S14 SR20 SAFC delete / Nistune install doesn't run

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Locus2779
Posts: 2
Joined: Thu Apr 26, 2018 11:24 am
Car: 1996 240SX w/ notchtop swap
Megan Racing full suspension build
Autocrossing on big fat ho-hos

Post

Okay folks, my brain's about done with this one. I've been running a zenki sr20 on an SAFC2 for years. I recently upgraded to 740cc injectors/ mazworx rail and the SAFC just can't cope. I decided to go to a Nistune board, cause I've read all good things about them. Should've been a simple swap, little soldering. Nope. I cracked open my ECU and found a cracked diode that failed and burned out a trace on pin 35 (park/neutral switch). It was obviously old and hadn't been keeping the car from running, replace the diode, jump the trace, NBD. I installed the Nistune board per instructions, dc'd the MAF sensor wire from the SAFC and plugged it back in to the ECU. Engine cranks but no fire. I popped out a plug, and I have spark, no fuel. The pump is running and the line is pressurized, so something's up with the injectors. I uninstalled the Nistune, reinstalled the SAFC, and it fired right up. At this point I thought maybe the diode going took out something further in the ECU, so I grabbed one off of eBay. It arrived yesterday and it was already modded w/ an old rom tune board, and "certified" in working condition. I swapped the nistune into it, threw it in the car, same thing. Spark, no fuel, no fire. I changed the jumper back to let it run on the internal map, plugged the SAFC back up, and it fired right up. I checked the wiring on the MAF, and that's all correct, and I have good continuity from the MAF to the ECU. Everything else from the SAFC is piggybacked off the harness and doesn't interrupt it. Original swap wiring was done by a now-defunct tuning shop because I didn't have the skills or knowledge at the time. I do now, I think.

Here's my setup:
Zenki sr20det
Zenki ECU (69F00) x2
z32 MAF
Apexi S-AFC2 (tuned great on the old 370cc injectors, not so much on the new ones)
Nistune board 3, imaged for z32 MAF & 740cc injectors
Mazworx rail, ISR FPR
Deatchworks 740cc injectors
AEM UEGO
intake/exhaust/intercooler/etc.

Side notes: Something in all my mucking about took out my tach, so I have to hunt that down. The SAFC has good RPM numbers, so it's something between the ECU and the cluster. This is the second tach I've put in the car, the first one was intermittent for a while and then completely died. I suspect more wiring issues. Also, the consult port in the car doesn't seem to have been wired right, it was backfeeding into the consult cable I bought and making my laptop reboot. I'm going to wire directly into the engine harness per Nistune's instructions.

So has anyone come up against something like this before? I'm reading through all the electrical diagrams today and I'll start checking through to make sure stuff is going to the right place. I'm mentally prepared to have to rewire significant parts of the car, but I'm really not looking forward to it. Would I be completely nuts to zero out all the correction in the SAFC and try to just run it along side the Nistune?


Locus2779
Posts: 2
Joined: Thu Apr 26, 2018 11:24 am
Car: 1996 240SX w/ notchtop swap
Megan Racing full suspension build
Autocrossing on big fat ho-hos

Post

Replying to my own post, just in case someone needs to figure out this exact problem later on.

The MAF is bad.

A friend of mine is running an SAFC on his 91 talon, and commented that his tune runs from +20 to -8 correction. That got itchy in my brain because my tunes have always run from around -3 to +11-ish. I thought maybe the MAF was wired wrong, so I started checking voltages. Turns out with no flow the MAF signal read ~0.3v, when it should be 0.5v. With a fan blowing across it it read 0.7v, when it should've been at least 1.4v. At WOT it was reading between 0.9 - 1v, and should've been in the neighborhood of 4.5 - 5v.

The car ran with the SAFC because that was putting out a baseline of 0.5v, and straight up faking the voltage through the rev range. I tested this theory by starting the car with the SAFC and MAF plugged in. Then I unplugged the MAF and went for a leisurely drive around the neighborhood. The car ran like normal when it should've been in limp mode and running like hell. Then I plugged the MAF back in, unplugged the SAFC, and the car immediately died. I zeroed out all of the correction in the SAFC and ran it and the car struggled to make power in the low end, then bogged and bucked and wouldn't rev past 3800 under boost.

I'm ordering a new MAF today, hopefully that's the final fix.


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