Need RB help, guys

[AZhitman]

We've been dealing with an odd issue in our RB25DET (stock) in a '72 Z.

It's backfiring on decel (sporadically). Also, it's not idling as smoothly as a typical RB, and when the electric fans kick on, the idle goes high-low-high-low-high-low until the fans kick off.

I've been trying to find a good program (like Conzult) to check all the parameters, but my copy doesn't work. We have an old scan program that shows the basics, like O2 sensor function, temp, rpms, etc - and everything appears to be in range.

I'd like to find a program that will let me do a power balance test (shut off individual cylinders) and such, but no luck so far.

Anyhow, any ideas what's up with our RB, or where I should start? Thanks in advance!

[Darius]

It sounds like you might have a couple issues going on at once. First, check the IACV line to make sure there are no vacuum/boost leaks. Then, set the screw to fully in and then back it out a quarter to a half turn. I'm hoping this addresses the idle surging when the e-fans kick on.

Second, if it is popping during off-throttle, it could be running a little rich in the decel part of the map. Do you have the ability to modify the fuel map?

Also, verify that ignition timing is set at factory specs 15* BTDC @ 650 rpm. Some unplug the TPS and some can't get the idle to stabilize with it unplugged.

[AZhitman]

Thanks, D. I'll check the IACV tomorrow morning. I'm pretty certain there's no leaks, but I'll give it another check, as well as the set screw.

I don't have the capacity to edit the fuel map (stock ECU) that I know of.

I really wish I had a program to scan this thing (to check timing and such).

Help me understand that last sentence, I'm not clear on that part re: the TPS?

[Darius]

Checking the IACV is a good thing to do in general. I had my setup running for years and it would occassionally surge at stop lights (embarassing ). After CarlH mentioned it in a post, I went out and toyed with it and realized that the set screw was way off. I set it per the FSM and the surging stopped. It started surging about a year later and I rechecked the screw and it was fine. Then I traced the 1-inch air line and saw that it had rubbed against a hole I cut in the battery tray area causing a cut in the line. I replaced the line and the surging stopped.

As for checking the timing, look at page EN-15 of the FSM. It describes how to set the base timing. The ECU is going on the assumption that the CAS is set such that the base timing is 15 degrees. (3rd white tick mark from the orange one on the crank pulley) If the timing is off by say 5*, the entire timing map in the ECU is trully off by that same amount. Having too much or little timing on decel can be just enough to cause popping and sputtering. So get a timing light out and verify the base ignition timing.

The FSM says to disconnect the throttle position sensor (TPS) plug from the throttle body to set the timing. Whenever I do this, the idle speed bounces all over the place. So I have to leave the TPS plugged in. Mine is a S1 so an S2 might be a little different. Try it both ways and see if it makes a difference.

[240z4u]

Sounds like a vacuum leak to me.

[RRRRB]

id also check the TPS adjustment that might cause it to pop on decel if it doesnt see 0 throttle

[elguapo260sx]

ok, this just may be dumb logic. have you checked your battery and alternator? heat kills those items quick especially if you are in arizona.

[TimTurboZ]

OEM Coilacks? bad spark...unburnt fuel

[gmac708]

I really wish I had a program to scan this thing (to check timing and such).

Greg

Did you ever find a program? I have a copy of Nissan data scan from blazt that works pretty well for me. USB patch cable and CD. You need to plug into the grey nissan plug on harness close to computer or rig up adapter to wiring. Simple TX/RX and power and ground. You can shut down cylinders, play around with settings. Might help. LMK, PM I can send to you.

Gord

[AZhitman]

VERY interested. I already have the cable and it's wired into the ECU.

I have Dropbox as well, if that's easier.

[l0nestar]

Greg, I PM'd you.