Acceleration surge - traction control?

[gmcgann]

My 98 750 has started something new. If I accelerate quickly from a stop it starts to surge badly. It acts like the traction control kicking in, but there is no wheelspin and the light doesn't come on. Anything more than about 75% throttle acts like it's full on, full off, full on, full off, every second or so. If I back out of the throttle it stops. If I wait until I'm in second gear or rolling about 25 mph I can stomp it to the floor and it's fine, so it isn't a fuel problem. My best guess is something with the DTC. Anyone ever experienced this?

[darkstarmedia]

When was the last time you changed the tranny fluid... sounds like the tranny may be slipping....

Richard

[IcemanBHE]

Try the DSC throttle module.

[gmcgann]

When was the last time you changed the tranny fluid... sounds like the tranny may be slipping....

Richard

It's definitely not a slipping transmission.


Try the DSC throttle module.

How would I check that? It isn't giving me any codes.

[Edwin NL]

Central bearing or guibo's?

What does the car do when you kick down around 40mph do you feel a thump?

[gmcgann]

It's not a guibo. You can replicate exactly what my car does with any car. From a standing stop, start to accelerate, then jam your foot to the floor. One second later take your foot off the gas completely. One second later slam the throttle to the floor again. Once second later take your foot off completely. Repeat until the car shifts to second gear, at which point everything is normal again.

Or, with your E38, go to dirt or wet pavement, turn on the traction control, and again slam the gas open. The difference here is that the DTC light blinks, and it will do it in any gear as long as there is wheelspin. Mine only does it in first and there isn't any wheelspin.

[pizzaman09]

This may seem far fetched but I could be an O2 sensor. I had similar symptoms with my Jeep for several years on and off. It was in all gears though. The fix was the O2 sensor.

[gmcgann]

I solved my problem! This one was a little obscure, but makes perfect sense. The reason it only did it in first gear was because that's when the torque is strongest, and when the motor twists the most in the mounts. In my case it twisted just enough to open up a good gap where the left MAF hose was not secured to the intake. So you slam the gas open, the engine twists up, the gap opens, and suddenly the left MAF gets almost no airflow because all the air is coming in the gap, while the right MAF is fine. The DME freaks out, the engine slows, the gap reseals, the engine surges again, the gap opens, etc, until the torque is no longer enough to open the gap. Of course, when the gap isn't there, which is while you're looking under the hood, you can't see it. I was tugging on things and found the hose clamp wasn't tight. I reattached it securely and everything is fine.

[HIBANX]

Nice catch! That one could have been elusive for a long time.

[IcemanBHE]

OK..so now...

Is your motor mount broke, thus twisting to far with the torque?

[gmcgann]

OK..so now...

Is your motor mount broke, thus twisting to far with the torque?

Nope, I checked. The mounts are fine.

[AwesomeIsLuke]

Nope, I checked. The mounts are fine.

There should be minimal engine twist even at sudden WOT.

Could also be trans mounts.

[Edwin NL]

^+1 if your engine does this then they are shot, btw you can see bad motor mounts

[gmcgann]

I tested by power braking it while someone measured the movement. The left side at the intake comes up just over 1/2 inch. I'd say the mounts are worn, but still tolerable. That doesn't translate to much at all at the mount.

[IcemanBHE]

Sounds like alot to me...