Rough idle on startup

vscalais · Postby **vscalais** » Mon Oct 05, 2015 8:06 pm

I have an NB8A with 115,000km.

When I start from cold she runs poorly - almost like she has too much choke. Power is way down and you can hear the idle 'pulsing' from the exhaust. Give her a rev and she almost stalls.

However once fully warmed up (after about 10-15 minutes) she immediately runs perfectly. Revs rise to about 780rpm and she runs smooth and with full power. I thought I heard a relay or something click when fully warmed up, but I could be mistaken.

Is this typical of the idle air controller valve under the throttle going bad? Don't think it is a vacuum issue as she is perfect when fully warmed up.

Help please??

David, Carlingford NSW 2118

Ice88 · Postby **Ice88** » Tue Oct 06, 2015 12:09 pm

Check and clean your idle control valve. The B series engines have a wax palette which adjusts the idle air based on water temp

vscalais · Postby **vscalais** » Tue Oct 06, 2015 1:23 pm

Thanks Ice88. Please excuse my lack of technical knowledge. Is the idle control valve easily accessible? I'm not sure what I am looking at. And once I locate it, will unplugging (or bypassing) the idle control valve electrical connection make a difference i.e. help me isolate the exact problem?

In terms of cleaning, once I do find and disconnect the idle control valve, can I use throttle body cleaner. Will this dissolve the wax palette or is it safe to use?

Again, thanks for your help.

Cheers, David

Ice88 wrote:Check and clean your idle control valve. The B series engines have a wax palette which adjusts the idle air based on water temp

Ice88 · Postby **Ice88** » Tue Oct 06, 2015 2:57 pm

Picture of throttle body with the valve attached (this is for a nissan but similar on a BP):
http://www.freeengineinfo.com/media/nis ... 20body.JPG
Idle air is on the bottom.

Scroll down the cleaning section:
http://www.miata.net/garage/isc.html

It's not hard to access on a mx5, but will require some basic hand tools. Even if its not the iac, its something that should be cleaned as part of semi regular maintenance.

If you remove the throttle body and/or iac for cleaning you will need new gaskets for each, there pretty cheap

vscalais · Postby **vscalais** » Tue Oct 06, 2015 4:23 pm

Great help, many thanks. Will try cleaning first down the air supply tube with throttle body cleaner.

Cheers, David

Ice88 wrote:Picture of throttle body with the valve attached (this is for a nissan but similar on a BP):
http://www.freeengineinfo.com/media/nis ... 20body.JPG
Idle air is on the bottom.

Scroll down the cleaning section:
http://www.miata.net/garage/isc.html

It's not hard to access on a mx5, but will require some basic hand tools. Even if its not the iac, its something that should be cleaned as part of semi regular maintenance.

If you remove the throttle body and/or iac for cleaning you will need new gaskets for each, there pretty cheap

vscalais · Postby **vscalais** » Mon Oct 12, 2015 7:48 pm

No joy unfortunately. When the fault comes on, the NB8A is running on two cylinders only for some reason. I have replaced the coil, leads and plugs to no avail.

Ideas please. Would a sensor that was playing up cause the coil to fire on two cylinders only?

Is this a known NB8A problem? ECU maybe?

Ice88 wrote:Check and clean your idle control valve. The B series engines have a wax palette which adjusts the idle air based on water temp

93_Clubman · Postby **93_Clubman** » Tue Oct 13, 2015 12:04 pm

At the rear of the head on the B6 ( 1.6 ) & BP ( 1.8 ) motors there are two coolant temp sensors, one to the gauge & one to the ECU. A common NA6 problem is for the sensor to the ECU to become US & cause the issues above, but this doesn't seem to happen on the NB8A.
Appreciate that you've replaced the coilpack, but did you replace it with a new one?
Have to ask as there are many 'spare' NB8A coilpacks out there where one of the two coils in the pack become US, resulting in running on two cylinders.
Regarding the coilpack, have you tried changing the arrangement so that the cylinders not firing changes to the other two, & you are then able to specifically identify which of the two coils in the pack may be US.
Regarding the ECU, usually only an issue if moisture has got anywhere near them.
Regarding the possible relay sound, this might be the large green EGI relay in the engine bay fuse box, which can commonly be a source of issues.

93_Clubman wrote:Most common problem items on the NB8A tend to be:
coilpack (usually just one of the pair);
EGI relay;
cam angle sensor;
crank angle sensor; &
weakened wires in wiring loom near coilpack due to being bent without support, ie a lack of support to the wiring where it bends near the coilpacks results in wires breaking inside the plastic insulation. To check for this, after a decent drive, with vehicle idling, move the wiring loom in this area - if you get a variation or stumble in idle revs then some of the wiring could be on the way out.

vscalais · Postby **vscalais** » Tue Oct 13, 2015 9:56 pm

Thanks Ice88 and 93_clubman for your help. Turns out to be the coil pack - TWICE.

We replaced the original coil pack with a brand new reputable unit - only to find that it was faulty! Swapped it over and all is well (touch wood).

EGR relay? Might get a replacement just for comfort.

What about the other sensors including the O2 sensor? Worth replacing after 16 years? Or go on forever?

Open to your thoughts please.

Cheers, David

93_Clubman · Postby **93_Clubman** » Wed Oct 14, 2015 11:45 am

EGI relay - indeed many of us do keep a spare.
O2 sensor - if not replaced since new then it won't hurt as they do tend to be something of a consumable. That said, if US the upshot is usually increased fuel consumption due to running rich, although hks_kansei also found replacing the O2 sensor on his NB8A did sort a 'shudder etc' issue for him.

speed · Postby **speed** » Thu Oct 15, 2015 5:36 pm

The O2 sensor is responsible for a small amount of adjustment (especially compared to coolant sensor) but yeah, it wouldn't hurt to change it.

vscalais · Postby **vscalais** » Thu Oct 15, 2015 5:41 pm

And the cam angle sensor and crank angle sensor?

speed · Postby **speed** » Fri Oct 16, 2015 8:17 am

vscalais wrote:And the cam angle sensor and crank angle sensor?

The other responders are more knowledgeable then I, however I believe that the cas doesn't need changing unless you have difficulty starting.

Can others please confirm?

Thanks

93_Clubman · Postby **93_Clubman** » Fri Oct 16, 2015 11:29 am

If it's running fine leave it - by all means do visual inspections, & may be even keep spares, like the EGI relay, so that you have them easily to hand if there's a future issue. But this is also getting into an area of personal preference, so do what you are comfortable with.