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My Eunos (NA 1.6) over the last couple of months has developed a misfire on overrun.

It doesn't miss under load (or at least, if it does it's so subtle I can't feel it), so I'm discounting the idea of an ignition fault, as ime these tend to show up under load.

Possibly related, ever since I have had it, once at operating temp and normal idle (around 900rpm) it wil randomly fluctutae at idle. This improved when I reset the base idle, but it still does it, and the miss on overrun is always there, cold or warm.

The guy before me (& Madda), replaced a few things to try to cure the erratic idle, but not really sure what he tried. So may have to start from scrathc in the respect.

Any thoughts? Vac leak maybe? I need to get hold of a Mazda tech manual so I can check signals from MAF, temp sensors, etc - but until then, just after some ideas? Is there anywhere to download a tech manual?

Also, I seem to get a lot of vapour from the exhaust at idle, even when warm. It's not gathering moisture when I put my hand in it and I don't loose any water, so I don't think it's that. It does smell pretty rich, but then I'm told these engines run a bit rich anyway?
 

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Discussion Starter · #3 ·
Yeah, good call, I suppose it is related to closed loop conditions.

I don't have any references for checking the sensors, etc - is there a Mazda manual I can download?
 

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Not sure on the MX-5's O2 set up, but on another JDM car with a 4 wire narrowband the O2 signal wire should be carrying between 0.1 and 0.9V (and fluctuating in between those two) on idle.

If the signal is significantly above those or rock steady, then the O2 sensor is goosed. Some of the self tuners on here should know what signal voltage the stock O2 gives out.

What's your mpg like - it normally reduces substantially when the O2 sensor goes.

You could also take it to an MOT garage or dyno and ask them to put their sniffer up the tail pipe whilst it idles, that should give you an idea of whether the fueling on idle is ok.
 

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Misfire on overrun shouldn't be possible - when you lift off, the fuel injection cuts off (until revs drop to somewhere around 1,600-1,800). Can you describe the symptom in any more detail?
 

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Well, it's as I say. Imagine how a missfire feels when accelerating, it's like that but when decellerating.

Basically the car judders & bogs a little, like you have kangaroo fuel!!

I can't say for sure, but I think it is a little worse as you get lower down the rev range. I'll try shutting off today at high rpm and seeing if it does it at any particular point in rev range.

Elduce - I did check mpg with rich running in mind, and it was giving over 30mpg iirc so don't believe there is a problem there. I will check the O2 sensor anyway.
 

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Interesting. If the car's still injecting fuel on the overrun, then I guess it doesn't know the throttle's closed. I don't know how the 1.6 does that. (On the 1.8 there's a "closed" switch in the TPS, but as far as I know the 1.6 only has a "wide open" switch.) Maybe your airflow meter isn't closing fully. (Just wild guessing now - does it show?
)
 

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The easiest way to check the O2 sensor is ok may well be to take it to a garage that does MOTs and ask them to check the AFR on idle.

On the laters cars they have to idle within a set per centage of 1.000 Lambda (14.7 AFR) to pass, not sure this is the case on the ealier mid-90s models though?
 

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The 1.6 TPS also has an 'im closed' switch so it knows when to idle, and presumably when on the overrun.
Aha! Could be that, then. Shouldn't be hard to check with a multimeter whether it's switching or not.
 

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Discussion Starter · #11 ·
Looked more closely at it today, and it does indeed run rough on overrun at any rpm. It feels worse around 2k, but that could just be down to speed/gearing maybe.

I'll check the TPS, the fact that the engine runs so rough on overrun must mean that there is a changing air/fuel mix. And if it shouldn't be fuelling with throttle shut above idle condition then that's a good place to start!!!

Only other weird thing I noticed today was this. I got out of the gym, started car and had to make a phone call. Whilst I was chatting the idle had dropped to a steady 950-1000rpm, with just the odd lump. Drove home, car reasonably warm now, pull up and the idle is sitting at 1250rpm. Didn't drop back to 1000rpm. Surely once it's dropped to a warm idle it shouldn't then go back to a cold idle!!?? So that's two things to check maybe, TPS & temp sensors!!
 

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My Eunos (NA 1.6) over the last couple of months has developed a misfire on overrun.

It doesn't miss under load (or at least, if it does it's so subtle I can't feel it), so I'm discounting the idea of an ignition fault, as ime these tend to show up under load.

Possibly related, ever since I have had it, once at operating temp and normal idle (around 900rpm) it wil randomly fluctutae at idle. This improved when I reset the base idle, but it still does it, and the miss on overrun is always there, cold or warm.

The guy before me (& Madda), replaced a few things to try to cure the erratic idle, but not really sure what he tried. So may have to start from scrathc in the respect.

Any thoughts? Vac leak maybe? I need to get hold of a Mazda tech manual so I can check signals from MAF, temp sensors, etc - but until then, just after some ideas? Is there anywhere to download a tech manual?

Also, I seem to get a lot of vapour from the exhaust at idle, even when warm. It's not gathering moisture when I put my hand in it and I don't loose any water, so I don't think it's that. It does smell pretty rich, but then I'm told these engines run a bit rich anyway?
take the idle screw out and clean= thouroughly,if not that its the lambda probe
 
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