Thread: Having Trouble With Idle

Okay, this has been puzzling me for a few weeks. I will go into detail over everything I've done so far. I'm not sure what the problem is. PLEASE READ ENTIRE POST

1994 180sx with a built SR20DET redtop swap. Top mount, big turbo, Apexi FC, aeromotive 340lph, 550cc injectors, etc

When I first start the car from cold it starts up no problem and runs excellent. No noises, no issues. If I wait about a minute, OR I try to give it some gas the engine will bog and start to run like crap. '
If I try to apply consistent throttle it will rev up but it sounds like an ignition cut rev limiter almost.

I've checked the fuel pressure, tested the MAF, tested the CTS, tested & replaced the TPS, checked the plugs, checked the coils.

I found the TPS sensor to be bad and reading 5V all the time (dumping fuel, bogging, etc) so I replaced that to a working unit. Sypmtoms have improved but not much.

There is currently no o2 sensor plugged in. In my experience this would not be caused by a faulty/non existed o2 sensor.

I honestly have no idea what could be causing this right now especially cause when it first starts it runs perfectly and then starts to get messed up.

Any ideas? TIA!

It sounds like it tries to enter closed-loop mode and the lack of O2 sensor is the exact issue here. I doubt any of us is going to be able to help you until you replace that O2 sensor.

You can probably confirm it's the lack of O2 sensor by driving the car and see if it completely "wakes up" above ~5k RPM.

Okay i will try what youve suggested tomorrow. I will also get an o2 sensor. The sensor i pulled out of the downpipe had 4 wires and the stock sr plug only has 3 wires? Hmm

So, I put a brand new o2 sensor in and the symptoms are still exactly the same.

Can I assume you're using the stock ECU, and the Apexi FC to be able to run those larger 550cc injectors?

Did you get the correct O2 sensor for the ECU? It sounds like you need a 3-wire O2 sensor?


This might not be ideal, but your next step might be to remove the Apexi FC and install stock injectors? Or otherwise test the Apexi FC to see if it's working properly. I've never trusted those things...
I'm pretty sure your issue is fuel related.

The Power FC came with the swap. I bought it from a friend who had bought it from a friend. I'm going to double check that they are actually 550cc injectors right away.
EDIT:
Just checked. The injectors after all are the stock 370cc ones.

I bought an O2 sensor for a 98 240sx (had the 3 wire plug) and it plugged right in, I just had to extend the wires. The weird thing is, when I go on the powerFC controller it is giving me no reading for the o2.

Just curious, why do you say fuel? What I find strange is that for the first bit on a cold start it runs just as it should. Idles well, sounds great, exhaust doesn't smell too rich, etc.

What I'm thinking is that you're right about closed loop... when it tries to enter closed it starts to run like shit (overly rich, stumbling idle, revs up weird, but it DOES "wake up" around 5k rpm). Which would lead to a sensor, no?

Is there possibly a way to bypass closed loop all together?? I tried to disable the o2 feedback through the powerFC controller but nothing changed.

Thank you for your help so far

Because it's starting and running great for a little bit. That tells me the compression and ignition is fine. The only thing that changes after some time is the fuel delivery, once the ECU leaves warm-up enrichment and enters closed-loop control.

Yah, the ECU goes into open-loop around 5k RPM and if it runs fine again after 5k then it's because you're back in open-loop. Yes, this would for sure point to a missing/malfunctioning O2 sensor or something else causing bad O2 readings.

Other than during cold start and above 5k RPM? I'm not really aware of any other way with the stock ECU, sorry.

I wonder what is the make of the O2 sensor? For bogging issues maybe inlet has oil, PCV is easy to check and IACV I had to clean myself and it was an unidentified IACV, also the neutral sensor not working made things funky for me (combined whith a working IACV but not the one for my SR20DE, buying a parts bin car solved that). Also do you still have that detonation pot, I removed mine and also exchanged my dried rubber hoses (car was sitting in a barn 5 years), 20 deg (I know against advice), ron 95, no probs after 10k miles.

After all I wrote and a new lambda (160 coins, but I get it you don't use a single band), idle behaved normal before ECU was still revving up and down, low idle made it bog too. I should get my Datascan running again and check my timing I suppose.

If this isn't helpful I'll shut up on DET engines.

I'm thinking its electrical based on how well it runs off the hop but I will check.

Here's an update: I went over the harness again. Buddy I bought the car from had the harness from one car and the engine/trans from another. The little brown wire near the fuse box on the RHD passenger side was not connected (therefore no power to the o2 sensor). I wired this in to a switched 12V+ source and the o2 sensor is working. It has, however, made the problem worse.

Now when I start the car it will run fine but only for 5 seconds or so and then it will die out completely. There is poor throttle response but if I'm lucky it will rev up and run okay if I continue to apply throttle. I've tried to disable O2 feedback from within the Power FC controller but nothing changes.

edit: repost