NC ENGINE MISFIRE

Postby **mx5racing** » Thu Sep 27, 2012 9:46 am

The early model NC’s (2005-2006) can suffer from an engine misfire caused by damaged wiring to the coils. This is usually first noticed as an intermittent engine misfire that develops into a full misfire in time. Another symptom can be “popping” from the exhaust on overrun.

Easiest way to check is lift the engine cover which just clips down.

The top of the engine is revealed and shows the 4 engine coils

With the engine running gently wriggle the wiring around the plug. If the engine stumbles or misfires you have weakened/broken wires.

This is number 4 coil plug and this issue mainly effects 3 or 4.

We carry a spare plug all set up and ready to go in the event of a failure. As you can see 2 of the wires are a much thicker gauge than the last one. The last one (coloured) is always the one that breaks and the break is just as the wire exits the grey plug.

The insulation is never broken so a visual inspection will not show anything.
The main cause of this is the wiring is unsupported and the weight of the harness over time causes the wiring to fail.

Phil Ashton sent me the link to a clip and length of wiring to replace the OEM unit

http://tools.nissantechmate.com/Detail.aspx?id=657&cid=1

or if you have access to a wrecker the Mazda 3 is the same.

An easy preventative measure is to cable tie a splint of support to the harness over the coil plug.

minipete · Postby **minipete** » Thu Sep 27, 2012 6:52 pm

Really great info, thanks

caravanner · Postby **caravanner** » Mon Nov 25, 2013 6:04 pm

I have just purchased an early NC and have just read this post.

I notice that a "fix" suggested is to use a splint to carry the weight of the wires.

Has anyone done this? If so can you please provide a photo of the set up.

Thanks.

Sailor · Postby **Sailor** » Tue Nov 26, 2013 8:01 am

Another preventative measure is to strip back the insulating wrap on the harness.
Apparently being wound so close to the plug transfers all the vibration and movement to a very short section right at the plug.
Removing the wrap allows a longer section to flex reducing point loads.

Mr Morlock · Postby **Mr Morlock** » Tue Nov 26, 2013 8:57 am

a splint is likely to be what the name implies- merely a support that is strapped or clipped to the wires leading to the coil pack. Whether it makes any difference is anyones guess. Interestingly a friend has a Maxima with a coilpack on each plug and this an Achilles heel on the car - they fail and expensive. Judging from the forum it would seem that the NC does does not have a big issue with it. Others with exp on NC would know better

Charlie Brown · Postby **Charlie Brown** » Tue Nov 26, 2013 1:00 pm

I have replaced #3 & #4 coil plugs and leads. Chris's post shows where I sourced the plugs from in the US.
The only problem is that they only accept delivery to the credit card address in the US, so you need to know someone there to do your ordering for you and them to send the plugs onto you here in AU.

You can unclip the pins from the plug base and solder on a new lead but it is a pain in the ar$e. 9 times out of 10 you'll give up trying to find the release to get the pin out.

It should be noted that this problem, caused by over crimping of the cable just past the end of the pin during assembly, only occurs in the 2005 & 2006 NC's. Mazdaspeed had replacement harnesses available for around $400US early on that were used in the NC Cup cars but these are no longer available.

For a bridge or support try a paddle pop stick and cable ties. Also leaving off the cover engine cover seems to help, however if the miss is there already, no amount of support will fix the problem.

caravanner · Postby **caravanner** » Tue Nov 26, 2013 1:33 pm

Thank you all for your responses.

I do not have a miss. The car has only done 47,000 km and goes beautifully.

I just wanted to stop a problem from occuring if I could.

I shall leave the engine cover off and look at putting a splint on the cable loom.

Thanks once again.

Charlie Brown · Postby **Charlie Brown** » Tue Nov 26, 2013 4:24 pm

caravanner wrote:Thank you all for your responses.

I do not have a miss. The car has only done 47,000 km and goes beautifully.

I just wanted to stop a problem from occuring if I could.

I shall leave the engine cover off and look at putting a splint on the cable loom.

Thanks once again.

If you look at Chris's second bottom photo and the white wire.

2mm from where the white wire enters the coil plug is where the break normally occurs so this is the area needing support.

I'd tape one end of the support to the top of the coil plug and then tape the other end back on the main part of the wiring loom. Then put a cable tie at the midpoint around the three wires and tighten them up to the support.

Make sure it's heat proof tape otherwise it won't hold.

Dan · Postby **Dan** » Thu Sep 24, 2015 1:27 pm

For anyone that comes across this and wants to fix this issue you can buy replacement plugs pre-crimped with 12" leads from here http://www.bmotorsports.com/shop/produc ... ts_id/2177

I just ordered 5 of them to replace the plugs on mine as preventative maintenance which ended up costing me $105.04 AUD including shipping.