Help needed diagnosing a hanging idle

[grannyknot]

1973 510 /Nissan KA24DE swap, 4cyl, 2.4L, fuel injected, MAF controlled, stock ECU, freshly rebuilt.

Finally got the engine running consistently, idle is about 900rpm, it's suppose to be 750 but can't seem to get it lower without it starting to stumble. Advance at idle is 19 degrees right where it is suppose to be but when I blip the throttle the revs go up to 1200-1300 and hang there for 4 seconds or so before backing down to 900. The throttle cable isn't hanging up anywhere but the revs are. Does that sound like a recognizable symptom of something?

Thanks

[Namerow]

I wonder if it's just a programmed-in emissions thing specific to manual transmissions.  My car (2006 GM product with a manual transmission) does the same thing.  The hang period is similar -- a few seconds.  Really annoying.

[Patcon]

What is manifold vacuum?

How is the throttle slam (for lack of a better word) controlled. On the EFI Z's it's a dash pot. How is it handled on the KA? I suspect it's all programming, but my first suspect would be air leaks

[Reptoid Overlords]

Here's some guys with S13's that seems to have the same problem.  Seems like they are leaning towards the ECU being from an automatic car used in a manual transmission application.

https://zilvia.net/f/showthread.php?t=533340

I remembered stumbling on it while troubleshooting a low/rough idle  on my D21 pickup years ago, but the single cam and dual cam intakes and throttle bodies are different.  Turned out to be crud on my temperature sender.

[Zed Head]

Have you confirmed proper fuel pressure?  The stumbling as you lower air flow (adjust the idle screw) suggests that you're low on fuel.  Might be pressure.

Reptoid's link suggests that timing is being controlled by the ECU to control idle speed since that's the only thing it can control electronically.  You might put a timing light on it and see what it's doing when it's hanging.  

What year KA?  What type of engine control?

[grannyknot]

7 hours ago, Patcon said:

What is manifold vacuum?

How is the throttle slam (for lack of a better word) controlled. On the EFI Z's it's a dash pot. How is it handled on the KA? I suspect it's all programming, but my first suspect would be air leaks

Good question, I'll check the vacuum tomorrow.  The throttle return is handled by a spring mounted on the side of the TB. When I blip it I'm turning the throttle cable rocker by hand so I know the butterfly is closed,  I have been looking for air leaks but haven't found any yet.

6 hours ago, Reptoid Overlords said:

Here's some guys with S13's that seems to have the same problem.  Seems like they are leaning towards the ECU being from an automatic car used in a manual transmission application.

https://zilvia.net/f/showthread.php?t=533340

I remembered stumbling on it while troubleshooting a low/rough idle  on my D21 pickup years ago, but the single cam and dual cam intakes and throttle bodies are different.  Turned out to be crud on my temperature sender.

Interesting, seems like it is a problem others are having. No idea if it came from an Auto car or not, I'll see if I can contact this kruked fella and send him a pic of my unit.

3 hours ago, Zed Head said:

Have you confirmed proper fuel pressure?  The stumbling as you lower air flow (adjust the idle screw) suggests that you're low on fuel.  Might be pressure.

Reptoid's link suggests that timing is being controlled by the ECU to control idle speed since that's the only thing it can control electronically.  You might put a timing light on it and see what it's doing when it's hanging.  

What year KA?  What type of engine control?

Normal fuel pressure is 38psi and that is pretty much where it is right now, as the throttle opens up wide the pressure goes up to 43psi. Good idea, I will put the timming light on while it is hanging.  The engine and ECU are from a 1991 240SX.

[grannyknot]

Okay did some more tweaking today, worked the idle down to 800rpm, brought the advance up to 20, the idle is still hanging for a few seconds and when it does the advance goes up to 30-35

Vacuum gauge is reading 19-20 in. at 800 rpm. Everything is healthy so I guess I'll have to live with it for the time being.

Thanks all.

[Patcon]

Vacuum looks good. I wonder if you can program it out?

[Zed Head]

I have the Nissan 91-94 240SX FSM if you want to use.  I'd have to upload it.  Got it from ??? before Nissan shut them down.  Describes the basics of the ECM timing control.  Might be your IACV. 

Mike might want to consider Nissan copyright stuff, not sure why they shut Xenon down.  I could put it on a Google drive too.

Edited July 18, 2021 by Zed Head

[Zed Head]

Actually it was Nicoclub and it looks like they resolved their issues with Nissan.  FSM's are back up.

https://www.nicoclub.com/service-manual?fsm=240sx%2F1991%2F1991_240sx.pdf

Here's the main list - https://www.nicoclub.com/nissan-service-manuals

Edited July 18, 2021 by Zed Head

[grannyknot]

Yes NICOclub download section has been a great help to me putting this engine system together.

3 hours ago, Patcon said:

Vacuum looks good. I wonder if you can program it out?

The ECU is very basic, you can read all 8 fault codes from a flashing led on the back of the unit.

[grannyknot]

On 7/17/2021 at 6:10 PM, grannyknot said:

No idea if it came from an Auto car or not, I'll see if I can contact this kruked fella and send him a pic of my unit.

I contacted kruked at the Zilvia forum, sent him a pic of the circuit board and he confirmed the ECU is for a manual car so my next move is to replace the Coolant Temp Sender and see what that does.  Apparently some of these engines hang on idle and some don't.