Acceleration Cutting Out.

Peezen · #1 12-17-2021, 02:05 PM

Hi all, happy holidays

As I dig deeper into getting my new beast back into shape I am dealing with the next big issue: Acceleration cutting out.

I have a 2004 4.8is.

What is happening is that more often than not now, after idling for a while to warm up the X or after driving for a bit, I lose my ability to accelerate. Originally I thought it may be a transmission issue, but it shifts fine when everything is going well, and I can feel it shifting through all the gears in manual mode, and there are no issues going into reverse or drive right away.

What happens is that when I push the pedal I accelerate very slowly and even the revs are slow to climb. I can barely get past 20km on a small hill, and the revs will slowly climb to a shift point, and the X will shift, but continue with the slow acceleration. If I turn on and off the X the problem vanishes and I can drive normally until it starts again; push on the accelerator and very little response.

Today to tried tapping on the pedal and I was able to get a bunch of grunts and surges before it cuts back to nothing.

There are NO transmission codes, no engine codes at all. There are NO dash lights or messages in the message centre. I have the Schwaben/Foxwell BMW reader and I am getting no codes when this occurs. The only code related to the drive at all is a stubborn oil wear code for ten tanager case.

When it’s not happening the thing is a beast and accelerates like a monster.

Am I looking at an issue with the accelerator pedal? Or the throttle body? To be able to trust my new friend in the winter here I really have to get this issue sorted out, but without any codes to start with I’m wondering if anyone in here has any experience with this?

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andrewwynn · #2 12-17-2021, 02:07 PM

The scanner will show you real-time data for both the pedal and throttle sensors so start there. See if they read differently first start.

80stech · #3 12-17-2021, 02:25 PM

The pedal has 2 sensing elements operating opposite from each other so almost impossible for that to be a problem and not show a code. There is not a lot that won't set a code. Maybe igintion timing is being retarded.

RocketyMan · #4 12-17-2021, 02:41 PM

I experience this same exact issue on my X. The throttle sensor can develop "dead spots" where the two signals from the sensor deviate past the threshold for "rational" data. The DME will interpret this data as false, thus, resulting in what feels like no throttle. The dead spot for me is right in between like 25% and 30% throttle--mostly in the cruising range area. This makes sense because that's where the pedal is most often, other than 0%.

You should be able to see a code for this set in the DME for the throttle pedal sensor. It may take some time to flag it as a fault...

Peezen · #5 12-17-2021, 02:45 PM

I’ll try and scan both and see. As for the culprit, does it sound like the pedal assembly then?

Peezen · #6 12-17-2021, 02:46 PM

I’ll try and scan both and see. As for the culprit, does it sound like the pedal assembly then?

RocketyMan · #7 12-17-2021, 02:54 PM

It would be nice if you could graph your data as you SLOWLY press the accel pedal. See if there are any gaps or spikes in the data.

As weird as it might be, my pedal is MOST finicky when its cold and I have heat blowing on my feet from the HVAC. As if it unevenly heats up the sensor giving erratic results.

Usually you should see a rational code set for this. I cannot remember the exact name of it.

Peezen · #8 12-17-2021, 02:59 PM

Quick scan and some info that might indicate my problem… I hope.

Here is a snap shot of the live data:

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And here is a code that my Creader picked up that I wonder if could be the problem:

[IMG]

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Could it be that the X is thinking that I’m pressing the brake pedal and the accelerator at the same time?

andrewwynn · #9 12-17-2021, 02:59 PM

Quote:

Originally Posted by Peezen

Could it be that the X is thinking that I’m pressing the brake pedal and the accelerator at the same time?

I'm sure the computer will try to smooth out spikes to avoid false errors. You are on top of exactly what I was suggesting. Look for one of the two to report a hiccup in a smooth transition. On the foxwell you can graph both at the same time. I'm sure a large part of why the two sensors is to compensate for hiccups.

andrewwynn · #10 12-17-2021, 03:03 PM

That would absolutely do it. The brake on will kill the engine power.

I think you found your problem. Will be much easier to fix. See if your brake lights are on to start with then work on figuring out why tye brake switch is stuck. Some times the plunger just gets some dirt/goo in it and stops working properly

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