Mystery stalling issue solved--coolant temp sensor - BMW M5 Forum and M6 Forums
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post #1 of 2 Old 1st July 2019, 12:09 AM Thread Starter
Tyrone D.
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Mystery stalling issue solved--coolant temp sensor

I've had a serious stalling issue for about a year now with no obvious signs of anything wrong. Thanks to this board and the collective data across some other forums, I knew the different things to check. The problem is that with an issue like this, there are so many possible causes and, therefore, just as many potential fixes. My hope is that I explain this in enough detail so that the next person with this issue will be better able to pinpoint the problem without going through a huge process.

Vehicle: 12/92 production U.S. M5 with Fahey 3.9 stroker, approx. 171k miles

Symptoms:
1) Across entire range of operation (cold or warm start, any sort of ambient temp/humidity/weather conditions, engine at any operating temp), engine would stall with sudden removal of throttle with clutch disengaged (or engaged with gearbox in neutral). For example, when approaching a red light and declutching to come to a stop, the fall in revs would continue until the engine stalled. The only way to avoid this was to keep the car in gear and come to a near stop, disengaging the clutch at as low an engine speed as possible, only then declutching. An alternative method was to declutch and perform a coordinated dance between brake and throttle to carefully control the drop in revs as you slowed down.

The stalling was most pronounced with sudden lift-off of throttle from high engine speed. The problem worsened over time, then eventually the Check Engine light started coming on at idle and disappearing with a blip of the throttle, then the problem got even worse to where it was no longer enjoyable to drive the car.

2) The only other symptom was occasional surging during low gear/low engine speed coasting.

Causes:
I read this sort of issue can be caused by fueling issues (pressure regulator, fuel pump, relays), ignition (plugs, distributor cap, rotor, coil, wiring), vacuum/air leaks, cam/crank sensors, MAF, throttle position sensor, and one of the coolant temp sensors. My MAF tested okay, but I had it rebuilt anyway for peace of mind for the future. I took apart the intake manifold and all my silicone vacuum lines were good. While exposed I replaced all of the dried pipes and some other components under there for good measure and did some cleaning. The throttle position sensor tested okay, so I left it alone. I didn't test the fuel pressure regulator on the rail or the coolant temp sensor (the one to check is the one on the coolant pipe just below the oil filler cap with a teal-colored plug), but I bought those two parts anyway. After doing everything but the fuel pressure regulator and coolant temp sensor yesterday and putting the car back together, it ran the same as before! I left the pressure regulator alone because it's on the surface and easy to change at any time, and I wanted to go through a process of elimination so I could identify the culprit. I couldn't do the coolant temp sensor yesterday because I didn't have the 19mm wrench, but I got one today and did the job in a couple minutes. The one on the car appeared to be the original with a 92 date stamp on it. Problem solved! No more stalling or check engine light for the first time in over a year.

Parts:
Coolant temperature sensor, 13 62 1 401 945
Washer 07 11 9 963 130

Tools required:
19 mm combination wrench

Total cost:
$55 as of February 2019 invoice date
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post #2 of 2 Old 3rd July 2019, 11:57 PM
Rikko
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Thank you for sharing your knowledge. I'm about to start assembling my s38 in a month or so, good to keep in mind these issues can creep up. 👍
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