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Tiny Misfire LS400


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It seems that at a certain age, these old LS400s start creaking like their owners! Now I have a tiny misfire on part-throttle pickup at about 40mph. Just an annoying hesitation, which vanishes if I put my foot down a bit more. I have replaced the plugs (correct iridium) and the garage diagnostics showed no codes at all! Seems more prevalent once the motor has properly warmed up on a significant journey. Any ideas anyone or experienced it too?

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How about the rest of the ignition system HT leads, rotor arms, distributor caps, are they original ?

I replaced all mine a little while before selling it and considering it appeared all original at 25 years old the only physical damage was hairline cracks in the rotor arms and it did cure the same problem you have.

All parts came from Rockauto (NGK/Bosch)

 

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I had mine from 96 to 99 and the only fault at about 100k miles was ignition HT leads, similar sounding fault and fixed it.

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Since I've been using E5 99 octane fuel I'm finding all these slight  " feelings " of summat not quite right have just about vanished .........  is this an illusion do you think ?

Might type of fuel make a difference for you MalcolmDC ?

Malc

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  • 2 weeks later...

Thanks, Malc, but tried 3 tanks of max, but once motor is warmed up the little hiccup returns! Have got a set of leads and coils waiting to be fitted, and may do the distrib caps and rotors at the same time. Cheers.

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The distributor caps are a bit of a pig to do as you have to remove the belt covers to get to the screws, I used a 1/4” drive ratchet set with long extensions to get to the cover bolts.

The passenger side coil is also awkward being on a slant.

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  • 5 weeks later...

During a break in the weather I did a quick coil change to the (easy) top coil, this sorted the stutter for about a week but now its come back! In my parking space with no shelter, anyone know the easiest way to get at the (awkward) bottom coil pack, pending a full dismantle job to do the leads later?

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There is no easy way, IIRC I removed the Battery and the tray under it (soak the nuts prior to trying to remove them in WD40 as they tend to shear off).

Plenty of light and a dental mirror, 1/4” sockets with short extension bars and double jointed fingers😀

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