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Hi All,

Just wanted some assistance with an idling problem that has been getting worse over the last month.

Car History:
Car has done 105,xxx kms, major service performed @ 100,xxx by Mania about three months ago.

Symptoms:
Car will randomly start idling @ 1500+ RPM or fluctuate between 300-1200 RPM. It doesn't matter if the engine or cold or hot. Will usually occur when driving and will notice lack of power under 2500 RPM. Engine struggles and splutters when trying to accelerate and will be fine above 3000 RPM. Coming to a stop, engine will be idling @ 1500+ RPM.

Action taken to resolve:
Stopping and starting the engine does not help. Neither does revving the engine. Tried different types of fuels and brands, 91>95>98 octane and Shell>Mobile>Caltex with no change. Engine will not roll start either.

Called NRMA tonight, battery is fine, so is the alternator. Tapping on the the "base idle box" (?) next to the throttle body intermittently causes the engine to idle correctly, at about 800 RPM. Unplugging the "base idle box" will drop the idle to about 1000 RPM. Disconnecting the AFM sensor and the engine will cut out. NRMA "mechanic" thinks it's the "base idle box" or the base idle "sticking".

Please help with any suggestions or comments. Hopefully I'll be able to get this sorted before the Ice Berg Cruise this weekend!

Thanks in advance,
Alan

EDIT - fixed 08/03/2010

Whats a "base idle box"? The idle speed controller or the throttle position sensor?

The idle speed controller is a small electric motor mounted under the throttle body that opens a valve to allow more or less air into the engine to adjust the idle speed. The computer controls the position of the valve.

The throttle position sensor is a black blox mounted on the throttle plate shaft on the opposite side to the throttle cable. On an NA6 it has switches inside to tell the computer when the throttle plate is in the idle position or wide open. On the 1.8's it is a potentiometer that changes resistance as the throttle plate is moved so the computer knows exactly how much throttle is being used.

If either were faulty they would produce eratic idle but on an NA6 I'm not sure that they would affect performance. It's also possible that you may have a bad electrical connnection rather than a faulty component. You may also have a vacuum leak somewhere.

Have you tried setting up your base idle speed by shorting out the GND and TEN terminals in the diagnostics box then setting the idle speed to 850rpm after the engine has warmed up to normal operating temperature. Once again this may help eratic idling but wont explain your performance problems.

Another thing to check is the flap in the air flow meter (AFM). Does the flap move all the way? Does it move freely without binding, does the spring close it without binding. Is there enough spring tension to keep the flap closed with no air pasing past it. The AFM will have more affect on engine performance than the idle control system.

What garry says is right, and it sounds like a vacuum leak somewhere.
Also check your PCV valve and the tube it's connected to.

You could have a small leak in a vacuum hose somewhere so check these.
If any of the hoses look old and have gone soft or hard, they may need replacing.

Hi Garry,

Thank you for the adept reply

The "base idle box" I was referring to is the Throttle Position Sensor (TPS).

I'll check the hoses and diagnostics box tonight.

Thanks Benny, will try tonight.

this sounds similar to a problem I was having, check the leads or better yet if there's another mx5 owner near by swap them out and test the difference.

Finally got round to testing a couple or more things tonight. Installed new spark plug cable and spark plugs with no difference.

With a little help from Sasso I managed to rig up an LED to read the engine self diagnostic codes from the ECU.

On initial testing, I got nine flashes, pause, one long flash, pause and then three more flashes. This light sequence repeats (sequence 1). From my understanding, this light sequence should be 13 (un-known) and 9 (water thermistor). Looking at the self diagnostics table, the 13 does not exist. Removing the -ve terminal on the battery cleared the errors. I then un-plugged the TPS plug and no change, removing the water thermistor plug and no change, removing the AFM plug and the engine cut out (as it should) and I got one steady flash (sequence 2).

Can anyone please explain the 13 error light sequence or am I reading it wrong?

I'll post some videos of the light sequence in the morning.

Thanks in advance.

Cheers,
Alan

gobsmax wrote:Symptoms:
Car will randomly start idling @ 1500+ RPM or fluctuate between 300-1200 RPM. It doesn't matter if the engine or cold or hot. Will usually occur when driving and will notice lack of power under 2500 RPM. Engine struggles and splutters when trying to accelerate and will be fine above 3000 RPM. Coming to a stop, engine will be idling @ 1500+ RPM.

Spark plug leads. Very common problem.

Nine flashes, pause, one long flash, pause and then three more flashes

One steady flash repeating

bruce wrote:

gobsmax wrote:Symptoms:
Car will randomly start idling @ 1500+ RPM or fluctuate between 300-1200 RPM. It doesn't matter if the engine or cold or hot. Will usually occur when driving and will notice lack of power under 2500 RPM. Engine struggles and splutters when trying to accelerate and will be fine above 3000 RPM. Coming to a stop, engine will be idling @ 1500+ RPM.

Spark plug leads. Very common problem.

Hi Bruce, I have changed both plugs and lead with no change

I decided to unplug the TPS this morning. Driving to work, can did idle alot better but still flucuated to about 1200RPM intermittently. So i'm thinking it might be the coil pack?

From what you say, “Engine struggles and splutters when trying to accelerate and will be fine above 3000 RPM” there’s a reasonable chance its not the Coil Packs, as the problem would likely get worse above 3000rpm not clear out, or at least sometimes give trouble at 4-6k range.

How often does the problem occur, by randomly do you mean it occurs intermittently, i.e. fine a few hours or days, and when it comes, is it constant, and how long till it clears?

What is exactly happening when going to idle, do the revs drop below 1000rpm its rough, then revs kick up to 1500rpm, or does it stay above 1500rpm always.
At which point is it rough or smooth?

I saw alans car and I'd describe it as a misfire. It will rev high on idle (constant) then when he tries to accelerate the engine struggles while it revs, it will stutter and misfire (but still rev) and a crap load of black smoke comes out of the exhaust. Sounds like one or two of the cylinders are misfiring when the problem is occurring.

Alan have you checked that when the problem is happening the spark plugs are actually firing? Take out the leads out and hold it to a ground plate and see if it sparks. Then if they all do make sure the plugs are too. Where did you get your plugs from? Check to make sure they're all the same and are the right rating.

All you need is air fuel and spark, if you have spark you need air and fuel, the problem is intimittant, which is more an electrical thing.
Sounds to me like ECU is dumping crap loads of fuel in for some reason and its misfiring, perhaps it sees that there is something wrong with a sensor and its going into safe mode.

Like you showed me, unplugging the tps makes the idle ok, so it sounds ecu like. Did you say you changed your O2 sensor? Is the TPS rotated to the correct position?

Bring it over tomorrow and you can test it out with my ecu, afm, O2 sensor and injectors if you like.

It's been a while since I've updated this, but I've still got he same problems.

After doing some research on miata.net, some members have fixed this problem by replacing the engine temp sensor (green sensor), as did Uncle Arthur



My question is, how do you bloody remove the thing? The above pic is the back of the engine block. And the sensor sits about 5cms from the firewall. I've tried removing hoses to try and get access but its a real PITA!

Any help is appreciated.

I just replaced mine..
You need to remove the Coil PAck, then the job is actually quite easy.

Unplug the Coil Pack.
Undo one bottom bolt & 2 top ones.. i think 12mm?
Remove Coil PAck

Then you will see the Green Temp Sensor.
Unplug it from the clip, undo the sensor, and then just replace

It made my cold start midly better, but still hasnt fixed my cold running prob.

Thanks Mark. I'll give this a go this w/e. How much was your sensor? Still go the old one?

I get the problems regardless of engine temp