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Low RPM misfire

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Old Dec 30th, 2012, 19:44   #1
volvy740
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Default Low RPM misfire

New member looking for help! My 740 (1990, B230E, non-turbo) has been running rough at low rpm. Once above 1800 rpm or so it's fine. I've changed the Distributor cap and rotor arm (they were both shot), but the problem persists. I'm thinking injector relay or ignition stage amplifier next, but I don't want to just through parts at it. Testable hypothesis are welcome!

FYI: the plugs were changed 3 months/1,500 miles ago. The misfire is intermittent
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Old Dec 30th, 2012, 20:26   #2
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Hi and firstly welcome to a fellow oxford member.It can be many things that cause low rev misfires.When was the cambelt last changed as this can cause very lumpy tickover.Also how clean is the crank breather system?

Glad it isn't a major running issue anyway..
Regards Brian.
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Old Dec 30th, 2012, 22:17   #3
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Have you checked the plugs to see what state they are in now ? Any obvious difference in one of them might give clues to what is going on. It might just be a faulty plug.
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Old Jan 1st, 2013, 13:53   #4
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I've checked the timing and it's fine until the misfire happens, at which point the timing starts to shift around erratically :s The timing belt itself looks to be in serviceable condition (no cracks or damage and turns 90 degrees on the longest travel, but no more).
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Old Jan 1st, 2013, 21:29   #5
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Assuming your car has a crank sensor I wonder if that is playing up.
I don't think anything else has much bearing on the ignition timing apart from the ignition ecu.
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Old Jan 2nd, 2013, 16:09   #6
volvy740
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HT leads can now be ruled out too. I found a split in one of them, so replaced the whole set, but it's made no difference at all.

There's a senor below the nearside of the distributor at the back of the engine. Is this the crank position sensor? If it is, how would one test it?
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Old Jan 3rd, 2013, 13:48   #7
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Default And the problem continues...

Okay, I think the sensor the leaves the bottom of the distributor is the hall sensor and it's looking like the most probable suspect atm. This looks difficult and expensive. I can't even get my multi-meter in the back of it to test!
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Old Jan 3rd, 2013, 17:21   #8
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I doubt it will be you ignition system. A fault with an ignition system at idle would definatly be present when the engine is under load at higher RPM.

I would be looking for an air leak as these have the greatest effect at idle speed because manifold vacuume at idle is at its highest.

At the engine speeds up manifold vaccume moves towards atmospheric pressure an air leak has little or no effect under load.

Common location is the inlet manifold which then creates a lean condition on the nearest cylinder to the leak.

Spray brake cleaner of LPG in the location if the manifold and listen for a change of engine speed which is the entry point of the induction leak.
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Old Jan 3rd, 2013, 17:41   #9
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I changed the inlet manifold gasket 2 months ago along with the injector seals to cure a lean misfire, so I'm doubtful. It's random and intermittent, which to me seems like the hallmark of an electrical problem. I'll give it a spray anyway as I'd be quite happy with a vacuum leak - much easier and cheaper to fix!
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Old Jan 5th, 2013, 13:50   #10
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Default Is this normal?

Sprayed around the injectors, inlet manifold and any other vacuum line I could see - no leaks.

Readings for the hall sensor were 10.5v (ground to positive) and 3.45v (ground to switch wire). Not sure what to make of that...
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