch with it OFF as the wheels just sit there and spin smoking.


I looked at the plot of speed/pedal position/rpm of one of my 0-60 runs with the new tune and could see the change in the shift points since I let it go from 0 to around 90.

Amazing how much fasted this car has become with the Predator - great job and my hat is off to the company


That was bakground info to show that I am in fact running with that tune because I need some advise.

2 days ago, I was driving out on a highway in Western Maryland that just begs for speed.

At 50 mph, in Automatic, I floored tha car and as it was nearing redline, the engine started doing a wierd kind of stuttering/missing that was almost like it was misfiring but it was not a misfire. I immediately backed off the throttleI wish I could describe it better.


This has me worried so I put the car back to stock tune (what a difference that felt) and have been more 'gentler' on the gas pedal.


Is there anyhting I could record in the Predator to see if I can figure out what is causing this?

I am having an SLP exhaust put on in about 3 weeks so I need to have this runni ngright before then Plus ,as I said, I am running without the Predator now.

Any help would be really appreciated.

[Mike_Levy]

Put the tune back on the car and log it for a while. Chances are good that what happened was that you broke traction and were spinning the tires. Then the traction control was individually braking wheels and pulling spark to get traction back.

I used to do that ALOT with my CSRT8. Mash the gas at 65-70 and she'd kick down and smoke the tires.

[LXSRT8]

Thank you for replying.
I will put the tune back as you suggest and see if it goes away.

I am able to be more specific on this after my drive today.

Had a solid hour drive up and back to visit my folks and the highway has a 65 mph speed limit. I had plenty of chances to mash it.

It did not do this all the time. When it happens, it is identcal: what it feels like is exactly like the car is starting to run out of gas for a split second, goes away and then repeats the feeling. So it is like it is kind of like it is stuttering as though I might imagne how the car would behave if it is hitting the rev limiter over and over again but I am nowhere near that rpm.

Sometimes it would stutter like this for just a few seconds and just as I was thinking to get off the gas so as to not hurt the car, it would smooth out.
Other times, it would continue doing this until I got worried about engine damage and I would let off the gas.

I would floor the car at around 50 to 70,it would downshift and as it started to rev past 4000 RPM, somewhere in the 4K to 5K range, it would do this stutter trick. But not all the time as I mentioned
Generally I would let off the gas as I approched the century mark but not for any stuttering reasons.


Last week I did hook this up to my laptop so I could add a lot more items to track as I would do multiple 0-60 timed runs. Therefore, I do know how to setup the laptop for logging.

Tomorrow, I will put the standard tune back in and make some test runs and also I will try the 'milder' 91 octane tune.

Can you think of any parameters that I should be logging onto the laptop to help get to the bottom of this?


Can't imagine that I put out that much power at 60 MPH to break loose the tires but is there a parameter which would indicate such?


Joe

[CaptainJ]

Hope you get an answer as I am also having a similar problem.

[Quick]

Log it and see if you can see anything unsual. Maybe rpm, vehicle speed, throttle position, timing at cyl_1 for a start.

[LXSRT8]

I did get lucky and managed to have reset my laptop log,restarted logging and within 2 minute,I got that stuttering problem.

By lucky I mean that I have a log file that is only 3 minutes long and has only 2 places where I floored it so it makes it easy to study the data.

I have enclosed a screen snaphot of the logged data which covers the period of time before,during and after the time it was stuttering.


The stuttering area occurred when the speed/rpm ranged from ~65mph/3840RPM to ~80MPH/5500RPM.

I let off the gas a few seconds after it started the stuttering but within a short time, I got on it again but the second time,it did not stutter. I lety off at 95 mph.





I don't expect anyone to make sense of the colors but I wanted you to see what the graph looks like when it does that stuttering.

It is really jagged.

The top period of time of the red square wave is the period at which I have it floored. The rpm/speed/LT/ST knock and some others have a very distinctive and repeatable set of curves.

In this run though, there are a few that are really out of wack.



Normally the curves in the plot of the handfull of parameters I logged when I 'floor' the car look very smooth. Obviously, such things as RPM does drop and start to increase again because of the shift to next gear but everyhting has a non jaggy shape to the curves.

I am hoping that one of the experts can point me to more data to log after seeing these results:
1 - While it is stutterring, the Manifold Air Pressure and the P-Ratio Map values track each other BUT they are fluctuating really badly as can be seen in their orange curve. Normally, during an accleration run where I leave the pedal to the floor, they are a smooth curve since their values are not changing all over the place.

I suspect that this is caused by the fact that the engine is stuttering rather than it being a cuase of the stutter but I do not know what events cause the MAP to change.

2 - The other odd thing I see is that normally the Accelerator Pedal position and the Throttle blade position track failry close to each other. The blue curve is the Acclerator Pedal Position and notice that it is very jaggy.

So, maybe one of the folks that make this device who are providing the support in this forum can give me some parameters to log based upon what I have opbserved.

[SRT8Tech]

Hmmmmm, Why in the hell is your throttle plate fluctuating while your foot is to the floor!!??? Thats the reason your map and baro is fluctuating. Did you lower the rev limit in the Predator instead of raising it??? LOL. That is one weird log. I dont like the way it looks. Also your throttle blade never gets to 100% its max was at 74%. Im thinking your throttle body is screwed. Only weird thing is that there are no codes. 99.9% of the time a bad throttle body would throw a code. If it was my car I would do a real throttle blade calibrate with the StarScan. Easy for me to say because I work at a dealer. Who knows how much the dealer would charge you to do it. Even then, that might not even fix it.

[Quick]

I believe the throttle blade showing less than 100% is normal.
It's physically all the way open (or as designed for "all the way open").
I forget the exact reason. Something with the actuator and the fact that it never goes 100% closed or something like that.
It's the way the measurement is calculated. The throttle fluctuating looks pretty odd.

[Mike_Levy]

Right, the blade will never go to 100%. 74% is WOT.

[LXSRT8]

"Hmmmmm, Why in the hell is your throttle plate fluctuating while your foot is to the floor!!??? "

Thanks for the VERY useful reply.

In my writeup above, I picked up on the fact that the throttle plate position was bouncing around even through I had the car floored.

From what you say, I understand that the fluctuating plate is causing the other items to fluctuate so I should concentrate on what the deal is with the throttle plate ?

I have looked through all the logs I have and the throttle plate is just fine EXCEPT for two logs and those were I felt the car doing the stuttering that I have been going on about.


I have not troouched the rev limiter at all. All I have done is try out the various tunes that come with the Predator and have used them unmodified.


Get this. I am feeling this occasionally with the stock tune but more often with the Predatore's tunes.
Ie. I do remember just a handful of times where I felt the car do this stutter under full acceleration with the standard tune long before I got the Predator but it was extremely rare.
The car has about 9,000 miles on it.


I went back and carefully looked at the rpm's when the throttle position starts doing that wacky thing and sure enough, the RPM is going up and down slightly during that time. That RPM changing is what I am feeling as stuttering.

On the log I posted above, it is happening between 5200 and 6100 RPM. I swear I can fell it happening a lot longer than just the time it accelerates only 900 RPM



I understand that the Acclerator pedal position is measuring the gas pedal paoistion that ones foot is on.
I also understand that since the car is drive by wire that the gas pedal is not hooked to the fuel system but rather that poistion % is used by the computer to decide what the driver is doing.

What does the throttle blade do?
Where is it?
What did you mean about how to test it?

Thanks - I am Finally getting somewhere!

[mikel]

Try raising the rev limiter 2-300 rpm to see if it helps.

[Quick]

I understand that the Acclerator pedal position is measuring the gas pedal paoistion that ones foot is on.
I also understand that since the car is drive by wire that the gas pedal is not hooked to the fuel system but rather that poistion % is used by the computer to decide what the driver is doing.

Actually the the gas pedal is hooked (indirectly by wire) to the throttle blade. When the throttle blade opens the manifold air pressure (MAP) sensor detects a drop in manifold vacuum and the PCM increases fuel. There are many other inputs that go into all this but the gas pedal pretty much only controls the throttle blade. (it's a small point).

What does the throttle blade do?
Where is it?
What did you mean about how to test it?

The throttle blade is that butterfly valve that sits in the throttle body. The throttle body is on the front of the air intake manifold and is the thing that your air intake connects to. It has a wiring harness connection to it. When you press on the gas pedal the PCM will take a number of factors into consideration and activate the electric motor in the throttle body that controls the throttle blade.

So normally, if you put your foot to the floor, the throttle blade would open completely allowing max air flow into the air intake manifold. Under certain conditions the PCM will override the pedal position and modify the actual throttle position. For example, you should be rev limited to something around 4K rpm when in neutral. Floor the car in neutral and the PCM will modulate the throttle plate position to limit the revs. Even though your foot is on the floor the throttle blade won't be all the way open.

Testing the throttle calibration... highly unlikely that's needed. The throttle calibration can be done by a StarScan (that's the Chrysler diagnostic/calibration/firmware updater/etc. tool -- dealerships have them) and it is to synchronize the pedal position with the throttle plate. I've never heard of anyone's being out (lot's of people think theiir's was out but most, if not all, were due to confusion on how the throttle blade position is reported).

[LXSRT8]

Thanks to MikeL - I will give that a try tomorrow and post back my results.

Quick - Thank you for that very detailed answer. Quite interesting how that drive by wire system actuially works.

I grabbed that answer and added it to my Word file that I have been buildiing which I am building. It is a 'How stuff works on the Charger/Magnum' that I have been compiling

[Mike_Levy]

If you raise the idle, make sure you keep your foot on the brake hard at stop lights. I've seen it many times where with a raised idle the car will surge against the brakes and if you're not holding it in place you could hit the car in front of you.

[IntenseImage]

Its EXACTLY what alot of us are having an issue with (ive posted about it several times). Seems like we are hitting the rev limiter -- and ive raised my rev limited by as much as 300 rpm and still hit it.