ECU & IACV tech question (Nisstech?)

Ok, having looked at the manual a 2nd time, maybe I misinterpreted the example scope readings.. looks like they are taking voltage from the ECU pin that grounds the IACV, so 12v would represent a closed valve and 0v would be full open, in which case what I'm seeing in my car would actually be correct, as much as I hate to believe that.

It seems very odd to me the the IAC would have that much effect... Have you looked with a scope at the pulse going into the IAC?... at idle and 2000RPM? Does it look like the manual describes?

Unfortunately, no, I don't have access to a scope. I'd love to be able to see what the duty cycle is at idle vs 2k RPM though.

Agreed. And are you sure the TPS is good? Measure it and make sure.

--scott

One thing that occurs to me is... Can I assume your check-engine light is on now that you disconnected your IAC? Have you checked the ECU for other non self-induced codes?

"JM" wrote in news:2jCNh.15139$ snipped-for-privacy@ursa-nb00s0.nbnet.nb.ca:

Not specifically familair with Nissans, but...

Are you sure that's the correct idle adjustment procedure? Some cars need to have other things done first, such as disconnecting the IAC, or to have a jumper shorted, when adjusting the idle screw. Failure to follow correct procedure will result in incorrectly-set idle.

You may have an air leak somewhere (like secondary cold-running air inputs, an unplugged vacuum hose, or a loose intake manifold). The ECM would then be adding fuel to correct the mixture, resulting in an elevated idle.

The CEL did eventually light, yesterday while I was testing its behavior above 2000RPM, but I drove around a fair but with it unplugged previously without it coming on. Maybe it took a second trip for it to light the CEL.

No other codes are, or ever have been, stored, "unfortunately".

I'm 98% sure it's good. I've tested it with an analog meter as well as monitoring it with my Autotap and laptop, and it's smooth without any dropouts or spikes. The TPS has always been my first thought, because if there was a dropout at some point, I *would* expect the ECU to open the IAC valve up as if it were at idle again. Maybe I have such a small drop out that it doesn't register on the meter or the Autotap... although you would think if the ECU sees it the Autotap would as well.

Yep, the procedure is to disconnect both TPS connectors, rev the engine a few times over 2k I believe, and then set the idle. I did it by the book at the time.

Yeah, I've looked for any sign of a vacuum leak, but never found one. I've sprayed TB cleaner around the vacuum lines and intake manifold, with no change in revs. I even tried pressurizing the intake manifold by sealing the end of the intake hose and blowing in the fitting for the aux air valve, and there was only a very very slight leakage, which could very well have been around my plastic bag/rubber band seal.

If there were a dropout, you would have seen it. Most of what happens on these actually are cracks in the element rather than dropouts, and that is very obvious.

Hmmm....

--scott

Agreed. I'm pretty much at the end of my rope on this one... I'd like to get a scope and see what the ECU is doing to my IACV... or failing that hear from another 2nd gen 5 speed Altima owner to find out if theirs works the same way. My only other thought is maybe I have the wrong PCV valve installed which is allowing too much airflow and throwing things off. That seems unlikely, but there's not much left!

I think scoping the IAC would be a good (and simple) action plan at this point. You seem to suspect this part... but are unable to prove a problem exists in that area without a look at that signal.

Well, I know the part is good; I replaced it because it's all part of one unit that includes the idle adjust screw.. which was seized when I got the car. But, that is the next thing I want to try, if I can find a scope.

I really dont know if this would pertain but on the sr20de engines regrounding the map sensor (solder onto the wire out of the map and to a good chassis ground) smooths out a lot of idle problems, maybe this is worth a shot?

Yeah, I have heard of that fix, and although I dont think I actually tried it, I did measure the voltage between that pin and ground and there was none, so I think I have a good ground. Thanks for the suggestion though!

Check the throttle stop screw for tampering.

The AIC compensates for idle variations, cold start idle and acts as a dashpot when decelerating, right? I doubt it is the cause of the jump in rpms though.

Have you checked for any obstructions in the air intake? I am thinking of something acting as a flap.

Good luck!

Tegger wrote:

I played with it myself before I replaced the idle adjust screw in order to get the idle down.. but it's now back to where it was originally based on the TPS reading at idle.

But it is, because with it unplugged, the rpm change is gone. I've wondered if it had more to do with the amount of airflow reaching a certain threshold and causing something to change in the intake stream myself, but I always come back to the fact that unplugging the valve eliminates that behavior.

Yeah, I've been down that road and everything looks fine. Someone suggested at one point that maybe the throttle plate was loose, but that's not the case.

Thanks!

For what it's worth, I just found a post by a fellow with a 96 Primera 1.6L with *all* of the same symptoms I am getting in my Altima:

I've fired him off an email, so hopefully he's still around (it's an old post) and with any luck can point me in the right direction. If he was able to solve his problems, I'm sure the solution will be the same; his description of the problem is exactly what I'm dealing with.

Sometimes it's nice just to discover you're not just imagining this stuff!

Although I haven't found a scope yet, measured the voltage at the ECU side of the IACV yesterday, and it read around 10V at idle, and decreased at the exact point the revs jump up to 2000 RPM.. what's more, the voltage continued to drop to about 0V beyond 2500-3000 RPM, meaning the ECU must have been grounding the EACV almost 100% -- the IACV appears to receive 12V constantly on one pin and the other pin connects to the ECU which I assume is providing ground at varying intervals to control the opening of the valve

I can see where that might be normal, in order to keep the revs up if you suddenly come off the throttle, but the only problem is why does it jump in so abruptly around 1250 rpm, and the cut off as I release the pedal just as abruptly?

To eliminate the possibility of it having to do with the amount of airflow vs. the position of the throttle, I pulled the vacuum line off the brake booster, and held it to the PCV breather fitting on the intake duct. This brought the revs up to about 1500 rpm, so it seems the IACV is acting based on the input from the TPS, and not based on the air coming in.

But, TPS doesn't show any flat spots or dropouts in its sweep, and I've tried adjusting it either side of where it was from the factory, with no real change once the ECU has relearned the resting position of the throttle... Maybe I should see if I can pick up a TPS from a wrecked car and hope I see a difference??