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Engine Missfire


danielbrookes
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Brumster the MAP sensor is from Emerald and is calibrated as per their sheet, it states 100KPA with ignition on and engine *NOT* running which I believe is right (if that has any bearing on anything).  It’s an odd one, works been hectic so I’ve only managed an hour on and off.  Tomorrow I can concentrate on it and hopefully make some progress.

Edited by danielbrookes
Missed the vital NOT running word...
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18 minutes ago, brumster said:

As the revs drop (as it starts to presumably cut out, although in this instance he saved it/it picked up again) the MAP reading is going down on the graph but in reality it's a negative reading (negative pressure) so it starts off at -0.67 bar (rpm @ 1400) and goes up to -0.24 bar (rpm @ 500) as it starts to stall. Assuming it's all calibrated of course, I would take the actual pressure readings with a pinch of salt but that's what the values on the graph show as.

Ah ok that makes sense.  

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Eugh I am starting to get annoyed now.  My compression test kit doesn't fit, the thread is correct but its not deep enough to reach the thread in the block - frustrating to say the least.  Issue still persists this morning.  Battery voltage when idling is 12.6v and rises with revs, unsurprisingly the problem is still there.

I've checked the resistance of the injectors and they are all the same, 14.9ohms (they should be 14.5 according to the data I have, but they are all the same).

Does anyone know if I have the ignition plugged in should I get 12V at each injector plug on the wiring loom, because I am not I am getting less?  I have the stock plastic loom for the zetec where all 4 connectors are together so would need to open this up to do some investigation.

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Does anyone know if I should see 12V at the injectors with the ignition on?  I've checked the ECU earth and its good, 3ohms resistance to battery negative.  Not so with the negative on the injectors but as they go to the drivers I'm not sure if its normal or not.  

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1 minute ago, brumster said:

I believe you'll have 12v at one pin on the injector and, as you say, the other would go to the driver in the ECU which would short it to ground via a transistor driver. So you shouldn't see path to ground via those...

Even when the ignition is on Dan?

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Hmmm. Even more interesting! I don’t suppose you (assuming you have an emerald) could check to see if you have the same I.e when the ignition is on, engine not running, you get voltage across the injector pins? I wonder if I have a wiring fault and they are earthing out.

 

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I would say that 3 ohms to battery negative is a bad connection, at 1A return current there is your 3V drop.

Other sensors for the ECU that earth through the engine will give false readings with this sort of resistance.

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I'm going to run the ECU earth directly to the battery negative post to see if that helps.  I am guessing the voltage drop at the injectors is something to do with the injector drivers and I dont understand how they work.  Given there are 4 separate driver cables, I have a good + supply to each injector and they all show the same voltage reading if I connect pin1+pin2 I doubt there is a fault (unless it is indeed a bad ECU earth).  

 

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Whilst i do not know the details of your ECU I worked in electronics for nearly 50 years.

Normally this type of load is switched with an open collector driver pulling down to the negative connection. This will normally drop around 0.5V so you should be seeing 12.5V accross the coil with a battery charged to 13V.

Convention is that the electronics reference everything to the negative connection so if this is bouncing around 3 volts everytime the solonoids in the injectors fire the ecu could easily get confused.

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Thanks Paul, I ran the car upto temperature today and it worked fine - didn’t actually drive it but revs freely without issue upto 4K.  Tested all the wiring and found no issue, however, decided to take the loom off the car and inspect it. I don’t want to just drive the car with an intermittent fault.

If I don’t find the culprit I’ll give emerald a ring I think.

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4 hours ago, danielbrookes said:

Tested all the wiring and found no issue, however, decided to take the loom off the car and inspect it.

If you tested the wiring, what are you going to find if you take the loom out? And can you take the loom out? I'd have to dismantle my car to do that 🙂

 

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