Help! Forgot to disconnect the battery when cutting my mafs harness!!!

[HI NRG]

Hey guys I forgot to disconnect my battery before cutting the mafs harn. will this blow any fuses...if it will can some tell me where that fuse is or did I fry the ecu. FYI I was swaping my mafs out of my KA dohc to a KA sohc. I read this in ine of the topics that the sohc mafs will work better with my turbo setup is this true. Well anyways the car sounds like its running on two cyl. but I checked for spark and I'm getting on all. It's blowing alot of black smoke which tells me that its running way to rich. Please help!!!!!

[WDRacing]

Ok...first things first, I've never heard of anyone swapping a SOHC MAF as a mod. So put the other one back. Are they even the same part number? You can't switch MAFS without having some type of mod for the ECU to read the voltage correctly, or we'd all use the 300ZX MAF.

Put the DOHC MAF back and let me know how things turn out. By the way, you should be fine with the fuses and I doubt you fried the ECU.

WD

[C-Kwik]

Brian, I think he is saying he put a DOHC MAF on his SOHC motor. Either way, put the original back on. They may not be compatible. Nor is there any performance value in doing such a swap.

[WDRacing]

Oh yeah...lol. Either way, simply reverse what you did and get back to us. If things are still awry we'll help you troubleshoot.

WD

[AZhitman]

Yep. Put your original MAF back on.

[HI NRG]

I installed the stock dohc mafs and it is still running like crap. Black smoke and still sounds like it is running on two cyl. Did I fry my ECU? Or is it doing this because of connection issue's (I twisted the wires together, do I need to solder and heat shrink them) since the mafs wires are sensitive.

[WDRacing]

A dead ECU will not work at all most of the time, at best it goes into Limp mode which won't allow the car to be revved over 2500 rpm.

I've had my car run fine with a MAF wire harness twist tied...just make sure the wiring is all insulated and the connections are atleast "good twists".

Was the car running well prior to this?

Oh, is this on a single or dual over head cam motor???

WD

[HI NRG]

I is a dohc motor. When I install the sohc mafs the car started up and ran like crap so I put the dohc mafs in it and it still ran like crap. I unplugged the battery plus the ecu for about 10 min and plugged it back in and its didn't change. The only thing I noticed when I unplugged each spark plug wire is that it was arcing from the cap to the distrub. But the car was running fine before I did any thing. Th ewhole reason in why I was doing this was I read this on a tread and I thought it might fix my fuel problem. At wot the car would feel like it was getting to much fuel and bogging out until I let the throttle close a bit then it would run fine. So if you guys have any theory on this could you explain why that would be happening. My T-setup is a greddy T28, walbro 255, safcII, Lg. fmic, 370 inj. 2 1/4 exauhst.

[WDRacing]

Brian, I think he is saying he put a DOHC MAF on his SOHC motor. Either way, put the original back on. They may not be compatible. Nor is there any performance value in doing such a swap.

HUH...I was right Chano...mark it on the calender.

[Florida240sx]

Pull your plugs.......You probably have them fouled out....What is your spark gap? What is your air/fuel ratio at idle???

[HI NRG]

I've pulled my plugges and they are black, while at idle I pulled each plug wire one by one and noticed a slight change in the idle so I know that there is spark at the plugs...the gap is at factory spec I forget what they were when I installed them. The only cool thing about this whole thing is I have flame shooting out on idle from my exaust. I checked the voltage and I'm getting 12v for power and 2.5v +/- 0.5v from the out put on idle, is this good?....checked the fsm and it does not show me or I can't find what the output voltage should be. WHne pulling the Mafs harn there is a change in idle (it gets worse), checked my inj. and they seem to be working fine....lowered my fuel preassure and the idle gets better. Is the mafs that sensitive that if it was seeing voltage while the ignition was off that it could have blown? I haven't checked the exhaust sensor yet but I don't think that has anything to do with the problem I have. I'm out of ideas other than going out to the j-yard and picking up a brand new used ECU.

[C-Kwik]

HUH...I was right Chano...mark it on the calender.

Consider it marked. Good thing it's getting close to year end though.

[C-Kwik]

Are you using an adjustable fuel pressure regulator? And if so, did you turn up the pressure before?

the initial bogging that you were getting was probably because the spark was blowing out. With boost, you have to lower the plug gap as the denser mixture will provide more resistance between the electrodes. But first things first, get this running right again.

[HI NRG]

Are you using an adjustable fuel pressure regulator? And if so, did you turn up the pressure before?

the initial bogging that you were getting was probably because the spark was blowing out. With boost, you have to lower the plug gap as the denser mixture will provide more resistance between the electrodes. But first things first, get this running right again.

Yes I am using an adjustable reg. I had it turned up to about 40-45 psi.

[HI NRG]

Would I be able to plug in a sohc ECU to make the sohc mafs work w/the dohc motor? Just wondering if I could. You guys must think I'm crazy ha ha ha ha!

[WDRacing]

Yeah....crazier then a loon my friend. Keep everything DOHC and you'll be straight, I can't think of anything that would be considered a mod by swapping it over from a SOHC motor or car. Unless what you have is already broke.

WD

[HI NRG]

Yeah....crazier then a loon my friend. Keep everything DOHC and you'll be straight, I can't think of anything that would be considered a mod by swapping it over from a SOHC motor or car. Unless what you have is already broke.

WD

Yeah what gets me is that it was running just fine before i did any thing to it other then the engine bogging on WOT. Well it leave me to buying a new mafs and if that doesn't do it a new ECU. CHecked all fuses and even the relays all connections and nothing.....soooooo mad. .Well thats it for now and if any body has any othe rsuggestions please feel free to answer this post.

[WDRacing]

Ok, so you swapped the MAF over to a SOHC, then swapped it back to a DOHC MAF. Are you using a SAFC? Don't give up, aint nothing we can't figure out. Tell me exactly what you have done to your car, leave nothing out.

WD

[HI NRG]

Yes I am using an safcII.Ok what I did:1. Changed from dohc to sohc mafs=tread on one of 240sx forums said it was done and worked (my car car was bogging on WOT). Oh I cut the mafs harn. w/battery still connected.2. Started motor, motor ran like it was running on 2-3 cyl.3. Pulled each wire out to check for any change in idle=yes change in idle on all four cyl.4. Re-installed the dohc mafs and motor ran the same=poopy.5. Attemped reset the ECU=unplugged Battery and ECU 1/2hr.6. Re-connected and motor still running like crap.7. Checked mafs with a volts/ohms meter= Power wire black 12v-13v idle, signal wire white 2v-2.5v idle.8. Cleaned mafs cyl. no change.9. Checked inj. for proper operation=good all clicking10. Checked all fuse and swapped relays in engine compartment=no change11. Checked all fuse under dash=all good12. The only thing is I need a Nissan Diagnostic Meter for codes I believe or can I do this the old fashon way and get blinking lights.

[Florida240sx]

Change plug gap to .032 chnage your fuel pressure to 35psi.....See how things go then....I have a 91-93 dohc ecu for $50+ shipping if you need it....You don't have any vaccum leaks do you?? I had a line off under my intake manifold and car was running horrible...

[HI NRG]

Well first I'm going t otry out a new mafs and if that does not work I'm going for the ECU.....All i did was cut the mafs harn and after that all hell went loose. So let me get back to you on the ECU after t he MAFS change. Thanks for the offer, Yeah I changed all the spark plug gap and no diff..

[HI NRG]

Well got a Mafs from the Jyard and it still is bad. I pulled the codes off the ECU, Code 12 blink...blink.blink bad Mass Air Flow Sensor. Well hopefully the guy will let me exchange for another one that might work....I guess that you should really DISCONNECT the battery before doing anything to your car no matter how small or how big the project is......

[Edub1]

Not just that but also always cut one wire at a time. A short circuit of this type ussually damages the part from where the juice is coming and does not harm things down stream. The juice goes from the 12v lead and takes the shortest path to ground. In your case this was from the 12v lead through your cutters into another wire and where? Because you have 3 wires and some electronic circuitry this is tough to know. There is also a possibility that you melted a wire. 12v doesn't seem like much but amps shoot up real high and wires get hot fast at 0 ohm. Anyway, you still have 12v so that's one wire we know is good.

I think the MAF output, at least on the SOHC should read .5v at idle and 3v with a slight ~2500 rev. If your ECU sees voltages outside the normal perameters it will default to the TPS and go into limp mode. This is the SOHC though.

If you have a schematic it would help. Untill then, study the FSM for diagnostic proceedures. I'm betting on ECU damage.

[coolbone28]

also, the BLACK wire is the ground wire. White is signal and Blk/Wht is the power. according to Jim Wolf. also i checked the 95 fsm and it says that at idle it should be about 1.5V and goes up linear to about 4V

[Jookmasta]

that bogging problem during WOT can be due to a faulty knock sensor BTW....................

[HI NRG]

Well got a new MAFS and the SAFCII is reading 5.0v @ the sensor check menu. I also ohm out the temp sensor and it read 0.2 ohms @ operating temp is this right. I had to run the engine in limp mode in order to get it up to operating temp because the MAFS was reading to much air and the injectors was just dumping fuel. Help!!!!!!!

[KA24DETDRIFTER]

Hey i too need help with an MAF problem.Plz reply to my email. [email protected] .Thanx

[KA24DETDRIFTER]

Hey i need help with my car on pretty much the same topic. The MAF.It never was like this until today.I had my battery disconnected and then when i put the MAf back on i start the car and the damn thing wont let me rev it up past 3,200.Wuts wrong?1993 Nissan 240SX Fastback.

[Edub1]

It should only read 5V at max load not at idle. If it's 5V at idle something is wrong.

Your MAF has a self clean mode in which the wire is super heated to burn off any dirt. This happens when you shut off the motor so you could have wacked out anything connected to any MAF wires. You might have wacked out your SAFC.

I'd try removing the SAFC and see what happens.

[Edub1]

Loose wire? The ECU will go into limp mode if it doesn't get a proper MAR reading.