E34 M5 - Hesitation/Stall

[EvanUrS4]

So, it's been a little over a week since I've taken the M5 out... this has happened once before when it sat for a while. On the flip side, one week isn't a particularly long period of time, something must be wrong if the car can't sit for 7 days.

It starts and idles fine, when you go to accelerate in 1st gear, it wants to die. Complete hesitation, will stall if you don't put the clutch back in quickly. Once you put the clutch back in, it idles fine. By fine I suppose I mean a little rough but my understanding is that's just how these S38's idle? My last one idled the same as this one and never had this issue.

Assuming you can coax it up to 5 mph or so and get the RPM's up, it stops hesitating and runs great. Above 2500-3000 RPM the car acts like nothing was ever wrong, you can floor it and have a nice smooth pull to redline. Back down in the 1000-2500 RPM range, if you push the gas pedal too hard, it will die out completely.

Thoughts?

PS: Last time this happened, it had sat for a little over a week as well, and after a day of being driven hard again, the car operates totally normally. If I drove this thing every day I think I would never know this problem existed... but as it is, it's really just a fun garage queen.

[Layne]

Vacuum leaks, most likely.

[TouringDan]

+1

Time to take off the plenum and replace every rubbery bit that transports air (or a lack therein) that you see. Pay extra attention to the intake elbow - look for cracks.

[jetavdk]

Start collecting all the vacuum hoses under the plenum and when they arrive replace all of them. You can also clean the butterflies with the plenum removed. The longest part of the job will be waiting for parts.

[EvanUrS4]

Just to play devils advocate here.... why would a vacuum leak only show itself if the car isn't driven for a week or more? If I drive this car every 3-4 days you'd never know anything was wrong.

[93FIM5]

Just to play devils advocate here.... why would a vacuum leak only show itself if the car isn't driven for a week or more? If I drive this car every 3-4 days you'd never know anything was wrong.

Thats a good point, personally I would pull the plugs and check there condition to see if you have any sort of lean or rich running condition and also check the condition of the rotor and cap. Depending on the plugs condition I would move on. The vacuum lines on the S38 are easy to access and relatively cheap to replace so they are worth taking a look at and replacing while youre in there as a preventative measure. Is it possible to post a video of your idle for comparison?

[Grim Reaper]

Agree with comments on vacuum lines but due to climate it could be a moisture or other issue with the cap, rotor, coil or plug wires

Also, how strong is your battery? If you have a serious drain it could be weak upon initial startup...

[Spiller]

If you spray brake cleaner around the vacuum lines and you notice a change in idle, you can be sure about this.

[woods]

Start collecting all the vacuum hoses under the plenum and when they arrive replace all of them. You can also clean the butterflies with the plenum removed. The longest part of the job will be waiting for parts.

Huh??? It's vacuum hose. You can buy it by the foot at any parts store on the planet.

[EvanUrS4]

I think 93FIM5 is onto something. I had pulled the plugs on the car after driving it a while back and everything looked normal. But I pulled them again after it has now been sitting for a few days again, and there's oil all over 2 of them.

Sounds like possibly the valve cover gasket just has a little internal leak and maybe oil is getting down in there when it sits for a while? This would certainly be a nice easy fix, might as well do a valve check anyways.

[93FIM5]

Oil on the tops of them? If so that would point to the small gaskets for the spark plug position like you are mentioning and could definitely cause some issues. Good luck let us know what you find!

[woods]

Sounds like possibly the valve cover gasket just has a little internal leak and maybe oil is getting down in there when it sits for a while? This would certainly be a nice easy fix, might as well do a valve check anyways.

Whether it's related or not, I would fix it to rule it out. Enough oil in the plug wells certainly can cause weak spark. The valve cover gasket is in 4 pieces. The outer gasket and 3 smaller gaskets that seal the spark plug holes.

Pull off the dizzy cap while you're at it to make sure there's no moisture, etc that could be affecting the spark from the other end.

Cheers.

[EvanUrS4]

Update - had nothing to do with vacuum. Replaced the distributor cap and rotor, valve cover gasket and a valve adjustment (22 of 24 were out of spec - WTF)

Car runs like a CHAMPION now. Wewt.

I assume the Distrib-cap was the main issue, it looked pretty rough.

Edit: I suppose that makes "Woods" the winner? :-)

[Evole]

Did you do your valve adjustment yourself?

[B10 BITURBO]

Update - had nothing to do with vacuum. Replaced the distributor cap and rotor, valve cover gasket and a valve adjustment (22 of 24 were out of spec - WTF)

Car runs like a CHAMPION now. Wewt.

I assume the Distrib-cap was the main issue, it looked pretty rough.

Edit: I suppose that makes "Woods" the winner? :-)

I was going to tell you that would had be my guess,, my ex M5 behaved like that .. also my BT,, and in both cases the distr. cap + rotor were replaced ,,

M20 M30 M70 S38B36 ,, all of them use the same partnr. and all of them are extremly senetive for corrosion and unhealthy situation of these things,,

well done and ,, keep flooring your car

[shervin]

Mine has the same problem (E34 M5 S38B36 engine)
I have replaced:
- Distributor cap and rotor
- Spark plugs and relevant wires
- Checked valve adjustments (all are between 0.28 - 0.33mm)
- All vacuum hoses are replaced
- Fuel canister valve is replaced
- Fuel pump is checked and fuel pressure is in correct value.
- Idle control valve is replaced
- Throttle position sensor is adjusted and function well
- Air mass sensor and intake air temp sensor have been checked and are functioning well

But it still hesitate when accelerating from low RPM
above 2500RPM everything is normal but below 2000RPM it really die when I push the gas pedal.
recently replaced its solid flywheel with a dual mass. driving quality is greatly improved and hesitation become less annoying but it still existed and I can"t fix it.
any useful comment will be appreciated.

[Hogg]

Did you find the problem? Generally speaking for issues like this, where a vacuum leak of some sort is relatively likely, i am amazed that more people don't spend the $s and invest in a good smoke machine. This one (https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1) is only $200, a lot less than all the parts people randomly throw at such problems. And way better than some of those hacked paint can smoke machines. A proper one like this will pump smoke all day long, and vast amount of it, making it easy to find vacuum leaks. After working on many older BMWs, and finally breaking down and buying this smoke machine, i am of the opinion that anyone that buys an older BMW should add the purchase of a similar smoke machine to the list, by default. It will save you a ton of time and money over the long haul...