Help!!!! '93 M5 runs rough/stalls when warm.

[m3driver97]

I have a 93 M5 with 95K miles that recently developed a consistent problem. The car starts OK and runs fine until it reaches normal operating temperature. As soon as that happens, the car runs very rough if you let it operate below 3000 RPM.

In fact, this past weekend I was out and the car stalled at an intersection. It wouldn't start for over 90 minutes. I ended up having to jump the car the last two times I tried to get it to start (using the specific BMW procedure for doing this so I didn't fry anything) and it still wouldn't start.

Finally I ended up towing the car home and hooked up the battery charger. An hour later the car finally started, and I ran it just long enough to power it into the garage.

Note: I searched this and other BMW forums looking for any clue what might be causing these symptoms. Some of the items I checked below are a direct result of finding similar problems in the forum(s) and following those steps.

- I have done a visual inspection of all the vacuum hoses and they all appear to be in good shape. (I plan on replacing all of them just to be sure).
- I unhooked the battery and checked the ECU for evidence of burned circuit lands and found nothing suspicious.
- e-box fan works at both normal and high speeds.
- I removed and checked the rubber boot between the MAF sensor/plenum and it looked OK (it had been replaced 3 yrs ago).
- There were no stored error codes.
- Fuel and air filters have been replaced.
- Main Relay and Fuel Pump relay have been replaced.
- Considering replacing battery but don't really believe this is the source of the problem.

Started the car again and the same problem presented itself after warmup.

Here's what I think I can discount as a cause of the problem...
- Vacuum Leak: This should also have an effect on a cold engine (unless the leak develops as the engine warms up) or some vacuum line that is enabled at operating temperature has a leak.
- Fuel Pump: Why would it wait for a warm engine to act up? Engine will keep running (poorly) if revs kept above 3000.
- O2 Sensor: Considered this but wouldn't I get some sort of error code if this were the source of the problem?

I am desperate for any ideas that might help me put the car back on the road and enjoy it...

Anyone have any ideas where I can look next/try???

[VentoGT]

Throttle position sensor? Vapor lock? I am not smart with these issues, but I did cross post this to the e34M5 Yahoo group list--I will report back with my findings there from the experts' mouths...I suggest you go over to Yahoo and join that group regardless--they are a very knowledgeable bunch of guys. Good luck and stay tuned!

[m3driver97]

Thanks for feedback..

I checked the TPS also and it checked out OK... 0 ohms at both idle and WOT positions. I suppose that it could be a vapor lock of some sort, although I haven't seen any instances of that problem relating to my symptoms -- and there has been no rerouting of the fuel lines from factory positions and all heat shields are in place. Any clues on how to check out the possibility of a vapor lock?

I'll check out the e34M5 Yahoo Group -- thanks for the suggestion...

[632 Regal]

is this correct???


I checked the TPS... 0 ohms at both idle and WOT positions.

you dont have vapor lock dude.

[SnakeyesTx]

Vapor lock in the fuel system (you could try insulating the fuel rail to keep some of the heat out, or the check valve in the in-tank fuel pump went bad. The good news is that this is a 3 dollar fix with a 5 dollar shipping cost :D

http://www.bmwe34.net/e34main/trouble/535ihot_start.asp

That's the how to..

Here's the BMW part number which if you call BMA, they can get it to you, but you can't search this number on their website since its under construction. 16-14-9-068-988 I had a very similar problem on my 525, and after putting this in, I've been running, starting, and idling just fine. For so little, its peace of mind.

[rogerdad]

newbie answers

the stupid thing i did after cleaning my engine was i left water around the spark plg boots.
My 525i 93 acted just like yours. It should not have run at all but its a bimmer.

roger

[BillionPa]

the O2 sensors do kick in when the cats get warm, so they could be to blame. and they dont always throw a code. if you havent replaced them in the last 50K, do it and be done with it.

the system could "think" its running too rich, so it leans it out to the point where its not combusting properly. i would use something that measures all the DME inputs at once, and see which one goes nuts when the engine freaks out.

[m3driver97]

Thanks for all the feedback -- particulary to VentoGT for cross-posting to the E34M5 Yahoo group.

I plan to check the dist. cap for water/condensation per Gary Derian and also replace cap/rotor/plugs in any event as a maintenance item. Also plan on ordering a replacement O2 sensor since it doesn't appear to have been changed per the maintenance history.

Also plan on borrowing/buying a fuel pressure gauge and see how the fuel pump/FPR are doing. Have most of the vacuum lines on order and already have the o-rings for the throttle body/plenum attachment. Also plan to check out the temperature sensor per the Bentley manual.

If anyone comes up with something else I should be checking please let me know...

Regards, Brent

[BillionPa]

im going to replace my air intake sensor before winter, its like, $20.

if you run upper cylinder lube (Lucas or FP60) in your gas, your fuel pump will last significantly longer, and the injectors and engine like it too.

check the resistance of the ignition coil as well, 13 years later it could be making much less power then it did new.

[VentoGT]

Here are the responses from the e34M5 yahoo group--

1. Hi there,

I had a problem with mine much like this. I had thought fuel pump as well. However, I found the top of the ignition coil & ignition wire going to it completely burned into a pile of carbon dist. I have seen corrosion at this connector before also causing stumbles on acceleration. Hope this helps...or at least one more thing to rule out.

Andrei

2. Pull the distributor cap and dry the water in it. I had a similar problem a
few years ago. It was very odd. I checked everything, including installing
a fuel pressure gauge and watching it while driving.

The engine would misfire very badly once the engine warmed up, but not in
concert with the water or oil temp. I could start it and run it for 5
minutes, then shut it off and wait for the heat to migrate to the
distributor. That would do it, even though the water was cold.

Gary Derian

I saw that you are registered on the Yahoo board--it's a great group of folks. I wanted to post answers in this forum so that if anyone else searched opinions on a similar matter they would be here for everyone to see. Good luck and keep us updated!

[m3driver97]

I started checking out the ignition components and the cap and rotor had 'normal' wear. I am replacing them (cap is on order, rotor and plugs are on hand). What's interesting is that the plug gap on all the plugs was .044-.045"
instead of the .024" gap specified for this engine. I'm sure that replacing them will make a noticeable difference.

The plug wires all fell in the range of 5.5-6.0K ohm so I believe that they are OK.

Does anyone know the coil specifications for the S38B36 engine?????

I checked the ignition coil and it looks very clean (no corrosion). However the Bentley manual has no resistance specs for the S38B36 coil.

What has me puzzled are the ignition coil readings.

The Bentley coil specs for the M20/M30 are:
-- coil primary (between 1(-) & 15(+) are 0.50Kohm
-- coil secondary (between 15(+) & 4 (center tower) = 6.0Kohm

The Bentley coil specs for the M50/M60 are:
-- coil primary (between 1(-) & 15(+) are 0.4-0.8ohm
-- coil secondary (N.A. in specs column).

I measured 0.75 ohm between 1 & 15 (coil primary)
I measured 8.5K ohm between 15 & center tower

These might be right for the S38B36 (M5) coil, but I don't know the specs.

Hate to throw away the money on a new coil if I don't need to.

Anyone know the specs for the coil on this car..??

[m3driver97]

Hi everyone,

Just wanted to post the results of my repair activities in case anyone else runs into this type of scenario.

Sorry for the delay, but it took over 2 weeks for Pelican to provide all of the parts that I had ordered from them and then to compound the situation I ended up out of town almost 2 weeks.

I ended up doing the following to the car:

-- Checked/replaced all small vacuum hoses under the hood
-- Cleaned ICV but it was almost spotless inside before I started
-- Cleaned oil separator assembly and replaced O-ring & drain hose
-- Replaced valve cover breather hose
-- Replaced Cap/Rotor/Plugs with all Bosch parts (dist & cap had some corrosion but no moisture)
-- Replaced O-rings between throttle bodies and plenum
-- Replaced main relay and fuel pump relay
-- Cleaned all pluggable connections with and applied dielectric grease
-- Checked resistance & condition of plug wires and coil and they were OK
-- TPS checked per manual and measured 0 ohms at both idle and WOT
-- CPS (Crankshaft Position Sensor) checked out per manual
-- IAT sensor (in MAF box) checked out per manual
-- ECT sensor checked out per Bentley manual
-- Tightened all grounding lugs I could find under hood and rear seat
-- Fuel pressure checked out per manual

Bottom line, there were only 3 or 4 things that I am pretty sure were involved with the problem.

1. The upper ICV hose was totally loose where it connected to the center throttle body. This would be the prime candidate for a significant vacuum leak. Might have been from previous maintenence by FO.

2. The oil separator drain hose was rock hard and broken where it connects to the dipstick tube and was replaced.

3. Orifice in dipstick tube where oil separator hose connects was plugged solid. Cleaned out with appropriate sized drill bit.

4. Plug gap was 0.042 instead of nominal 0.024

So far, the car is running fine after all this work. I still plan to replace the O2 sensor as the records don't indicate that it has ever been changed.

Thanks again for all your input and areas to check out. MUCH easier to see what's going on with the engine when the plenum is removed.

Regards, Brent Dann

[VentoGT]

Thanks for the follow up Brent. My cap and rotor were toast when my car was running rough, water in the rotor, bad points, etc. Glad you got your problem figured out.

[Brandon J]

So it looks like basic maintenance did the trick huh. Also, don't forget to check if the resonance flap is working. One last thing, sometimes it is difficult to check the wires. One technique to check for bad wires and cross firing is to remove the plug wire cover, turn off the light in the garage, and run the car through the rev range. Any cross firing is easier to see when it lights up in the dark.

Good Luck,

[BillionPa]

fyi coil specs should be .5/6000, so you are running at about 70% ignition power.

[stu1]

Thanks for posting the results - I was fairly sure it was some closed loop thing, not even thinking about vacuum leaks when warmed up.

Cheers,
Stu.