I've still got a cold idle problem after having had half the engine bay out in recent weeks. The running has got a lot smoother after changing the turbo inlet pipe, air filter, ISCV/TB/inlet manifold gaskets and generally tidying and cleaning everything up. But still it won't settle to a decent idle when cold and is prone to stalling.

There are any number of sensors that could be causing this so here's a bit of info to get you started on the diagnosis:
MY98 WRX
120k miles
Blue 'F' label MAF
Power FC with Commander, mapped for 1.4 bar with VF35/440cc injectors
ISCV just cleaned out. Lambda replaced recently.
TPS voltage range is fine (0.45 to 4.3v).
O2 signal looks fine (0-1v).
Not sure about what coolant sensor voltages to look for, but the temps read out fine on the Commander, and show normal on the dashboard water temp meter.
Sensor check function on Commander shows AF1 at 0.22v with engine off/ignition on, 1.25v when idling around 900 rpm. From memory this doesn't seem right, although I can't be sure. IIRC the idle voltage used to be more like 0.9-1.1 v. Shouldn't the 'off' voltage by 0v?
There is also a BST1 readout of 2.34v with engine off, 1.32 v at a 900 rpm idle. No idea whether this is correct or not, presumably it's a boost pressure sensor?
AIRT (air temp?) has always been steady at 4.88v ever since it was mapped, presumably because my WRX doesn't have an air temp sensor as standard therefore it's just a dummy voltage that represents an average ambient temp. I wonder whether the Power FC expects a real sensor to be hooked up to this input, that in this cold weather its basically causing the dodgy idle - i.e. water temp is showing as cold, air temp is showing as warm, so it just gives up and oscillates the revs back and forth?

And lastly, being a MY98 MAF, it's the expensive one and I can't take a chance on buying a £300 sensor from Subaru only to find that it still won't idle. It looks like loads of Nissan MAFs are very similar and can be had for a lot less. Is there a Nissan equivalent to a blue label 'F' MAF from a MY98? And if the MAF does need replacing, what's the best option - a straight replacement from Subaru (it's recently been hitting 4.996v at 1.55 bar with my new inlet pipe/filter/cold weather), or a Nissan Z32 and just get it all remapped again?

Cheers!

And btw yes I have done a search and nowhere does anyone just come out with a straight answer of what the MAF voltage should be with the engine off.

 

Hi can't help to much but i got a purple expensive maf for a MY98 from Glenburgie breakers for about £120 delivered but be prepared to wait as it took about 10 days to arrive.

 

The easiest solution, rather than post lengthy detailed posts on here, is to phone me, as I have datalogs of exactly what your car should be doing WRT maf voltage at idle etc.

If your maf was badly damaged or contaminated I doubt it would run very well at all, and the problems would be worse on boost.

With your new inlet pipe, I would also say you need to adjust your boost control values as you're overboosting rather a lot. Your max MAF voltage sounds about right though for the boost I imagine is being held, and the cold weather we are having right now. The idle voltage was around 1 volt in moderate weather, but cold weather will have an effect. At low voltages, the voltage change results in much smaller changes in measured airflow than near 5 volts. The voltage with the engine off will not be zero.

It could be that there was a small split in your inlet pipe when is was mapped, and after running lean when th esplit initially grew, it now runs rich with the fixed pipe.

What RPM does the idle oscillate between?
What other changes have you made since mapping (no matter how slight)?

 

Hi Paul, yes I would give you a ring but I tend to be really busy during the day (as I know you are) and I can type very fast, hence why my posts are quite long, and I do most of my problem solving in the evening and at weekends. :-)

I'm really pleased with the warm idle now, it's rock solid and better than it's ever been, but the cold idle still fluctuates between around 600 and 900 rpm. This only seems to happen for a little while, until the water temp gets over roughly 20 C then it seems to settle down, although whilst warming up it still seems to struggle to return to a proper idle if you blip it - i.e. let off the throttle and it overshoots right down to 500 rpm/almost stalling before coming back up to a proper idle. The MAF voltage now looks right as do all the other sensors when I test them, even the IACV which is showing 8.5 ohms each way. I think I was looking at the airflow reading on a fast idle at 900 rpm, where it was 1.2-1.25v. On a normal warm steady idle it's showing roughly 1v consistently. It won't start without the MAF plugged in.

Engine work I've done since the last mapping you did for me:
Gaskets replaced for inlet manifold/throttle body/idle valve
Speed sensor replaced (bouncing needle problem)
Split/loose inlet pipe replaced for silicone
STi panel filter replaced with Green
Swapped PFR6B plugs for 7's
One plug not screwed right in
Leaking rocker cover bolt sealed up properly
Oil and filter changed
Cleaned out slight oil film in IC/TB
Compression test (all cylinders 155-170 psi)
Matched inlet manifold ports to gaskets, ported throttle body a little

I've set the boost to the 1.2 setting for the time being, and the max overboost I'm now seeing is 1.3.

 

Today I've adjusted the duty cycles for the higher boost settings of 1.2 and 1.4 bar - down from 65 and 82 respectively to 61 and 74. The highest overboost I've seen so far is 1.44 and it's pretty cold out.

 

For future reference if anyone does a search on this, the problem has been totally fixed by a replacement idle valve. Idle is now rock solid, starts off around 1100 rpm when the engine is stone cold, gradually lessens as you get up to temperature until it holds at 750 when water temp is up above 70 degrees.

Blipping the throttle is no problem at all while warming up, it will return perfectly to the proper idle speed without overshooting or getting close to stalling.

It seemed to struggle to find its level during the first warming up cycle with the replacement idle valve. After this it settled down to perfect functionality. This was using a Power FC, not the standard ECU.

 

Hello silent. My car (98 STi) has also developed an unsteady idle when cold. Additional to this, the dash coolant temp guage doesn't seem to work unless the engine is running a bit hotter than normal. For example, if I drive it hard or leave it idling whilst stationary, the needle will go up to the 'normal' position. If I cruise steadily the needle will then dip down to on or below the 'C' marker. It didn't used to do this i.e. it used to work as per normal.

I'm not hijacking your thread (I don't think ) but could you tell me where the idle control valve is, how much they cost new and if it's simple enough to fit as I'm not too mechanically adept and I wondered if anyone had any ideas about the temp guage? Many thanks

 

silent running, i am no expert but the problem you had with cold idle may have been the bi-metal strip in the icv itself which expands and retracts with water temp hence the two water pipes connected to the valve and that might have been the reason for bad cold idle and good warm idle. just a thought, bit late now though i know