NB8B starting and running issues

[speed freak]

Hey there

Having a few issues with my 2001 NB8B again and looking for some help.

Firstly, sometimes (not everytime) when I go to start it from cold the engine very slowly cranks over and at the last minute it just fires up and the revs will slowly rise to idle point (not rev up to around 2k rpm and drop to cold start idle point) When it does do this it will always fire up still just very slowly cranks over.

Secondly, after its been dríven, if I go to start the car again about 5 to 10 minutes after Iv turned it off it will crank over normally but wont fire. I have to turn the key back to ign and try again then it will fire up as usual.

Thirdly, still have the issue where if Im slowing down or the revs get to 1500 rpm while Im in gear but off the throttle it will jerk back and forth violently. Automotive Plus have sorted this by resetting the base idle. They have had to do this about 4 times now, when the base idle gets reset it does fix the issue and I can slow right down to idle in gear off throttle with no issues but, this only last for about a month or so before it starts to jerk back and forth and be rough around this rpm. How do you adjust the base idle and is there something that can wear out to cause the base idle to keep getting put out?

I thought the battery could play a part atleast for my first issue but if it were the battery Id expect it to do that on every start.

Iv also replaced the cam position sensor and the crank position sensor. Spark plugs and spark plug leads have also been done plus all other major service items.

Any thoughts and suggestions will be muchly appreciated

[Okibi]

Can you borrow an AFM from a mate and see if that fixes the issue?

[tescoking]

IAC sensor too old or dirty, thats why it cannot detect the air. Take it out and clean it carefully or get a second hand one on ebay. Let us know if you have more question.

[speed freak]

I was thinking AFM, does the NA6 use the same as nb or no since its a different engine? I might be able to find one off an nb to try.

IAC has been cleaned numerous times now as well as the electrical plug and that does not help. Plus I cant remove the IAC from the TB as the screws are stuffed (old age) I tried getting a new/used one in the WTB section but no one seems to have one. Plus resetting the base idle fixes the 1500 rpm rough spot.

[speed freak]

also there are no CEL lights and its always run on 98.

Could it be the coil packs are slowly dying?

[tescoking]

IAC is hard to take out just because of the angle, remove only 3 screws to clean inside, worth a try.

[93_Clubman]

1. If your NB8B is still running its black Panasonic OEM battery, then they generally last about 12 years, which could mean yours is ready for replacement.

2. If you haven't already replace the green EGI relay in the main fuse/relay box in the engine bay - usually best to pick-up a couple of second hand ones from a wrecker as the same one is used across a number of makes & models.

3. Setting base idle:
http://www.miata.net/garage/KnowYourCar/S7_Idle.html
It shouldn't need resetting as often as you're finding, but if depressing the clutch pedal slightly higher, say at 2k RPM, then that might be a easy out.

Ref coilpacks slowly dying - the NB8B didn't have coilpacks like its predecesors. NB8B had two coil on plug leads & two normal plug leads, which usually are fairly reliable.

[93_Clubman]

Meant to add, if the problem persists check back & front of the ECU printed circuit board & its components for any signs of dampness or browning, as there were more than a couple of NB8Bs in QLD that ended up with water damaged ECUs (for a variety of reasons) that experience electrical gremlins.

[Mr Morlock]

A coil pack or whatever one wants to call it ( 2 of them) can certainly fail though getting a loan of one from a mate or a repairer is a good ploy of possible to check. Coils and plug leads can be checked on a bench as well.

[speed freak]

Cheers for the responses guys. As mentioned I cant remove the IAC valve off the throttle body as the screw tops are stuffed, its been cleaned as well as it can be while on the TB and TB has also been cleaned many times now.

Where abouts is the ecu located? Is it up under the dash or something on the passenger side?

Ill check the battery, its all black so its possibly the factory one so might be on its way out, mine is 12 years old this year.

Might grab one (or a few) of those EGI relays and replace it see if that helps.

With the third issue (1500 rpm rough spot) slightly depressing clutch or even very slightly on the throttle will bypass the issue but that's a pain in the ass lol plus its even worse in stop start traffic second gear at around 20 kmh is 1500 rpm and going on and off the throttle with the varying traffic flow is really difficult unless you just ride the clutch the whole way. Also I might add this issue goes away all together when the air con is turned on. So it seems with a little load on the engine this issue goes away but turn air con off and it returns.

Might try to find a pair of coils and try them too but I guess if it were them it would probably mis-fire too, it is running a little rough at the moment but its not that bad. Its got about 106000 km on it too just for reference.

[tescoking]

Ecu is under the passager side, you need to take out the carpet first and you will see a "shield", remove it and you will see your ecu.

Since you have cleaned so many times, perphas the IAC is actually getting too old, therefore it detecting weak signal, go to ebay and get a second hand one, it won't cost much. Also the screws are just too tight, it requird screw driver and your big arm only. Trust me, it can be done, I did it myself. I also believe, if you removed the IAC to clean is much better than just spraying thing into the TB.

[tescoking]

viewtopic.php?f=29&t=61437

[93_Clubman]

Might grab one (or a few) of those EGI relays and replace it see if that helps.

Ref relay, in addition to wreckers, you can also find them on ebay, where a number of us have bought them - eg:
http://www.ebay.com/sch/eBay-Motors-/60 ... 5b4&_sop=3

Diagnostics on page 2 of this thread might also be of use:
viewtopic.php?f=29&t=59479&start=15

[speed freak]

Ecu is under the passager side, you need to take out the carpet first and you will see a "shield", remove it and you will see your ecu.

Since you have cleaned so many times, perphas the IAC is actually getting too old, therefore it detecting weak signal, go to ebay and get a second hand one, it won't cost much. Also the screws are just too tight, it requird screw driver and your big arm only. Trust me, it can be done, I did it myself. I also believe, if you removed the IAC to clean is much better than just spraying thing into the TB.

Thanks for your suggestion but as I said, I cant remove the IAC from the TB as the tops of the screws are buggered (rounded) and even cutting a slit to use a flat head screw driver doesn't work either. The screws are old as you said and are stuffed, so yes they are tight but that's not the real issue here no size screw driver and no matter how big my arm is these screws aren't coming out. Plus I can get TB cleaner into the IAC valve anyway, before the butterfly valve there is a hole that goes into the IAC valve so I sprayed some in there. Done this again yesterday as well as the TB and runs and starts smoother. TB cleaner and rev limiter might have fixed the starting issue lol.

Might grab one (or a few) of those EGI relays and replace it see if that helps.

Ref relay, in addition to wreckers, you can also find them on ebay, where a number of us have bought them - eg:
http://www.ebay.com/sch/eBay-Motors-/60 ... 5b4&_sop=3

Diagnostics on page 2 of this thread might also be of use:
viewtopic.php?f=29&t=59479&start=15

Cheers mate, might get some off ebay then. Do local auto part stores sell them?

That link has some very useful info, gives me a few things to check. Thanks for the link on setting base idle too.

On cold start what should the idle be? Usually after the base idle is set again cold start will idle at 1500 rpm then drop to about 900 rpm when warm. Now cold start is about 1100 rpm and warm is 800 rpm or so. I was getting idle dip on cold startup too, it would fire and rev up to about 2000 rpm drop to about 1000 rpm then back up to around 1100/1200 rpm for cold start. I cleaned the TB and IAC valve again yesterday (I seem to get a lot of blowby, oil sitting in the TB and intake manifold) now no more idle dip. Start the car this morning, revs up to 2000 rpm and drop to about 1200 rpm.

When the air con is on and lights on etc is that when the IAC valve is working? If so since the 1500 rpm rough spot goes away with air con on does that mean IAC valve works properly? If the base idle is out could that cause starting issues? It seems heat related the cranking but not firing after its been dríven but Iv replaced both cam and crank position sensors, unless that's the EGI relay causing this. Then there is the slow cranking on cold start and just firing up.

Also I looked at my battery and its all black but I couldn't see Panasonic on it anywhere. Didn't take it out though so might be on the side of the battery?

[93_Clubman]

Do local auto part stores sell them?

Some of the bigger ones might, but last time I heard a price quote for a new one from Mazda was $150 in 2007.

On cold start what should the idle be? Usually after the base idle is set again cold start will idle at 1500 rpm then drop to about 900 rpm when warm. Now cold start is about 1100 rpm and warm is 800 rpm or so. I was getting idle dip on cold startup too, it would fire and rev up to about 2000 rpm drop to about 1000 rpm then back up to around 1100/1200 rpm for cold start. I cleaned the TB and IAC valve again yesterday (I seem to get a lot of blowby, oil sitting in the TB and intake manifold) now no more idle dip. Start the car this morning, revs up to 2000 rpm and drop to about 1200 rpm.

NA8 here, so from memory not exactly sure what the cold idle speed should be for the NB8B, however there should be a black & white sticker on the inside of the bonnet, which details normal operating temperature idle etc, & might include cold idle, although 1500 RPM sounds close. Normal operating temp idle for NA8 is 850 RPM +/- 50 RPM, so 800-900 RPM.

When the air con is on and lights on etc is that when the IAC valve is working? If so since the 1500 rpm rough spot goes away with air con on does that mean IAC valve works properly? If the base idle is out could that cause starting issues? It seems heat related the cranking but not firing after its been dríven but Iv replaced both cam and crank position sensors, unless that's the EGI relay causing this. Then there is the slow cranking on cold start and just firing up.

My NA6 & NA8s have had no air con or power steering, so I've not delved into what various affects they have. However, as you've advised it's been to Automotive Plus a few times to set base idle - I think it's unusual that base idle has had to be repeatedly set, which to me suggests something else is faulting or going out of adjustment, which subsequently requires base idle to be reset.

Also I looked at my battery and its all black but I couldn't see Panasonic on it anywhere. Didn't take it out though so might be on the side of the battery?

I'm not currently running a Panasonic so can't check, but from memory Panasonic is on it somewhere. That said it sounds like it, especially if it's also got the breather pipes attached at the top of each end.