Engine stalls under low speed braking

[Alpinewhite 633]

Is this common? Is there a somewhere I should be looking to sort that. Is there a deceleration pump or something?

300 633

[Taylor Marshall]

What does it idle at ?

[Alpinewhite 633]

750 most of the time.

[M633CSi]

Dude my car cant idle. I have to keep the RPMs above 1k for it to stay running. and mine has the same problems but mine are the injectors

[HayekFan]

How low is the speed and what gear are you in when this happens?

My first guess would be a vacuum leak, especially if you have a rough idle too.

[Taylor Marshall]

Nothing wrong with a 750 idle assuming it's smooth.

If you don't have any vacuum leaks then the next things to check are that your throttle plate (aka butterfly) and AFM flapper door are not dirty and sticking. Next that your FPR is OK and keeping sufficient fuel in the rail when you go off throttle to brake.
Does it only stall when braking or does it stall too when you simply go off the throttle ? Do you have an AT or a 5 sp ?

[Alpinewhite 633]

5 Speed. Upgraded rising rate fuel regulator. Overbored throttle body with larger butterfly. Are you thinking that I might need to boost the fuel pressure? It is off throttle stalling. But does not stall at idle. Stalls only when slowing.

AW 633

[Taylor Marshall]

I forgot that you have an RRFPR. If I remember correctly from a previous post yours is a standalone and replaces the stock FPR. This setup is quite different from mine in which my RRFPR (aka Fuel Management Unit=FMU) is in series after the stock FPR and the two work in tandem. I am just not familiar enough with how yours works to make any useful comment on it other than WAG's. My FMU is used to tune the air fuel ratio as well of course as provide correct fuel pressure for a bank of 37# injectors through a wide range of air manifold pressures from minus 15 lbs thru plus 15 lbs. I really doubt that, no matter how egregiously I misadjusted it, that it would ever cause stalling under decompression since the stock FPR is there as a first defence.
Has this problem been happening ever since you got the car or was it running happily and this is a new development or did it only start immediately after you played with your RRFPR adjustments ? Can it even be adjusted ?
Have you checked and cleaned your AFM flapper and butterfly ?
Also, once the TPS gets long in the tooth its idle switch can get a lot more sensitive.
My old one made me crazy for a while as I just couldn't get it to stay consistently responsive to the closed TB plate position input. I would adjust the elevation so the idle (and WOT) switch responded perfectly but then as soon as I tightened down the retaining nuts it would cease responding consistently as it had lost any range of tolerance. I might have been able to kludge it into working by adding a washer or two to the stud but I need reliability in this car so I got a new TPS. Is your ICV sticking ?
Have you pulled the rubber air intake boot off and really CLOSELY examined it under a good light for hairline cracks in the accordion folds ? This is a notorious and very common source of serious intermittent vacuum leaks that will ruin your day.
Logically, since your idle is smooth and steady at 750 (and IF you have no vacuum leaks) you need to look at what is the difference in the car's operating environment from when it is happily and smoothly idling off throttle and when it is going to idle (but overshooting the mark into a low RPM stall situation) from load to off throttle. I have covered in my above points most of that environment's fungibles that I can think of.
Finally if you have covered all of the above bases you can adjust the stop on the throttle plate to ensure that it stays open a crack off throttle and provides sufficient air to the motor to prevent stalling. I've BTDT and you can read about my education on this subject at the below thread when I had exactly your problem.

http://forums.roadfly.com/forums/bmw/bmw-6-series-e24/7757934-1.html

It worked but I was never totally happy with this fix as it kept my idle a little too high for my taste. Finally what worked for me to get my idle down to a very reliable, smooth and quiet 650 was to get a used TB and really clean it up including replacing with all new (and chromed for max bling effect ! ) the springs and levers. This combined with previously installed new TPS and ICV did the trick.

http://mydrive.roadfly.com/photos/pic.php?u=65692NBQmL&i=9194


http://mydrive.roadfly.com/photos/pic.php?u=65692NBQmL&i=9633

[Alpinewhite 633]

I had the car running good for about a week and then the stalling got more and more common. Had a hard time even starting it yesterday. Yes, the FPR is adjustable. It is the one from BAVauto. I boosted it about 16 pounds yesterday, which seems like a lot to me. But it is running better and stalling less.

When I have the cash I am going to take it in and have it fogged to see if there are any leaks.

The problem first started when I sealed a leak and hooked up the vacuum that wasn't attached under the intake manifold. Have been trying to find the right fuel pressure ever since. Gone to lean, thought I got it to rich. Going to check my new plugs these next weekend and see if they tell me I am running too rich.

AW 633

[Taylor Marshall]

Sounds to me like you have a vacuum leak and are fighting with and trying to compensate for unmetered air.

You should get yourself a vacuum gauge . Egauges sells the VDO one very cheap for 30 bucks. You can just slap it on a T-fitting to any of your air manifold vacuum lines and perch it up on the windshield facing in so you can note the various readings at different RPM's and in dynamic situations like coming off throttle. Anyway, good luck.

http://autorepair.about.com/cs/troubleshooting/a/aa112401a.htm

[jbd5015]

taylor, did you have fun trying not to chip paint on that shiny new TB when putting the assembly back together? And im guessing the extra big opening is for your turbo business?

-Jeff

[alpinacsi]

The problem first started when I sealed a leak and hooked up the vacuum that wasn't attached under the intake manifold. Have been trying to find the right fuel pressure ever since.
AW 633


What Vacuum line was it? Was it plugged before or just disconnected? If it was the cabin air sensor, I think it should have a restrictor at the manifold and it should not be hooked directly to the manifold.

[Taylor Marshall]

taylor, did you have fun trying not to chip paint (http://forums.bimmerforums.com/forum/showthread.php?t=1079543#) on that shiny new TB when putting the assembly back together? And im guessing the extra big opening is for your turbo (http://forums.bimmerforums.com/forum/showthread.php?t=1079543#) business?

G'morning Jeff !

It's powder coated so I didn't much have to worry about chipping it. I had that extra length of pipe TIG welded on because I kept blowing off the silicon sleeve from the intercooler to the stock TB under high boost conditions.....just about the worse time there is to develop the world's biggest vacuum leak :( . I tried T-bolt hose clamps, snugging it on at wierd angles, shamanist chants..you name it...on the stock length TB flange and nothing worked .
The demented Dr. Barber (PhD in Pure & Applied Smegmatology - University of Poughkeepsie) in a rare moment of lucidity at Watkins Glen in the Fall of 2007 put on his Skip Barber surgeon gloves and tried his hand at connecting it (see pic) but it didn't last a week.
With the pipe extension there is now no way the sleeve can blow off ...2nd pic.
Cheers !
T.

http://mydrive.roadfly.com/photos/pic.php?u=65692NBQmL&i=12170

http://mydrive.roadfly.com/photos/pic.php?u=65692NBQmL&i=12169

[Alpinewhite 633]

What Vacuum line was it? Was it plugged before or just disconnected? If it was the cabin air sensor, I think it should have a restrictor at the manifold and it should not be hooked directly to the manifold.

That is the one. I could hear it sucking air when it was not plugged in. The line slipped onto a brass nipple. Is that the restrictor?

300 633

[alpinacsi]

There should be a 1-2" long large diameter cloth braid hose coming off the manifold with a plastic restrictor attached to it and then the cabin air sensor line (smaller dia. hose that might be black with a red stripe) will mount to the restrictor.

Try plugging/capping the port on the manifold to see if that makes any difference.

[Alpinewhite 633]

Grrr. It is getting worse. Bumping the fuel pressure seems to solve it for a bit.

Again, when I had nothing attached to that nipple on the bottom of the manifold, the car didn't stall.

I will seal it and see what happens.

AW 633

[tjn182]

Try searching mye28.com as well, I'm sure they have many similar situations on e28s as you do.

Couple of questions....
(if its not a vacuum related problem...):
------
Are you holding the clutch in when you are braking or letting the engine help?

Is your steering difficult at times, or does your power steering pump make noise? I could be way off, but it sort of sounds like the engine doesnt have enough juice to provide power braking at lower rpms because of something resisting too hard.

If you have the clutch pedal pushed in (car @ idle), and youre just rolling along down the road and hit the brakes does anything happen?

------
(if it is vacuum...):
When it stalls - does your tailpipe smell like gasoline/unburned gasoline?

Just trying to get a better idea :) ... strange that having the nipple at the bottom without anything on it causes it to not stall... makes me think that it's running very rich and the extra air around the MAF "balances" it better.

good luck!

[Alpinewhite 633]

Try searching mye28.com as well, I'm sure they have many similar situations on e28s as you do.

Couple of questions....
(if its not a vacuum related problem...):
------
Are you holding the clutch in when you are braking or letting the engine help? Clutch in. Should I do other?

Is your steering difficult at times, or does your power steering pump make noise? I could be way off, but it sort of sounds like the engine doesnt have enough juice to provide power braking at lower rpms because of something resisting too hard. Steering is only hard when the engine turns off.

If you have the clutch pedal pushed in (car @ idle), and youre just rolling along down the road and hit the brakes does anything happen? Nope. It seems less involved with brakes, than off throttle.

------
(if it is vacuum...):
When it stalls - does your tailpipe smell like gasoline/unburned gasoline? I can't tell. Does Premium smell any different than regular unleaded?

Just trying to get a better idea :) ... strange that having the nipple at the bottom without anything on it causes it to not stall... makes me think that it's running very rich and the extra air around the MAF "balances" it better.

good luck!

No stalling today. But that is because I pulls that vacuum line off the bottom of the manifold, which lets in a certain amount of unmetered air.

But drivability suffers, from time to time, where the idle goes stupid and goes back and forth between 1000 and 1500 rpm. Both other times it idles perfectly at about 850.

I did turn down the fuel pressure about 12 pounds. Didn't change anything with drivability.

Previously I have leaned it out so far that it stutters, bogs and backfires. So trying to find the right pressure.

I don't like drivability versus stalling. I want good drivablity and no stalling.

But today the car was freaking awesome on the Freeway at 60-75 mph and has good power under acceleration. Sucks trying to drive it around at 30-40 mph between off and on throttle.

Thanks for you help guys,

AW 633