NB8B intermittent low/rough idle + power loss

aviper4u · Postby **aviper4u** » Sat May 28, 2016 8:41 pm

so the car intermittently had a rough idle today around 500rpm and loss of power on acceleration and hesitates to rev though does rev and then all of a sudden comes back to life!!!

this seems to be common place on the web though there are people changing fuel pump/ignition coils/cat converters/cam angle,crank angle sensors and not having luck......

does anyone know a way to read the diagnosis port which could shed some as there are no dash lights anyways

my NB8B is 2000 build

Locutus · Postby **Locutus** » Sat May 28, 2016 9:15 pm

http://www.mx5diy.com/2015/04/retrievin ... r-mx5.html

aviper4u · Postby **aviper4u** » Fri Jun 17, 2016 12:34 pm

thanks for the link Locutus

well i've finally had a chance to reset the codes solder the LED to test though sadly the issue continues and the ECU thinks nothing is wrong LOL

i tried to record a video on my phone though the audio isnt good enough for diagnosis

when the car has low/rough idle and hesistates to rev its if as though i can smell fuel?

anyones suggestions are appreciated

StanTheMan · Postby **StanTheMan** » Fri Jun 17, 2016 1:21 pm

I would go back to basics.

Give the engine & idle valve an internal clean.
Check your air filter for crud
vacume leaks
check spark plugs
check spark gap
re adjust idle.

give your car some TLC short of going up the rear pipe. wash it. polish it.....

check all the basics. then work your way up the chain replacing things like ignition wires & stuff. but work out what's least extensive to fix. maybe even borrow parts to see if it makes a difference

then go from there.

Trackphotos · Postby **Trackphotos** » Fri Jun 17, 2016 1:49 pm

If it's idling low and lumpy, hesitates on throttle, drones, and smells of fuel, you've got a misfire. Fuel is going in and out again without being burnt.

The question is whether it's one cylinder or two. If it's two cylinders, a coil pack isn't working. Then you need to determine if it's the pack or the connector. If it's one, it could be leads or plugs. If you can induce the problem and then disconnect spark leads, and the idle does not change, the lead you disconnected was already not firing. Don't get too close to the lead sockets ;) they will hurt a lot if they arc to you. Best way is to disconnect the coilpack plug instead, but it will drop 2 cylinders instead of only one. If you disconnect it and the idle stays exactly the same, you know neither of those cylinders were firing before you unplugged it.

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hks_kansei · Postby **hks_kansei** » Fri Jun 17, 2016 2:36 pm

If it's misfiring on one cylinder you'll notice it, idle will be more than just low, the whole car will shake and throb.
Acceleration will be awful and very rough.

If it's dropped two cylinders it will often stall, and likely have barely enough power to move.

First stop for me would be new spark plugs, it's $20 and they're probably due anyway.
When you remove the old ones keep them in order as they come out and have a look at the ends of them, google how to read spark plugs (normal for an MX5 is to be brownish, or light black soot)

If it's misfiring on a cylinder you'll see one plug noticably more black than the rest, or even wet if it's been doing it recently.

My car had similar symptoms develop a while back, idled crap, on and off would drop power, and eventually started dropping a cylinder every week.
I did plugs, worked for a week or so until the new plugs got fouled.
Then did leads with a known good set, worked for a month, same thing.

In the end mine was the o2 sensor.
The diagnostic codes didnt know there was a problem, since the ECU was still getting a signal from the sensor, just the wrong one (it kept saying "lean" so the ECU kept adding more fuel until it choked)

First step for me though would be plugs and leads (give it a good hard rev or two after changing them to make sure it'd burned off any unburned fuel in the cylinder if any), then see if the issue returns after a while.

Trackphotos · Postby **Trackphotos** » Fri Jun 17, 2016 3:06 pm

This was what mine sounded like (but with headers only) when my MSPNP ecu was playing up and not triggering a coil pack. 2 cylinders not firing, idles roughly but does not stall. Also can maintain speed on highway, but not accelerate.

And this is it cutting from 2cyl back to 4cyl as recorded by dashcam, can hear the cutover at 0:31

aviper4u · Postby **aviper4u** » Fri Jun 17, 2016 5:40 pm

This was what mine sounded like (but with headers only) when my MSPNP ecu was playing up and not triggering a coil pack. 2 cylinders not firing, idles roughly but does not stall. Also can maintain speed on highway, but not accelerate.

wow mine sounds exactly like this when having the problem!

is there any way to bench test the leads/coil packs or can i remove the lead then start the motor to test

also if anyone has a spare set that is willing to loan/sell

also the TLC comments above are valid though I have just done a thorough wash lol!

hks_kansei · Postby **hks_kansei** » Fri Jun 17, 2016 5:45 pm

You can test coils with a multimeter.

I know there's an NB8A spec sheet floating around the web, with test points and resistances listed.
I'd assume there would alos be one for the NB8B coils.

aviper4u · Postby **aviper4u** » Fri Jun 17, 2016 6:30 pm

You can test coils with a multimeter.

yep found the below in the manual

Trackphotos · Postby **Trackphotos** » Fri Jun 17, 2016 6:34 pm

From what I've heard, most of the time when a coilpack fails it will test fine on a multimetre and fail when in use. Heat-sensitive failures most of the time, apparently. If you do find a problem then great, fault found. If you don't, it may still be defective. What you can do to test it is, assuming you can make it happen in the garage and not only while driving it, unplug one of the coilpacks while its idling. Don't need to touch the spark plug leads at either end, only the connector at the back of the coilpack. Unplug one pack and see what happens. No change = that coil pack was not firing even before you unplugged it. Swap the coil packs around, recreate the fault again, and see if the fault followed the coilpack, or stayed with the plug.