Cold air intake problem

[BarryB]

Morning all

I have fitted a K&N cold air intake to my NB8B and I now occassionally have a soft "popping"sound when the engine is cold and idling or being started when warn. It is a sound similar to a car spitting back though the carby. Mazda have checked the car over changed the plugs and reset the engine management system but without rectifying the problem. There is no loss of power and it does not happen on accelaration.

Does anybody have any suggestions?

BarryB

[93_Clubman]

Sounds very much like this:
viewtopic.php?f=29&t=46548

viewtopic.php?f=29&t=45889

[BarryB]

Thanks Clubman. All of the info sounds like it applies to mine. It is a "soft" sound and not as sharp as a backfire. I'll try turning the wheel to the left or right and see what happens. As I said in my original post it is not constant so it is not like a faulty plug with a constant miss.

Thanks for your advice

Barry B

[manga_blue]

The CAI would make any popping that was already there much more audible. So it's possible that you've had it for years without it being a problem.

The links that 93_clubman posted were applicable to NA series cars which had quite different idle and timing control systems from your NB8B's. In general, backfiring into the intake manifold can be caused by lean fuel mixture or ignition/timing problems or valve timing problems.

I can't give a direct answer, just a brain dump of hypotheticals. Don't read any further if you don't want to hear about a whole raft of potential issues.

Assuming you're running a standard ECU and engine sensors then a lean mixture problem could in theory be:
- a weak injector (failing to deliver a charge occasionally or else consistently under-delivering)
- a faulty engine coolant temp sensor, so the ECU thinks the engine is warmer than it really is and doesn't request a large enough charge for cold running
- a faulty intake air temp sensor, so the ECU thinks the air is warmer than it really is and doesn't request a large enough charge for the larger mass of cold air

NB8B timing is fully electronically managed. An ignition/timing problem may be due to:
- faulty/wrong spark plugs
- poor spark plug earthing (did you put a coating of non-conductive grease on the spark plug threads?)
- faulty plug leads
- faulty ignition coilpack
- faulty cam angle or crank angle sensors

Valve timing problems can also cause idle misses. I suspect that you don't have strong enough symptoms of the timing belt being a tooth out. VVT may be an issue though. Roughly speaking the VVT in an NB8B works by giving the inlet valves full advance from idle to about 3500rpm, then full retard from 3500 to 5500 then full advance again above that. Retarded ignition at idle could cause some popping. Does anyone know if it's possible that a sticking VVT actuator causes partial/full retard at idle? I recall that miata.net has references to heavier oils or mineral oil deposits causing VVT actuator sticking. What grade oil are you using? Clogged/leaking VVT actuators are usually easily cleaned and re-built.

[BarryB]

Thanks NA8 I'll give it some more consideration

Barry B

[93_Clubman]

Mazda have checked the car over changed the plugs and reset the engine management system but without rectifying the problem.

Importantly you've had your MX5 checked by a Mazda dealer, so if they are competent in their checks, resetting, & diagnosis then you can have some confidence. However, if you're still concerned perhaps get a second opinion from a MX5 specialist.

[jonosx]

Try resetting your base idle.

I had a similar thing when i installed my intake and from memory it got better after doing this.

Could also be worth giving your throttlebody and IAC a cleanout as well.

Still notice it everynow and then but not as prominent.

[BarryB]

Thanks

I'll give it a try

Barry B