98 ER34 GTT idle races, if AAC is unplugged, then idles perfect

[JC71]

98 ER34 GTT idle races, if AAC is unplugged, then idles perfect.

 

1998 ER34 GTT rb25det neo, 

rpm's race at idle, around 1200 rpms, I cleaned MAF, AAC/IACV. no change.

if I unplug the ACC 2 wire plug, it idles correctly, around 900 rpms.

 

questions:

1. will it damage anything or effect characteristics by driving the car with this AAC unplugged?

2. is this 2 wire AAC component replaceable, or must one replace entire IACV assembly?

 

appreciate any help

Edited April 15, 2024 by JC71

[GTSBoy]

The "2 wire AAC component" is just a solenoid, that opens and closes the idle control valve. It's PWM controlled by the ECU. It is hard to see how you can have the symptoms that you describe without it being caused by the valve itself being dirty and so not moving smoothly. Yes, I understand the rest of your description.

Will it hurt to have it unplugged? No. You will not have idle control so idle speed will be able to fall below stall point if and when you put too much load on it (AC on, PS turned to one side, etc).

Is it replaceable? I dunno. I think if mine ever goes that bad I will just be doing the whole ECU. Good excuse to go e-throttle and sidestep the whole idle control bullshit circus anyway.

[JC71]

I replaced the entire IACV assembly with new unit from Nissan, idle acts the exact same, starts and runs with idle rock solid at 900, as soon as I plug in the solenoid from the new IACV the idle races up to 1400, even with idle adjustment on side back of intake manifold maxed out for lowest idle still way too high around 1200.  Again if I unplug the IACV, turn idle screw to about half and it runs fine at around 900. thinking about just leaving it unplugged, do not ever plan to drive it when its cold, and always stored in climate control garage, just sucks to spend 500 dollars and have no change, could have left the original at this point. any other thoughts as to what it could be? maybe not even worth pursuing? that is if cold start idle aid was the only reason for Nissan to have this part anyway. tried AC and other loads and it never seemed to push idle below stall.

[GTSBoy]

Is there coolant flow to (through) the IACV?

[JC71]

appears to be, I loosened the bleeder on top IACV and coolant came out.

[GTSBoy]

Yeah, that doesn't necessarily mean that it flows. If the exit side pipework is blocked somewhere (in the hose, in the nipple that it returns into the motor/plumbing, etc), but the supply side is not blocked, then when you crack the bleeder you will get flow from the open side. But when you close the bleeder - no flow. And.... therefore no heat.

You might have to work a little harder to prove it to yourself. When I bought my engine the entire IACV bleeder was choked with cooling system schmutz. I had to clean it and the pipework out before it would work.

[joshuaho96]

11 hours ago, JC71 said:

I replaced the entire IACV assembly with new unit from Nissan, idle acts the exact same, starts and runs with idle rock solid at 900, as soon as I plug in the solenoid from the new IACV the idle races up to 1400, even with idle adjustment on side back of intake manifold maxed out for lowest idle still way too high around 1200.  Again if I unplug the IACV, turn idle screw to about half and it runs fine at around 900. thinking about just leaving it unplugged, do not ever plan to drive it when its cold, and always stored in climate control garage, just sucks to spend 500 dollars and have no change, could have left the original at this point. any other thoughts as to what it could be? maybe not even worth pursuing? that is if cold start idle aid was the only reason for Nissan to have this part anyway. tried AC and other loads and it never seemed to push idle below stall.

You definitely have a big problem if with the adjustment screw closed up you're idling at 1200 rpm. Have you verified you have no vacuum leaks? No throttle shaft seal leaks? Nobody has messed with the throttle body set screws? I would love to see what Consult reports when your engine is idling that high.

[Dose Pipe Sutututu]

I recall you're going to launch the OEM ECU into the bin in another post, unpopular opinion but throw the AAC, IACV, cable throttle & pedal all into the bin and convert to DBW e-throttle & pedal.

For quick install (minimum fab work) these guys have a great bit of kit:

https://outsidergarage.com/products/nissan-rb25-oem-dbw-conversion-kit

Hit up @outsidergarage for more details.

(I'm not affiliated nor do I use them)

[joshuaho96]

9 minutes ago, Dose Pipe Sutututu said:

I recall you're going to launch the OEM ECU into the bin in another post, unpopular opinion but throw the AAC, IACV, cable throttle & pedal all into the bin and convert to DBW e-throttle & pedal.

For quick install (minimum fab work) these guys have a great bit of kit:

https://outsidergarage.com/products/nissan-rb25-oem-dbw-conversion-kit

Hit up @outsidergarage for more details.

(I'm not affiliated nor do I use them)

Sure, but personally I'm a big fan of when you have a problem, don't make it harder to diagnose by changing more variables all at once.

[GTSBoy]

54 minutes ago, joshuaho96 said:

Sure, but personally I'm a big fan of when you have a problem, don't make it harder to diagnose by changing more variables all at once.

That's perhaps not even correct in this instance though. If the OP were in fact to bin all that troublesome crap and convert to DBW, and then if the problem persisted, we would at least know that it was not caused by any of that troublesome crap and he could go looking for it in more likely places. AND... he would have a significant upgrade in niceness to go along with it!

[Dose Pipe Sutututu]

Once you go non OEM DBW e-throttle, you'll never go back to a cable throttle.

Just so good in every single way.

[joshuaho96]

1 hour ago, GTSBoy said:

That's perhaps not even correct in this instance though. If the OP were in fact to bin all that troublesome crap and convert to DBW, and then if the problem persisted, we would at least know that it was not caused by any of that troublesome crap and he could go looking for it in more likely places. AND... he would have a significant upgrade in niceness to go along with it!

Sure, but you could just pull enough timing out of the map to make it idle at whatever RPM you want. Or close the throttle until it's about to bind to compensate for vacuum leaks.That's kind of why I'm not a fan of this approach, especially if OP isn't experienced at tuning these things.

[JC71]

I couldn't get this to work right, New clean IACV, coolant seems to flow and solenoid functions, but again as soon as I plug it in the idle raises, I ran the car for a good 1/2 hour messing around with various things it, no vac leaks.

At one point with the IACV plugged in I was able with the adjustment in the side of IACV turned all the way in get it idle down a little to around 1000, but no lower, and it would surge, I unplugged the IACV engine rpms drop to about 350, turned the adjustment about 3 turns in , set idle to around 800 rpms' and it idles smooth.

I set the idle stop screw abut the same so it cant drop below under load. the engine starts, restarts, with no load or with a load seems to run fine. I will have to leave it like this for now as I cant get it to function correctly with the IACV plugged in.

One other thing I discovered that might be the reason for some of the issues, the engine in the car is a RB25det neo from a Stagea along with the Stagea ECU. so alot of wiring, plumbing etc that is typically there in a R34 is not. 

[Dose Pipe Sutututu]

Has this been converted to manual by any chance?

[JC71]

no, the car was a manual 5 speed from the factory and still is.

[GTSBoy]

Spray carby cleaner around all gaskets and other possible vac leak locations on the inlet manifold. See if the revs change somewhere. If so, vac leak found.

 

Plug in Consult and see what the ECU says it is doing to the IACV. It will tell you how many "steps" it is commanding it to open. If it is not commanding it to open more than normal, then the problem is elsewhere. If the ECU is trying to drive the IACV really far open, then you know that it is either ECU cray cray or some input to it telling ti be stupid.

[Dose Pipe Sutututu]

14 hours ago, JC71 said:

One other thing I discovered that might be the reason for some of the issues, the engine in the car is a RB25det neo from a Stagea along with the Stagea ECU. so alot of wiring, plumbing etc that is typically there in a R34 is not. 

can you check if this is an auto ECU?