JCF

Hi,

I have a very strange problem on a 86 3.2 Carrera. The cas was running perfectly (like always before a problem happens ). The problem begins when the engine, at idle, dies. Ok, tried to start again the engine, the starter is working, and the engine "seems" to start but.... dies one or two seconds after. Just like the regular over-rich mixture of starting lets it start, but no after. Actionning the throttle don't change anything.
Sometimes, the engine starts and there's no problem... until it dies.
It can happen at every rpm, cold or hot engine... nothing specific.

So I controled / did the following things :
- Test the DME relay and shunt the three wires : OK
- Test the fuel pressure in the rail : 2.5 when engine stopped and between 2 and 2.5 when the engine is running : OK
- Test ignition. No problem, I have spark when the engine dies. OK
- Test injector plug with diode : Like ignition, there's signal even when the engine dies. OK
- Control the head temperature sensor with ohmmeter : OK
- Control the AFM temperature sensor with ohmmeter : OK
- Test the AFM plate signal as describes there : http://the944.com/afm.htm : OK
- Test the two switches for idle and full throttle positions : OK
- Test the altitude comensation switch : OK (resistance of 1.8 kOhm as in wiring) OK
- Control and clean the grounds on the left intake, control the ground between body and tansmission : OK
- Check the battery plugs : OK
- Test the two reference / speed sensors with ohmmeter : OK (but there's ignition so they should work)
- Idle positionner : vibrate.
I checked all this at their plug, and then, at the DME plug. No cut wire, all is good. I controlled also at the DME :
- Oxygen sensor : there's no O2 Sensor but a factory shunt, which is OK at the DME plug.
- Grounds : OK
- 0V for the AC signal when off, 12V when on.
I checked all pins except the starter signal one. I've also have nothing at the two tachometer pins (21 and 11).

Then, I opened the DME. The three transistors solder were cracked and I fixed it. Did the same for one other transistor and one or two other solders. Put it again in the car and nothing changes : sometimes yes ; sometimes no... As a cut wire but all I checked were good (but not all, otherwise the car would start ).

Now I suppose the injection works properly except false injection time. I suspected that because when I unplug the head temperature sensor, the car runs (approximetely 2000 / 2500 rpm when throttle released) instead of dying because too rich. Sometimes it dies, and it happens when I can run the car with the sensor plugged...
I can also have the engine running (bad) if I remove the AFM plug... It dies once I plugged it again.

After several hours of test, this car becomes a nightmare for me... I forget something but what ? If somebody have an idea... a known location where the wires cut ? A similar problem with solution... All is welcome !

Thanks !

ricster

Pickup sensor on the flywheel. I had that problem a few years back....guaranteed you need them both replaced.

JCF

I planned to invert both sensors, but the upper was so hard that I gave up. I noticed that the plastic insulator of both sensor was so dry that it broked (almost) completely when I touched them. I can see the shield of the sensors...
The resistance of both is good. I measure the alternative voltage when cranking, appr. 2V for one et 0.2 for the other one. But haven't oscilloscope...
I prefered to be sure before changing it, so can a bad sensor lead to a start for 1 second and a cut-off engine ? Can the engine runs with bad sensor when I remove the temperature sensor ?

rusnak

It sure sounds like it could be the sensors. Out of curiosity, you say you have 12v at the AC? How much voltage at the injector connector? What is the battery voltage/ charge level? Did you check the positive battery clamp for corrosion?

JCF

When I say 12v, I mean battery voltage (or almost). It's the signal to the brain to tell it that... the engine will lose some horses.
Don't measure the injector connector voltage. My 12v diode flashes and I really can hear the injectors "clic".
The battery is good and charged (just charged before). I imposed it several seconds of cranking so it's very brave. I checked and clean (even they don't seem bad) the positive and the negative clamps.
After some search, if I remember, the tacho needle doesn't move when cranking (but shows the right rmp after). Maybe is it a symptom.
I will investigate more about the sensors, already, thanks for your answers.

2qwik4u

Has the car got wet recently? Rain or car wash? If so could be dist. cap and rotor, happened to me once.

JCF

Yes, the car was store in a garage (humidty) and the first engine cut went when we were cleaning the car...
But the ignition parts (rotor, cap, plugs and wires) were renewed 3 years and 2000 km ago... they are as new.

2qwik4u

I'm sure its cap and rotor! Very common problem, inexpensive and easy fix.

JCF

I'm sure not, I checked both. Moreover, I've spark at plugs and sometimes the car runs, sometimes doesn't run. If not working, just have to unlplug the temperator sensor to have the engine firing !

But I'll check again monday. It's not time consuming and easy to check so you're right that I have to be sure of this component. What had these parts in your case ? Cracked ? Broken ?

JCF

Removed the sensors yesterday and ordered 2 BMW ones (50€ instead of 120€, great deal ). Received them this morning and installed on the car after finding the right piece of metal for setting the gap.
Try cranking... OK, the car starts and idles as before (a good begin). But after 10-15 minutes, agian, just as before, the engine stops. And same problem, when trying to crank, the engine fires and stop immediately. I notice that when cold, the car always starts. It's only when the first "dying" happened that it won't start again (but still fires one or two seconds).
I still suppose an very lean mixture, but why ? Is it possible that an electronic component in the Motronic (or another component in the car) heats so much that it "freeze" ? I don't have another 3.2 Motronic for testing it. A also check the rotor / distributor cap ; in perfect condition.
One other thing I didn't notice : before this problem, I added a resistor in derivation with the temperature sensor of the heater (in fact, done like this at factory but the resistor was down). According to the diagrams, nothings links the heater and the engine management (I also tryed to remove the relays, the fuses but nothing changes), but can it be linked ?
Will try to look at the alternator....

I (now) hate this car but I would like to solve this problem.

rusnak

Do you have a way to test for ignition spark the next time it dies?

theiceman

fuel pump .... ?

rusnak

Yeah, I'd keep a fuel pressure gauge on there and watch it, along with spark. I doubt the alternator is the problem. In old Fords, if a coil is bad, it fails as it heats up.

theiceman

usually starts , stmbles and fails is fuel releated ..

rusnak

I could see how it could be the ignition switch too. Still...I agree. I'd be knocking on the fuel pump or watching the pressure with a gauge. The 3.2 has a nice test fitting for easy fuel pressure checks. Much easier than the CIS fuel pressure rigs.