R32 GTS25 high idle help?

[damesta]

Hey guys, so I'm working on a new to me 91' r32 gts25, mostly stock rb25de. I'm trying to sort out all the issues and get it reliable before any modifications. I have a high idle at 1200-1300 rpm that I can't get figured out. I have been through everything I can think of, checked every vacuum line/hose for leaks, cleaned mafs, cleaned throttle body, made sure throttle cable wasn't to tight and throttle body was fully closed, checked and adjusted tps voltage to spec, cleaned the iacv and then swapped a new iacv on. None of this makes a difference, even with the new iacv screw cranked all the way down the car idles at 1200rpm. I can unplug the brown plug on the iacv solenoid and get the idle adjusted down to a normal level at 800-900, but when I plug it back in the motor revs itself up to around 2500rpm and then settles back down to around 1200rpm. So it seems to me that the ecu is bumping the idle up for some reason. Trying to adjust the idle on the ecu has no effect either.

I think this was an auto to manual conversion (although I wasn't informed of this prior to purchase) and it is running the BB ecu, the only code I can pull from it is 54, which means the ecu is not seeing the auto tranny signal from what I can gather....could this be the cause of the high idle?

Thanks in advance for the help. 

Edited May 19, 2021 by damesta

[joshuaho96]

High idle shouldn't be caused by the ECU not seeing the transmission signal, but the fact that plugging in the AAC causes a major idle spike is a bad sign. At 1200 rpm is the AAC duty cycle 0-2%? 

[damesta]

I don't have a consult to read the AAC duty cycle, I pulled the code from the ecu led. It did the same thing on both aac valves I tried though, so what does that idle spike when I plug it in mean?

The other thing I haven't looked at is the coolant temp sensor, could the ecu bump the idle up if the sensor wasn't working and it thought the engine was cold?

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Edited May 19, 2021 by damesta

[blind_elk]

4 hours ago, damesta said:

... cleaned throttle body  ...

That is a common cause for the same problem in GT-Rs. (Maybe the previous owner did it)

[damesta]

1 minute ago, blind_elk said:

That is a common cause for the same problem in GT-Rs. (Maybe the previous owner did it)

What is a common cause, build up in the tb? Mine wasn’t that dirty but it made no difference when I cleaned it.

[blind_elk]

If it wasn't that dirty, then it's possible the previous owner cleaned it. And that is a recipe for disaster - and a fast idle speed! Cleaning the TB removes the material put there at manufacture to seal off the butterfly in the TB and stop the air leak that causes the fast idle.

[Ben C34]

3 hours ago, blind_elk said:

If it wasn't that dirty, then it's possible the previous owner cleaned it. And that is a recipe for disaster - and a fast idle speed! Cleaning the TB removes the material put there at manufacture to seal off the butterfly in the TB and stop the air leak that causes the fast idle.

Not for rb25s mate .it's just rb26 that use the sealant

[damesta]

4 hours ago, Ben C34 said:

Not for rb25s mate .it's just rb26 that use the sealant

Yeah, there was nothing but a little dirt in there.

So the only two ideas I have left are to look at the coolant temp sensor and then maybe the timing, I've read somewhere that the base timing is 5 degrees different between auto and manual rb25's. This is a manual conversion running on an auto ecu, so maybe the ecu is looking for different base timing and trying to compensate? I need to research that idea a little more but it doesn't really seem like it would cause it to idle at 1200-1300 all the time.

[GTSBoy]

Auto ECU idles the engine at 20°. Manual is at 15°. 20° will idle faster.

But.....the ECU appears to be controlling the speed and controlling it to be faster, not slower. If you unplug, it slows down, plug in and it speeds up. That suggests that the ECU is trying to increase idle speed (and achieving it) - not decrease it.

Just for clarity - please explain which valve you are calling the IACV. The actual idle speed control valve that has a solenoid on it? Or the other valve that lives under the plenum and is the cold start fast idle valve that is electrically heated to it is open when cold and closes up as it heats up?

 

You probably do need to get a Consult scanner onto it so you can see what the ECU thinks the speed is and how many "steps" the idle speed control output is running. And what it thinks the timing is running against what the timing light says.

[damesta]

49 minutes ago, GTSBoy said:

Auto ECU idles the engine at 20°. Manual is at 15°. 20° will idle faster.

But.....the ECU appears to be controlling the speed and controlling it to be faster, not slower. If you unplug, it slows down, plug in and it speeds up. That suggests that the ECU is trying to increase idle speed (and achieving it) - not decrease it.

Just for clarity - please explain which valve you are calling the IACV. The actual idle speed control valve that has a solenoid on it? Or the other valve that lives under the plenum and is the cold start fast idle valve that is electrically heated to it is open when cold and closes up as it heats up?

 

You probably do need to get a Consult scanner onto it so you can see what the ECU thinks the speed is and how many "steps" the idle speed control output is running. And what it thinks the timing is running against what the timing light says.

So to clarify this, are you saying that if using an auto ecu then the base timing on a r32 rb25de should be set to 20 degrees at the cas and if using a manual ecu on the same engine the base timing should be set to 15 degrees? Or are you just saying they should both be set to 15 but that the auto ecu will automatically advance the timing to 20 degrees at idle? I don't have an r32 rb25de fsm but on the r33 engine manual I'm looking at it calls for 15 degrees base timing on both autos and manuals....could be different on an r32 though I suppose.

When I say iacv I am talking about the complete idle air control valve assembly, mine looks identical to this ka24 valve with 2 solenoids, when I unplug the brown plug it will idle down and let me adjust the idle, when I plug it back in the idle jumps back up.

https://www.z1motorsports.com/intake-mas-and-hoses/nissan/oem-ka24de-idle-air-control-valve-iacv-aac-p-9986.html?gclid=Cj0KCQjwkZiFBhD9ARIsAGxFX8AwwUSevaQlUXJexHhG1coTcwqGE_fxA7xLS8f_ahLOk6NUct43CmEaAr_3EALw_wcB

I am not talking about the cold start valve but I did look at it already, I actually completely capped off the cold start valve at one point to test if air was getting past it causing the high idle but that made no difference either.

And that's exactly what I was thinking, the ecu is trying to increase the idle speed for some reason but I have no idea why at this point....unless its a bad coolant temp sensor or base timing issue.

Any recommendations on a good fully functioning consult cable/program to get started with?

Edited May 20, 2021 by damesta

[GTSBoy]

3 minutes ago, damesta said:

So to clarify this, are you saying that if using an auto ecu then the base timing on a r32 rb25de should be set to 20 degrees at the cas and if using a manual ecu on the same engine the base timing should be set to 15 degrees?

No. I am saying that the auto ECU tries to run the engine at 20°. That's the advance angle that is in the idle "table" in the ECU. The ECU knows where TDC is (and so knows where 0° is). And with that knowledge, it deliberately sets the idle timing to be 20°. The manual ECU runs it at 15°.

But also yes. You do need to set the CAS so that the idle timing matches what the ECU wants it to be. If you try to set the timing so that your auto ECU achieves 15° (real, on the crank pulley) at idle, then the timing will be 5° retarded everywhere, and you don't want that.

The auto ECUs run more idle timing to help the engine idle against the load of the torque converter when sat in gear with the brakes on at the lights.

10 minutes ago, damesta said:

When I say iacv I am talking about the complete idle air control valve assembly, mine looks identical to this ka24 valve with 2 solenoids, when I unplug the brown plug it will idle down and let me adjust the idle, when I plug it back in the idle jumps back up.

Ah yes. I short circuited my brain. Because you're talking about an R32 I leapt straight to the RB20 style valves - not the RB25 stuff that you actually have. The brown plug is certainly the control valve.

12 minutes ago, damesta said:

Any recommendation on a good fully functioning consult cable/program to get started with?

Nope. I have never used one. I have Nistune in my ECU so I use that, which is far superior to typical Consult software. When I said "Consult scanner" I literally meant a workshop grade Consult scanner - either an actual Nissan one, or Snap-On, or any of the other good ones that can handle most manufacturer's ECUs.

[damesta]

25 minutes ago, GTSBoy said:

But also yes. You do need to set the CAS so that the idle timing matches what the ECU wants it to be. If you try to set the timing so that your auto ECU achieves 15° (real, on the crank pulley) at idle, then the timing will be 5° retarded everywhere, and you don't want that.

So if I'm understanding correctly my base timing at the cas still needs to be set to 15 degrees at 650 rpm as the r33 fsm instructs?

I'll get to searching for a decent consult. Thanks for the help.

[damesta]

1 hour ago, GTSBoy said:

The auto ECUs run more idle timing to help the engine idle against the load of the torque converter when sat in gear with the brakes on at the lights.

This also brings up the question in my head, is it possible that everything is working correctly and I'm just chasing a non-existent problem here? Is it possible that the combination of decreased load from the lack of a torque convertor and the auto ecu idling at 20 degrees advance is causing this whole issue and bumping the idle to 1200 rpm automatically?

[joshuaho96]

5 hours ago, damesta said:

This also brings up the question in my head, is it possible that everything is working correctly and I'm just chasing a non-existent problem here? Is it possible that the combination of decreased load from the lack of a torque convertor and the auto ecu idling at 20 degrees advance is causing this whole issue and bumping the idle to 1200 rpm automatically?

it is possible, especially because you said the idle calms down when you disconnect the IACV. The torque converter has non-negligible drag even in neutral/park which is probably why base timing is advanced on the auto ECU. You may need to find some way to restrict airflow beyond what a dinky little idle screw can do. Possibly adjusting the closed throttle butterfly position, but that's a really big hammer for what could be a relatively small problem. I would only do that if you're 100% confident that everything else is correct and you're 100% confident that's the problem.

[damesta]

2 minutes ago, joshuaho96 said:

it is possible, especially because you said the idle calms down when you disconnect the IACV. The torque converter has non-negligible drag even in neutral/park which is probably why base timing is advanced on the auto ECU. You may need to find some way to restrict airflow beyond what a dinky little idle screw can do. Possibly adjusting the closed throttle butterfly position, but that's a really big hammer for what could be a relatively small problem. I would only do that if you're 100% confident that everything else is correct and you're 100% confident that's the problem.

Is there any reason not to just leave it idle at 1200 then? It’s a little high but if there’s nothing technically wrong I’ll live with it until I put a turbo on and get  it tuned.

[joshuaho96]

30 minutes ago, damesta said:

Is there any reason not to just leave it idle at 1200 then? It’s a little high but if there’s nothing technically wrong I’ll live with it until I put a turbo on and get  it tuned.

not particularly, but high idle is pretty irritating to drive around if it's bad enough.

[GTSBoy]

9 hours ago, damesta said:

So if I'm understanding correctly my base timing at the cas still needs to be set to 15 degrees at 650 rpm as the r33 fsm instructs?

No. The auto ECU wants to see 20°. If you set it to an actual 15° on the engine and the ECU believes that to be 15°, then your timing will be 5° retarded everywhere. As I said in previous post.

7 hours ago, damesta said:

is it possible that everything is working correctly and I'm just chasing a non-existent problem here? Is it possible that the combination of decreased load from the lack of a torque convertor and the auto ecu idling at 20 degrees advance is causing this whole issue and bumping the idle to 1200 rpm automatically?

No. The ECU "has" control over the idle speed. You can see that when you unplug the IACV the revs drop - meaning that that valve is closing down. When you plug it in, the speed increases. meaning that the ECU is driving it open. The 1200 rpm is a consequence of ECU action.

If, however, the ECU had good control of the idle speed, then it would be driving it down to 650 rpm, not bumping it up. It is either bumping it up because there is an input (like air-con idle up, or some other thing) or a fault in the ECU/wiring that is putting more (PWM) voltage onto the IACV than is warranted by the idle speed.

Here's another thing. I don't know if R32 RB25 ECUs have it, but delta idle speed control is a thing. If the idle is stubbornly high (ie, it won't come down because the IACV is not doing anything/blocked) then the ECU will reduce timing to pull the idle speed down. Certainly happens on R33/4 RB25 ECUs. For this reason there are things you have to do to ensure that the ECU is not trying to adjust the idle timing while you are trying to set the base timing. I'm not suggesting that this is what is happening in your case, but it is another excellent reason to look at the ECU via Consult to find out what idle timing target it is working to. It may be all f**ked up (ie faulty tacho signal) and think that the rpm is different to what it actually is and be trying to speed it up, for all we know.

2 hours ago, damesta said:

Is there any reason not to just leave it idle at 1200 then?

It's ridiculously high. I changed the idle speed setpoint down to 600 rpm in my Neo ECU (Nistune, remember) and saved an appreciable amount of idle fuel consumption, which I see in my actual fuel bill, because I drive through thick traffic on my daily 50km commute. 1200 rpm will absolutely pig the fuel.

[joshuaho96]

4 hours ago, GTSBoy said:

No. The auto ECU wants to see 20°. If you set it to an actual 15° on the engine and the ECU believes that to be 15°, then your timing will be 5° retarded everywhere. As I said in previous post.

No. The ECU "has" control over the idle speed. You can see that when you unplug the IACV the revs drop - meaning that that valve is closing down. When you plug it in, the speed increases. meaning that the ECU is driving it open. The 1200 rpm is a consequence of ECU action.

If, however, the ECU had good control of the idle speed, then it would be driving it down to 650 rpm, not bumping it up. It is either bumping it up because there is an input (like air-con idle up, or some other thing) or a fault in the ECU/wiring that is putting more (PWM) voltage onto the IACV than is warranted by the idle speed.

Here's another thing. I don't know if R32 RB25 ECUs have it, but delta idle speed control is a thing. If the idle is stubbornly high (ie, it won't come down because the IACV is not doing anything/blocked) then the ECU will reduce timing to pull the idle speed down. Certainly happens on R33/4 RB25 ECUs. For this reason there are things you have to do to ensure that the ECU is not trying to adjust the idle timing while you are trying to set the base timing. I'm not suggesting that this is what is happening in your case, but it is another excellent reason to look at the ECU via Consult to find out what idle timing target it is working to. It may be all f**ked up (ie faulty tacho signal) and think that the rpm is different to what it actually is and be trying to speed it up, for all we know.

It's ridiculously high. I changed the idle speed setpoint down to 600 rpm in my Neo ECU (Nistune, remember) and saved an appreciable amount of idle fuel consumption, which I see in my actual fuel bill, because I drive through thick traffic on my daily 50km commute. 1200 rpm will absolutely pig the fuel.

Is it even possible to get down to 600 rpm in most cases without messing with the throttle stop? My R33 is idling around 1000 rpm right now even with the AAC duty cycle reporting 0-2 in consult under most conditions.

[GTSBoy]

4 hours ago, joshuaho96 said:

Is it even possible to get down to 600 rpm in most cases without messing with the throttle stop? My R33 is idling around 1000 rpm right now even with the AAC duty cycle reporting 0-2 in consult under most conditions.

I can set mine down until it stalls. Just keep typing in smaller numbers.

[GTSBoy]

9 hours ago, GTSBoy said:

If you set it to an actual 15° on the engine and the ECU believes that to be 15°, then your timing will be 5° retarded everywhere.

Oops. Correct myself. I meant "if the ECU believes that to be 20°".