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Misfire with voltage drop


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I've been lurking around this forum for a while trying to learn as much as I can. I recently bought and installed a MS from Nigel (a great kit and suport I hasten to add). Before I start I'd just like to say what a great forum this is, a lot of effort invested by many - a virtual beer to you all.

I got my 3.9 serpentine up and running. I havent checked the timimg or adjusted the idle yet but she starts first time and idles pretty well.

Keen to get it playing with it I tried a bit of tuning the other night using TS and at around 3500 revs it started to run rough and auto tune really ramped up the fuel at that point.

I ran a log this morning and I noticed that where the misfire was happening my voltage was dropping, is this a symptom of something? I've attached a graph, would be grateful for some ideas.

Cheers Kev


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Could be as simple as a loose earth, but the fact it is dropping to 13.4V smacks of a loose connection to the alternator, or even an alternator on the way out...?

Not sure why you suddenly get misfires though, unless it is the alternator creating so much noise it is affecting a signal somewhere that MS doesn't like...

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Thank you for the quick response.

I had reset the tps and I wasn't holding full throttle. I was holding it where it was starting to misfire.

I ran the alternator wire separately to the ms loom but it is close as it runs along the left side of the left cylinder bank. I will try and relocate it and I do have a new alternator which I will now try.

All my earths are separately run to the battery and soldered but I'll re-check to see if one has come loose.

Would a faulty coil pack cause this, I have a spare one I could try unless its definitely not the culprit.

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Read this I have just posted as the "before you start tunning" is relevant

Also do check your timing with a light vs MS - timing makes a huge difference aND CAN CAUSE BOGGING ETC



curse of the caps lock fairy :D

Anyway, report on back - thanks for the thumbs up too - much appreacited !


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When we put the trigger wheel on we had a lot of problems with the timing. With the mark on the crank and the engine lined up the trigger wheel bolts didn't line up.

With rocker cover off, cylinder 1 valves closed and piston at the top we found the marks were off. The crank mark was about 3mm anticlockwise away from the pointer. With this set the trigger wheel bolts lined up.

My question is, how do I time it up now?

Nige, the pipe from the pmw valve is really tight, we had to heat it up to get it on, I'm worried it might be damaged if I remove it. Do you have the part number, or can you supply a replacement? Alternatively can I just use a hose clamp?

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The spare alternator I have won't fit, well it will but you have to take it a part and use part of the old case. I didn't do that but did change the brushes and the regulator. Unfortunately it didn't make a difference a part from getting another 0.5 volt out of the alternator.

I've got a spare coil pack, I'll try that next.

Anymore suggestions chaps?

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I'm not sure that's anything to do with the battery/alt. Bear in mind MS measures the voltage on the MS connector, not back at the battery, so small voltage drops in the wiring when (for example) it's drawing/sinking higher current would affect it.

If I'm reading the graphs right, you're mashing the pedal (TPS goes up, MAP goes up) and it's dumping masses of fuel in (duty cycle showing 88%, if this is true that's almost flat-out max for the injectors).

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Thanks guys for the feedback.

Fridge Freezer, what does that mean though? The injectors I have have been reconditioned and seem to be working fine. The injector loom is quite simple really, if we had made a mistake with the wiring it wouldn't run at all would it?

What do I need to check next? Could there be a fault with the board?

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What drives the duty cycle, what is it a function of? Is it from the map itself or does MS work it out on the fly?

On your setups do the volts drop at all when you open the throttle? What type of things could cause the MS board to drop voltage based on hitting a particular rpm?

I haven't tried different coil packs yes (as my mates got my truck this weekend for some welding) but is this still a possibility?

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  • 2 weeks later...

A quick update.

The misfire got worse, anything over 1500 revs it was hopeless and barely went over 30mph

I swapped the coil packs, no difference. Checked the wiring, all seemed ok and re-routed the 12v alternator output so it wasn't near anything ms related.

No change. Quite dejected at this point.

Then I re-burnt the tune I had saved and voila, all fixed. It revs out and pulls well. It should have been exactly the same but this time it runs a treat.

The engine starts well but I now need to tune the idle which is a bit high and slightly lumpy.

So all good, well alsmost, the inlet manifold gasket is leaking. We changed it when we were installing MS so its a bit disppaointing I've got tear it apart again. If it aint one thing its another, thats land rovers for you I guess.

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