Revs dropping when the engine is cold

[LMProto]

Recently, my 2005 MX-5 NB has been having issues when the engine is cold. 9/10 times I start the car the revs drop intermittently when idling. About half the time the engine is back to normal when I start driving, but the other half the revs keep dropping even while driving, making the car feel very jittery. One time the rev dropping continued long enough to make the car stall twice at a stop light while it was in neutral.

Here is a video just after start-up, while standing still and idling.

Does anyone have any idea what could cause this problem?

Edited by LMProto on Wednesday 2nd September 11:51

[anonymous-user]

I believe it is called "idle droop" and there are loads of websites that explain how to solve it. I fixed it on my NA by resetting the idle speed via the diagnostics connector I think, although it was a long time ago.

Google "mx5 Idle droop" and you should find the solutions.

[LMProto]

JimSuperSix said:

I believe it is called "idle droop" and there are loads of websites that explain how to solve it. I fixed it on my NA by resetting the idle speed via the diagnostics connector I think, although it was a long time ago.

Google "mx5 Idle droop" and you should find the solutions.

Thanks! Would that also fix the revs dropping while the engine is under load/not idling, though?

[GravelBen]

I had similar with an early NA, it affected the car while driving too - stopping at an intersection etc the revs would sometimes drop too low and stall.

I'm not sure about the process for adjusting it with an NB, with the NA IIRC you bridged two contacts to bypass the ECU idle control while you adjusted the idle air valve on the throttle body with a screwdriver. I'm sure you can find more detailed instructions somewhere online.

I also gave the throttle body and butterfly a good clean and polish at the same time, which made a noticeable improvement to throttle response as it was quite grimy.

Edited by GravelBen on Thursday 3rd September 11:33

[anonymous-user]

LMProto said:

JimSuperSix said:

I believe it is called "idle droop" and there are loads of websites that explain how to solve it. I fixed it on my NA by resetting the idle speed via the diagnostics connector I think, although it was a long time ago.

Google "mx5 Idle droop" and you should find the solutions.

Thanks! Would that also fix the revs dropping while the engine is under load/not idling, though?

On my car it never did it while under load, just when idling and / or slowing down and stopping, ie when the engine was attempting to return to the idle speed, so if its dying when under power then it sounds like there is another issue present. Maybe a coil pack failing or similar? (not sure if an NB has coil packs like an NA).

Maybe check the spark plugs and leads also in case there's some obvious problem?

I would be tempted to give it a full service, so plugs, leads, fuel and air filter and oil + filter to rule out the common and cheaper stuff, then see if anything improves / changes before you move on to expensive bits like coil packs etc...

Maybe there is a way to read codes from an NB? I think with the NA you had to attach an LED to the diagnostic connector and what it would tell you was very basic, but perhaps worth a go.

Edited by anonymous-user on Saturday 5th September 10:06