Another idle issue

[bobbo]

I did some reading but couldn't put anything together from what I found. Here's the situation.
I had the car tuned by Robin at Almasi and everything worked great.
I drove the car part of the day friday (normal city/hwy) with no issue.
Saturday I was at the track, I did two sessions with no problem and on the third ran dangerously low on fuel and when I got back to the pits I had a hunting idle.
I fuelled up and ran the car again with everything running great off idle but still a hunting idle with a A/F around 17:1.
As soon as I turn on the A/C the idle is perfect with a perfect A/F.
I drove the car home from Quebec and got an easy 27 MPG.
Thoughts?
Thanks!

[SkylineUSA]

Hate to say, MAFs? Have you cleaned them lately? Man, that is running lean.

[MarusGTR]

My first thought when you had mentionned the idling was the MAFs too (I find myself cleaning them each year, they blacken pretty easily) or the BOVs; with your new setup I've been wondering how your BOVs were holding up.

Since you say you're running normal idle, is that normal RPM or just steady idle? I'm also wondering if idle control affects the fuelpump modulation?

[SkylineUSA]

Marus92, I never heard of idle control affecting fuelpump modulation? Can you explain that a little?

[MarusGTR]

Just a random thought since the fuel pump voltage is modulated through a relay by the ECU, you don't get the full volumetric capacity of the pump (at 12V iirc?) unless at WOT. Probably should be disregarded, as it's not something I'm too familiar with.

[bobbo]

Still stock recirc valves. Stock FP and we confirmed its working well. MAFs were cleaned at the beginning of the season.
I forgot to mention earlier but I've got some funny readings on my rear o2 sensor. It never seems to read the same as the front one. Sometimes "0" but wouldn't explain why it idles perfectly with AC on.

[Skym]

0v = very lean. Unplug both o2 sensors and see if it runs richer (ECU goes into open loop). But it's not a good idea if have leaking intake or exhaust manifolds.

Usually 17 A/F ratio can be due to leaking intake, exhaust manifolds, so check them for leaks (soot deposits, etc).

[bobbo]

0v = very lean. Unplug both o2 sensors and see if it runs richer (ECU goes into open loop). But it's not a good idea if have leaking intake or exhaust manifolds.

Usually 17 A/F ratio can be due to leaking intake, exhaust manifolds, so check them for leaks (soot deposits, etc).

If that were the case wouldn't the idle still be lean with the AC on? As soon as I turn on the AC it idles perfectly.
Edit:
I started the car and it warms up at normal ratios (~13.5) and as it gets warmer it gets leaner. It's not until it reaches 80 degrees that the hunting starts.
I also tried unplugging the oxygen sensors but it didn't appear to make any difference. I suspected that would be the case as I tried shutting o2 feedback off with the PFC.

[bobbo]

I'm pretty sure I have the problem solved. I found a kinked vacuum line under the intake manifold. O2 values still aren't what I'd expect but at least they're both roughly the same and my wideband is reading a steady 14.8-15.1:1.
Thanks for the input!

[Skym]

You're welcome, good to hear.

[bobbo]

It appears as though I was wrong. I didn't fix the effing problem.
I'm going to check the water temp sensor. That may be a culprit. It's just very strange that the car would run perfectly and then after a run stop wanting to idle.
To be fair I did virtually run it out of fuel and MIGHT have leaned it out. I didn't cook anything, it's not smoking or loosing oil so I didn't melt a cylinder. After 3000 kms and about 6 hours of lapping the car used less than 1/2 a liter.
Any more thoughts would be appreciated.

[Skym]

That coolant temp sensor should pop up as a faulty sensor on hand controller. Can check at coolant temp sensor itself with multimeter (see manual for voltage specs).

Also check TPS setting, as TPS might not be on idle contact. The TPS does fail, as been through 2x TPS on my car and it did make engine randomly stall.

If aircon is on, voltage drops, but PowerFC adjust's to suit. So might want to check battery voltage settings on PowerFC ECU (need tuning software to access it) to see if it's compensating by running engine lean instead of richer.

[bobbo]

I am getting idle sensor contact. Shows on PFC hand controller. TPS is set to .5V.

I'm not having any issues with stalling. But will check the voltage using the PFC software.

My big concern is why it changed after a hard run.

[Skym]

Could be higher intake temps (settings on PowerFC are incorrect), as a GTR should have a IAT sensor.

[bobbo]

Could be higher intake temps (settings on PowerFC are incorrect), as a GTR should have a IAT sensor.

I would think if that were the case it wouldn't be consistent. Also we were seeing up to 56 degrees during the tune. Here I don't see more than 54 and those are under boost. I would never see more than 36 at idle.
Appreciate the input though.