NB8A rough idle.

[sweep77]

Im trying to work out what could be wrong with my mx5 at the moment. When its been running and then stopped for maybe 1-2hrs i go to start it again and it feels like its only running on 3 cylinders(rough idle, no power) for maybe 10-20secs and then kicks to life again and runs normally. If its been off over night it starts fine. Same as if its been only stopped for maybe 20mins. Its only for that 1-2hrs of sitting there that it seems to develop this. At first i thought spark leads but figured if that was the case then it wouldnt really matter how long the car had been sitting there. Any ideas? oh and it never stalls when its running rough.

[manga_blue]

I tend to blame coilpacks for everything but usually when it's coilpacks the problem happens within a few minutes of stopping. What happens is that when you switch the engine off the coolant stops flowing and the stored heat around the exhaust ports and combustion chambers spreads out, heating the coilpacks to something a bit hotter than normal running conditions. If the coilpacks are getting old and dodgy then this may affect them for 10 or 20 minutes after you stop, but not an hour or two later.

Can't think what else it might be. ECT sensor not returning to correct reading: when you restart the engine is cold but ECU is told it's hot? Unlikely. Heat soak in IAT sensor? ECU thinks the incoming air is hot and adjusts mixture down to lean? Has there been any change to IAT sensor position or exhaust shielding?

[sweep77]

as far as im aware its all pretty standard. i'll have a look at some of the sensors and see if anythings amiss

[Black_Penguin]

Leaky O-ring on an injector? Fuel will leak past and sit in the inlet, when you start it it will run pig rich on one cylinder until it is all drawn in. And over night it will evaporate away.

[sweep77]

Would the o-ring be leaky or the injector itself?

[Black_Penguin]

Could be either, wouldn't hurt to get both checked.

[93_Clubman]

Ash, some ideas here: viewtopic.php?f=29&t=47266

[samx5]

hey sweep, I've had a similar issue intermittently for some time, my coilpacks have checked out OK and I've done plugs and leads. The injector idea was suggested to me and I'm yet to check it out (not sure how much labour it would take to check all 4). Mine will occasionally run on what seems to be 3 or 2 cylinders after starting when sitting for anywhere between 30 minutes and 6 hours, but hardly ever from dead cold. Sometimes it clears in a couple of seconds, very occasionally it doesn't clear and I switch it off & restart a few minutes later. It only happens once every few weeks or even less, and is very random so almost impossible to replicate at a workshop when the mechanic only gets one or two shots at it. I've been driving it most days for the last week and it hasn't happened at all. It never has any problems whilst driving or going up hills under load, which ruled out some of the electrical problems that can happen, its only when starting the engine. I'm going in for a service soon and I'll ask about geting the injectors checked.

[dimarg]

Hey mate had the exact same issue! Ran through some injector cleaner and hasnt happened since.

Probably the cheapest option so worth a try at least?

[sweep77]

Yeah was thinking to try injector cleaning. Can't hurt anyway!

[sweep77]

Well took the car to the shops and then started to go home and it wouldn't come good and only ran on 2 cylinders. Back to the Mazda dealer then and get them to fix it under warranty hopefully. I'm thinking its the coil pack as its 2 cylinders not just one not firing.

[sweep77]

Turns out it was the coil pack aft all. Lucky the dealer replaced it under warranty! Saved myself $500!

[Insurgence]

yeah was going to say coil packs.

exact same thing happened to me. they aren't cheap but hopefully only need to replace once.