Highway driving for a few miles then gradually downshifts so that I need to pull over
 

I’ve got the e70 4.8. After a few miles of driving on the highway, the transmission will gradually downshift and slow down to the point where I need to pull over to a stop. Then when I try to accelerate, there’s strong resistance. Any ideas what this could be? I got stranded on the bridge the other day and had to get it towed home. It was a scary experience as the suv wanted to stop abruptly.

Dash warning? Check engine light? Engine codes? (You need to have a scanner that reads bmw codes to diagnose x5).

Typically a slow to a crawl limp mode is caused from overheat but you will get a dash alert when that happens.


–awr–

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Thanks for chiming in. I’m really sorry for not being more specific, but here’s the whole story: Around a month ago, I noticed some dash lights while driving. If I recall, it was the 4x4 indicator. It drove fine and everything, and eventually the light disappeared. Last week, I had to move the x5 out of the garage and when I started it, I noticed the brake pedal fell all the way to the floor. After shutting it off and back on, the pedal stiffened up again so I was able to move the suv. However, the pedal wasn’t the same as it would still slowly move downwards if I kept my foot depressed on it. Then the following weekend, I decided to replace the master cylinder and I bled all 4 wheels after. I’ve even used foxwell to bleed the abs too. I took it out the next day and that’s when I got stranded on the bridge with 4x4 and dtc on the display screen. I’m now wondering if it’s the brake booster contaminated by oil inside the vacuum line. I know years ago there was a recall about this, but my door jam sticker has the code punched out, so it’s been addressed before by bmw. There’s just 62k miles. I also noticed when idling in park, the brake pedal does gets stiffer when I depress it a few times. However, when I shift it to D, it feels normal again.

Sorry to interrupt, but I've been meaning to ask this question for four years...what doess this "hole-punched" sircular sticker pasted in the door jamb signify? I've no idea. '01 E53 3.0i
Attachment 85111

If the brake booster pin is misaligned it will apply the brakes usually somewhat slowly but eventually full application.

All the BMWs I've helped with have no adjustment so usually the fix is to shorten the pin or shim out the master cylinder.

You can confirm by cracking a brake pipe at the MC to release the pressure.

I forget the correct name of the pin from the booster to the MC but that very much matches your symptom and also almost always happens shortly after the MC was replaced.

This was a recent thread on XO and the fix was thin washers between the booster and MC.


–awr–

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I don’t know if the brakes were automatically applied, but I do know the first sign was the transmission downshifting a gear. I even saw the rpm fluctuate, then resistance with acceleration that grew stronger and stronger. It’s a good thing I had enough time to pull over to the shoulder. When it finally came to a halt, the suv was still idling just fine. Just that when I hit the gas, it was like trying to move a 5 ton truck. Even when I shut off the car and back on again, it would fire right back up. I’m wondering could it be the brake booster setting off the limp mode.

No you’re fine. This was what I mentioned of earlier. I checked my recall number on this sticker and what’s punched out was for the brake booster recall service.

It doesn't sound like limp mode sounds like brakes stuck on.

If you can get to the bolts that hold the MC to the booster loosen them a little and it should relieve the pressure in the line vs. crack ab hydraulic line.

You can pull the vaccum line off the booster to prevent it from "taking over" for testing but be prepared to use the parking brake to stop. Does the pre lci e70 ise the electric parking brake?

When the brakes are held the DME will cut power to the engine. As you slow down, it will downshift and once stopped you will likely not be able to move or just crawl.

The symptoms you describe m exactly match booster over zealous.

I've seen a couple times and always resolved.

I'm not sure you can pull the check valve for the booster that would be the easiest way to relax the booster to test it.

High confidence it's the booster over acting.

Loosen the MC mounting a little if you can, just 1/16 to 3/32" else check valve.

If MC is a little loose and maybe use a small shim like washer to hold it away from the booster it should immediately behave.

https://uploads.tapatalk-cdn.com/202...8b74b742a9.jpg
Looks like the check valve is just pressed into a grommet on the booster. Pull that, remove from begun l vacuum line so you can cork the vacuum line that will release the booster and turn brakes into unpowered brakes but will release.

That will confirm what I've been saying about the booster pin/rod being effectively too long.


–awr–

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Pre lci is electronic parking brakes
Number 2 on this photo is very difficult to pull out. I can’t get it out. I remember when removing the old mc, there was a gush of air heard.

I appreciate you Andrew and your suggestions won’t go to waste. This Saturday, I’m occupied by the family, but I’ll try to find time on Sunday to work on it. Thanks a million.