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Electrical gremlins: engine hard to start 850 1994

posted by  benglar on  Sat Sep 23 13:52 CST 2006
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last visit: Mon Jul 5 09:23 CST 2010
[RELATED]

I'm stuck and need some help.

I'm the original owner of a '94 850 sedan with 220k miles. Having a hard time starting: the starter motor can turn the engine over fine, but (usually when warm but sometimes even when cold) the engine will not fire. It sounds instead like pressure is pumping up in the cylinders for a short while and then the engine will fire once, but not catch. This happens 1-8 times in a row, then it will start just fine and run with no problems: full power, no hesitation.

Further complications:

The Check Engine light is on, but the diagnostic code reader only intermittently responds. On any and all of the A and B sockets, sometimes the LED will flash in response to the 1-second button push, and sometimes it won't.
In all cases, the LED flashes when I push the button, but only occasionally do I get a response from the equipment the diagnostic code reader reads. This is new: previously the diagnostic code reader worked reliably.

With patience I can get codes from the reader. So far, I've seen "engine speed sensor signal missing or faulty". Replaced the sensor that reads the flywheel/flex plate, after cleaning contacts and applying dielectric grease. No change in symptoms. Reseated the engine/tranny controllers in their little box. Symptoms remain unchanged.

Cleaned the battery posts, terminals, and the ground points at body just in front of the battery and at the engine. MEasure 0.003 ohms resistence between negative battery post and engine block, bare body metal, all other ground points I found.

Last bit of complication: the heater fan works only on position 4. I measured the voltage after the load resistor near the fan, and discovered the load resistor had failed. Replaced that, and the fan worked for a couple of weeks but then failed.


So, here are my questions.
1. Floating ground?
2. Wiring harness?
3. (It is about time to replace the timing belt, so:) timing belt?
4. Engine computer?

Thanks for the assistance. I can keep my 240s and 740s working forever with no problems, but these 850s are much harder to diagnose.








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    Electrical gremlins: engine hard to start 850 1994

    posted by  will rehsper on  Tue Sep 26 02:02 CST 2006
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    last visit: Tue Jul 3 14:55 CST 2012
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    If the OBD is pointing in the wrong direction and electrical stuff is leading nowhere perhaps the problem isn't electrical. Has the fuel filter ever been replaced? Can you check the fuel pressure? I've had two 850s experience severe driveability problems due to a faulty fuel pressure regulator or old fuel filter. In each case there were no codes from the OBD (except for the engine speed sensor one that you got).

    The 240s are nice and simple to diagnose. If it's not due to a short list of things, one can just jiggle the wiring harness at the firewall and hope the crumbling wire insulation or fatigued wires make just the right contact. Then you're ok...for now.

    Good luck,

    Will








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    Electrical gremlins: engine hard to start 850 1994

    posted by  someone claiming to be fixit2003 on  Mon Sep 25 02:00 CST 2006 [RELATED]

    Have you tried replaceing the cammshaft rotation sensor?








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      Electrical gremlins: engine hard to start 850 1994

      posted by  benglar on  Mon Sep 25 17:42 CST 2006
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      last visit: Mon Jul 5 09:23 CST 2010
      [RELATED]

      I replaced the camshaft position sensor about 2 years ago. I'm hoping that it is still good. But I've got a '95 850 wagon and I can swap the cam position sensor from that into the '94 to see whether it helps. The '95's engine speed sensor/flywheel position sensor is already in the '94, since that was what the 94's diagnostic code said was faulty.

      The saga continues. Thanks for the suggestion.








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        Electrical gremlins: engine hard to start 850 1994

        posted by  someone claiming to be 1 roll 66 on  Fri Sep 29 04:03 CST 2006 [RELATED]

        i would chk out put of ign coil, chk basics fuel pressure, compression.
        ign coils act up on cold starts, when full kv is required to start a cold
        engine.








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    Electrical gremlins: engine hard to start 850 1994

    posted by  someone claiming to be ksg on  Sun Sep 24 13:48 CST 2006 [RELATED]

    I bet you are on a good track checking connections and grounds. How about the ignition switch itself and also the alternator, brushes, and regulator? Since several symptoms seem to occur together or randomly the main voltage may be off or not reliably getting to parts like fuel pump relay or primary ignition. Engine controller failures are extremely rare, probably only due to some kind of electrical abuse if even then. Codes might get triggered by a component which isn't really bad if the voltage regulation is off. The problem with the fan might be a good starting point - if power is not getting there you may be able to trace all the way back to primary connections to find something open or corroded.








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    Electrical gremlins: engine hard to start 850 1994

    posted by  KlausC on  Sun Sep 24 06:00 CST 2006
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    last visit: Fri Oct 24 10:15 CST 2014
    [RELATED]

    If the timing belt is changed every 70K miles, I would say you are over due!
    As for the hard starting... There are so many variables it is hard to point to any one thing. Is it time for a tune up? Time to put a can of injector cleaner in the tank?
    Not being able to read codes is a real problem. Clean the sockets in both boxes and the probe, they are subject to corrosion. Also check the wiring leading into the boses for exposed wiring.

    Remember, just getting a code is not reason to replace a part. Only if the code re-ocuures in a short time.

    Klaus






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