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Ignition Issues: Hesitation/misfiring Under Load, Rough Idle When Warm

19K views 25 replies 8 participants last post by  Zed. 
#1 ·
Hey All

Hoping you guys can help me with some suggestions for an ignition issue I'm having!

I've done some searching across here and the Miata.net forums- wanted to see if my hypothesis was on the right track before I start ordering replacement parts.

Car
1990 NA 1.6 (Short-nosed crank)

Problem
Misfiring/hesitation

Symptoms
Not much when cold- seems to idle quite high, between 1000-1200 rpm.

As the car starts to warm it begins to hesitate with a misfire on acceleration, especially below 3000rpm.

Once up to temperature, if you're sat at a set of lights out of gear, without your foot on the clutch, the idle becomes very sporadic and you can hear it misfire- it sometimes gets to the point of wanting to stall. Pulling away requires a very light foot, else you'll end up with it misfiring and barely moving, even when you go through the gears. Once it's moving though, I can knock it down to 3rd at around 50mph and it'll accelerate cleanly going through the gears.

Occasionally- if you're pulling away with it misfiring, it will sort itself out with a noise that I can only describe as a 'clank'. It's not massively intrusive, but you'll hear it coming from around the bulkhead, near the coil pack. Doesn't happen all the time though.

Factors
The car wasn't very well looked after by the previous owners- it had developed an oil leak from the CAS sensor at the back of the block and as a result, the engine was covered with an oil/muddy residue, especially near the bulkhead- I'm thinking this has finally killed the coil pack. The wires aren't original and don't seem the best quality, the length alone is wrong. I did have a look under the bonnet with the engine running in darkness and I can't see any flashes or signs of arcing, but it could be happening in the plug chambers itself.

I'm pretty sure a change of leads and a new coil pack will sort it, but as I've already replaced most of the cooling system over the last month I can't really afford to be wrong!

I've read a few other posts that says it could be throttle body or a variety of sensors. Are there any other things I can do (cleaning electrical connectors, testing etc.) before I start buying up parts?

Thanks again guys, apologies for the war and peace!

ZB.
 
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#4 ·
I've got the same issue now on my Mk1 Eunos 1.8 S-Special, did you ever get to the bottom of it?? I believe my leads and plugs were replaced not too long ago by the previous owner who is on here and I've been discussing it back and forward with him, I'm just hoping its not the coilpack really don't want to spend that kinda money on 1 of those, I've got more plans for my cash


Cheers,

Robert
 
#6 ·
It has actually now went down to 3 cylinders on my way to work :-( I've not had the chance to investigate further yet, but whilst speaking to the chap that sold he says he changed spark plugs and leads not too long ago so I'm fearing the worst, just at this time of year as well.... Never happens in the summer eh,

Cheers
 
#15 ·
If you have a multimeter its easy enough to check the leads and coil pack Rob Graingers book explains what to look for
http://tinyurl.com/c....com/c8aahlf��
note the book excerpt is the 1.8 manual but the principle is the same
I've just got home from work, yeah I actually made it home, car sounds so sick though, actually sounds a bit like a boxer engine from a subaru lol feel so sorry for the wee thing, anyway this link you provided is brilliant, really appreciate the effort finding the information out and posting the link


Thanks for everyone for there input hopefully I'll get to the bottom of it tomorrow morning before work

Cheers,

Robert
 
#16 ·
Well I pulled the plugs out and they look absolutely ancient so I think thats as good a place as any to start with changing, I was getting a right yellow spark from them when I was cranking it so think they're definitely shot, I went to test the coil pack too and my multimeter prongs don't actually reach far enough down the terminal of the coil pack to touch the connection
. So I had already ordered new NGK plugs for it anyway seen they're cheap and always worth replacing so hopefully they get here in the post today or tomorrow, I'll also get myself some longer prongs for my multimeter so I can give that a check whilst I'm at it.

Cheers
 
#20 ·
Lol
well changed the plugs and still goosed, damn and there was me thinking easy cheap fix
, I'm going to at least test the leads with my multimeter tomorrow morning, starting to think may be the coil pack though, its permanently running on 3 cylinders so in theory should make it easier to trace rather than an intermittent fault, the leads do look like cheapo 1's although they do fit the car snugly and are handily numbered on the top of the coil pack end of them even though you'd have to pretty slow witted to be able to stretch 1 of them further than its meant to go or have 1 with loads of loose lead hanging around but still, its nice lol I might have to see if a friend has a multimeter with longer needles/prongs on them to reach down the coil pack terminal to the connection.

Anyhoo, thanks again
 
#22 ·
Car Sorted!!!! The lad I bought it off and his Dad came out with a spare set of leads and a coil pack, ended up it was just 1 of the leads that were knackered, so they gave me a lead so I can run around in the car til I treat myself to a set of magnecors me thinks.....

Panic Over!!!

Thanks everybody for your feedback and help, very much appreciated
 
#23 ·
Wow, only just checked the thread again! Seems you're all well embroiled in discussion, so I'll just quickly update:

I swapped my plug leads out for a set of Cobalt ones from Moss, it's now working perfectly. I managed to get hold of a coil pack from JunkYardDog that I'll be fitting anyway, as it looks a lot better than the current one I've got in there.

So as the OP- my problem was solved with a set of good plug leads!
 
#26 ·
Hi peeps got the same problem on my MX mk1. If its running on 3 how can it be the coil pack as there are only 2 coils in the pack?
check plugleads, sometimes they're not pushed into the coil properly & can cause a miss...

Rich.
 
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