Why won't this car idle right?

[CMF_Red Menace]

I can't get my car to idle properly from cold. Typically, it idles very high for some time, then it grudgingly settles to the base idle, but tapping the throttle brings the high idle back until the car is thoroughly warm. And it's not just slightly high, it's astronomical. So far:

- Replaced head gasket (car overheated in traffic)
- Replaced lambda sensor
- Removed idle air control valve, cleaned it, stared at it meaningfully for a while
- Stripped and inspected cooling system, new thermostat
- Replaced throttle body (with one from a scrapyard, condition unknown)

Nothing makes any difference! After I replaced the throttle body, it actually got worse. I noticed the following:

- Idle started high (it's cold and wet here), then settled down a little. I blipped the throttle, idle went back up and stayed high (over 1000rpm, no tacho).

I pumped the brake to cycle the vacuum servo; initially, pumping this brought the idle down as expected. Idle stayed low for a little while, then either it rose, or I blipped the throttle and it rose - I can't remember which but the effect was the same. Thereafter, it continued to climb; pumping the brake had no effect. I switched off.

Ideas would be gratefully received; this one's doing my head in! Could it be a vacuum leak perhaps?

 

[CMF_frank2]

red menace

have you tried procedure in the haynes where you disconnect the TPS, set the idle, then reconnect it ?

frank

 

[CMF_Red Menace]

Realised I'd not done that since fitting the new throttle body. Unplugged the TPS - and the engine just went ballistic. I'm not sure it's ever idled that high before. I screwed the idle adjust all the way in, but it's still roaring.

At the next opportunity that isn't wildly antisocial, I shall start the engine and then block the intake. It wouldn't be beyond my capabilities to have accidentally caused a massive vacuum leak somewhere and then to be too stupid to find it.

 

[CMF_frank2]

Red Menace WROTE:

"Realised I'd not done that since fitting the new throttle body. Unplugged the TPS - and the engine just went ballistic. I'm not sure it's ever idled that high before. I screwed the idle adjust all the way in, but it's still roaring.

was that with the engine cold ?
the waxstat holds the throttle open when cold eh.
haynes says to adjust the idle with the black plastic screw on the rear corner, but that is an air bleed screw (which bypasses the butterfly)

frank

 

[CMF_Red Menace]

That question reminded me that I might well be doing something stupid - and so it proved. I hadn't adjusted the throttle cable, and to my shame it was cracking the throttle open. Fixed that, and now the idle is incredibly low (aforementioned air bleed screw all the way in). Going to set it again, then we'll see where we are... Thanks for the pointer.

<edit>...and it idles beautifully! Thanks very much, Frank!

It seems that one of the most common Micra faults has taken me months to find and sort... never mind, I've learnt a lot about the electronics of this car in the process.

 

[CMF_BigJohn]

So when you adjust the idle at the rear corner screw, do you disconnect the TPS while the engine is running and re-connect it while the engine is running also?

 

[CMF_frank2]

BigJohn WROTE:

"So when you adjust the idle at the rear corner screw, do you disconnect the TPS while the engine is running and re-connect it while the engine is running also?

it just triggers "idle mode" in the ecu john, because the ecu also uses timing adjustments to control the idle if the i/c/v is fully in or out
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