Higher than usual idle when cold

[Rochester]

H-Dev Stage 2, Auto transmission.

When starting the car from cold these last few weeks she's been boosting cold idle, as designed obviously, save that instead of boosting it to 1200 odd rpm its going up to 1400 / 1500. This can cause the odd stall when coming to a halt - presumably from torque converter drag.

Once warmed up she's running just fine and as normal, save that static idle with lights off etc is now more in the region of 900 - 950. It used to be pretty solidly nailed to 850.

Is this a symptom / indicator of something going wrong / sensor starting to malfunction or other problem anyone else has ever encountered ? Water temperature sensor or other sensor problem, AAC valve or..... ?

If so then I'd be grateful for any advice.

[stacky]

Has your lambda sensor ever been changed?

[Jonny Wilkinson]

Have you taken it for a good run and got it nice and hot recently ?

[Rochester]

Lambda changed two or three times over the years, present beastie OEM Nissan and working fine.

Has a good run up to full temperature and up to 15psi boost not less than eight times a week back and forwards to work.

Only substantial thing changed recently is the Head and Head Gasket. Haven't disconnected / cleared ECU since. Is that perhaps it ? New head perhaps works better than the old one ?

Suspect its time I changed my Water Temp sensor for a new one in any event, just for the peace of mind thing. It's done 140K so it certainly owes me nothing....

[Rochester]

Quick thread update, the problem was that my idle valve / AAC assembly housing was full of black crud. Cleaned out, did the screw thing, back on with the AAC plug and run perfectly ever since. Until now... Idling at 1400. Still clean as a whistle inside and AAC valve good, so its something else... *sigh*

[Jonny Wilkinson]

What have you tried, Evan ? Does the revs change when you disconnect the electrics ? Have you been messing in the vacinity ?

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[Rochester]

Hi Jonny. 'tis an odd one.

All was running well, idling on the button, when I found a split in the pipe that runs from the T join with the cold start boost thingy under the throttle body up and around through the inlet manifold to the AAC valve body.

Replaced the pipe, and then the trouble started - when you would have thought it would have been the other way round !

Disconnect the AAC valve connector and it sets perfectly on the screw at 800 - 850, put the plug back on and it goes up to 1350 - 1400 and won't settle lower. Not the ECU or chip, swapped over to my spare and back and it does it with both ECUs and both Horsham Stage 2 and OEM original chips.

No fault codes on ECU either.

So far I am left with possibly either the water temperature sensor having failed, so the ECU thinks the engine is cold, or the cold start boost solenoid sticking open and letting in too much air. But then neither of these have been fiddled with or in any way affected by simply changing a rubber pipe. I also swapped over the AAC solenoid for the spare you let me have - for which many grateful thanks ! - but no difference. Both valves were and are obviously working, as otherwise putting the plug on wouldn't have done anything.

Quite mystified at the moment. New temperature sensor on its way care of good old Conceptua, and if anyone has a spare working cold start boost thingy (PM if you do plz ?) I'll swap that over as well. If none of the above then its time to work my way through the wearisome list of possible causes of high idle in the manual. Have checked all the other pipes for splits and no sign of any leaks, plus fitted new gaskets to the IIAC assembly when refitting.

Any ideas ?

PS. In the meantime is it better to drive the car with the AAC plug off and idling normally, or is it essential it is connected or summat else will be damaged and add to my woes ?

[Jonny Wilkinson]

Have you checked the wires to the AAC with a multimeter ?

Odds on its something you've disturbed while changing the pipe.

Also check the wiring to the ETS. It may not be the sensor.

[Jonny Wilkinson]

Also, and this is a long shot, check the breather pipes and PCV are clear of mayo as it builds up over time and I'm just wondering if the leaky pipe was compensating for the air that is supposed to draw through the PCV at idle.

[autosri]

Sounds like a vac leak maybe one of the inlet manifold gaskets could have been damaged during the head gasket change try smoke testing or boost leak test

[Rochester]

Sounds like a vac leak maybe one of the inlet manifold gaskets could have been damaged during the head gasket change try smoke testing or boost leak test

I can't see it myself, though I will have another check when I finally have the time to get back to the car on Monday. This work thing, really interferes with the real business of life, ie. car maintenance. Will re-check but...

The weird thing is that it was idling perfectly on base and with AAC connected when there definitely was a leak, ie. the split in the pipe to the idle control valve which I repaired. Why and how it was idling perfectly when there was a split I can't fathom, nor why fixing it should have resulted in high idle !

If it were a vac leak then I would expect it to continue to affect the idle whether or not the AAC valve is connected or not. The fact is that she is idling perfectly and dead stable at 850 on the manual set screw with the AAC valve disconnected. It is connecting the AAC valve that is then bunging the idle up to 1400 odd.

Which means, if I follow the chain of logic correctly, that it is the signal from the ECU to the AAC valve which is causing the high idle. Assuming that neither the AAC valve or the ECU or chip is defective (and all three have been swapped for double checking purposes) then it must be some signal being sent to the ECU which is erroneous and therefore causing it to generate high idle.

In passing Jonny the job I was doing on the car when I found the split pipe was to swap over the Alternator for your replacement, which is working beautifully thank you. Old one caused some head scratching as intermittently failing, sometimes charging fine, then running the battery down and then back to fine. Wasted a few quid replacing the battery before confirming that it was the Alternator. On the other hand a new battery at this coming time of year never really goes amiss in the winter peace of mind stakes. One less worry.

Anyway, that is the job I was doing, which included taking the intercooler to throttle body piping off for easier access around and down to get at the Alternator. That included taking off the the piping to the T piece, cold idle booster and up to the metal pipe through the inlet manifold to idle control valve body. That's when I found the split in the pipe.

Alternator changed, pipe replaced and all back in place. Then high idle. And as its plainly an erroneous signal to the ECU, hence my two principal suspects are the engine temperature sender and the cold idle booster valve.

Otherwise, if swapping those in / out for new / known working replacements, I am stumped

PS. In the meantime is it safe to drive around with the AAC valve disconnected ? or should I keep it connected and put up with the high idle ?

Also, and this is a long shot, check the breather pipes and PCV are clear of mayo as it builds up over time and I'm just wondering if the leaky pipe was compensating for the air that is supposed to draw through the PCV at idle.

Pretty sure everything is clear, no mayo in my engine any more thank you very much, but I'll give it a go when I can get back to it on Monday. Not got a cold idle boost valve thingy hidden in your (dwindling) forest of supplies by any chance have you ? 'twas just luck that I took you up on the spare AAC valve back in June. Thanks as ever for your help

[pointz]

Got the plugs on correctly? Easily mix up aac connector with other connector.

[Rochester]

Got the plugs on correctly? Easily mix up aac connector with other connector.

Skyshack has already pointed out the possibility but I'm pretty sure that it hasn't been, as I have long since disconnected and blanked off the other plug as unnecessary (only needed for an air conditioning equipped car as I understand it) and the plug wasn't budged or removed in the course of changing the Alternator etc. But will double double check again on Monday when I next have the time to play with her. Thanks for the input and trust all is well at your end, Cheers

[Jonny Wilkinson]

I don't have a spare cold idle boost I'm afraid. It might be worth checking that the tps wiring is good and that the butterfly is closing properly on the throttle body as you have been working round that. I don't hold it much hope on that one as the disconnected idle is fine but I do have a spare TPS of you need it.

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[skyshack]

The cold air "boost" device under the throttle AKA air regulator is a bi-metal strip that closes a shutter when warm. It is powered by the fuel pump supply. The fuel pump works for 5 seconds when the ign is switched on and this won't heat the bi-metal enough to move it. The fuel pump is switched on when and while the CAS is turned. The air regulator should take about 5 min to go from fully open to fully shut when it has a supply.

Check it's plug has continuity to the fuel pump relay.

The plugs that tend to get swapped are the air reg and the AAC.

If you removed the FICD check that whatever you used to seal the FICD port in the idle air assy is still in place.

[Jonny Wilkinson]

I have gaskets for the FICD if you want them...and the throttle body.

[Rochester]

The plugs that tend to get swapped are the air reg and the AAC.

Air reg definitely hasn't been disturbed or unplugged as long as I can ever remember, so it won't be that one.

Looking at the AAC plug and the water temperature sender (two pin one to ECU, not the single pin to dash indicator) I can see why and how they might get mixed up. Both have the same profile and slot in the side corresponding to a protrusion on the socket of each in the same place. Easily interchangeable by mistake. Not a good piece of design. If it can be fitted wrong sooner or later it will be fitted wrong as the engineering variant of Sod's law goes.

Just double checking, which is which and how to check ? At present one connecter is dirty pale yellow and on the AAC, the other on the water temperature switch is dirty pale green.

Are these the correct plugs and the right way round ?

Any easy way to double check which is which with a multimeter etc ?

[Jonny Wilkinson]

My very unreliable memory suggests that the one on the aac should be green but you may want to wait for skyshack to come along and give chapter and verse

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[skyshack]

Yellow is the temp sensor.
Pale green is the AAC.
Pale blue is the air reg.
No 1 injector is grey.

Yellow taped injector is no 2. Green is no 3.

[Rochester]

Yellow is the temp sensor.
Pale green is the AAC.

Ooops. Swappetty swappetty time I think.

Oddly enough with the plugs the wrong way round miles per gallon have been really good !

Where is the numpty icon when you need one...

Thanks guys.