High Idle Investigation

[Teesetz]

I’ve always had a higher idle and I’m pretty certain it has to do with IACV partly, but there is another weird aspect to it. It idles high, and when I turn on the headlights, the idle comes down 500ish rpm and holds steady. I think there is some sort of electrical gremlin as well as the IACV needing to be done.

 

Thoughts? Is there a known thing in this department with these?

 

Thanks in advance.

[Frenchy]

There is a check for the Idle Air Control Valve that would be good to preform. To give you a general idea the idle air control valve does get help from the secondary air valve when cold start happens. The colder it is the higher the idle RPM for starting before going down to specifications(700 +/-50). The idle air can be adjusted with a Phillips screwdriver. Before adjusting it you should perform the check specified in the book. 

[Slartibartfast]

That's a weird one. I've had low idle/stalling issues with mine when the MAF connector was acting up, but I've never had it stick on high idle. And the headlights playing into it is bizarre.

 

There is an adjustment on the IACV, and the procedure to adjust it is in the service manual.  If you think it's acting up because the last guy messed with it, it might be worth ruling out. The crosstalk with the headlights tells me something else is wrong. Adjusting it to mask the current issue may complicate your diagnosis of what's actually wrong, or give you other issues.

 

For some damn reason, these have two idle control valves. The IACV-AAC (the one under the back of the intake, with the adjustment screw) has a solenoid that's pulsed by the computer to fine-tune the idle. The IACV Air Regulator (the tall one behind the EGR, with the plug on top) is wired to the fuel pump relay and acts like the electric choke on a carburetor, except backwards (starts wide open for high idle on cold starts and slowly closes as it heats up). Test info for the two valves is on EF&EC 117 and 119 of the '95 manual.

 

I suspect the valves themselves are alright, but a weak connection in the headlight circuit is confusing one of the sensors that tells the computer when it needs to raise the idle. The computer controls the IACV-AAC according to input from the sensor in the dizzy (which tells it the engine speed--you'd have other symptoms if this was bad), coolant temp sensor (idles up when it's cold--check the sensor and its connector), ignition switch (start signal, doubt that's got anything to do with it), throttle position (presumably so it's not thinking about idling when it's not idling--it is idling, so I doubt it's this, though it wouldn't hurt to check its adjustment), neutral position switch if it's an automatic, aircon switch (idles up when the aircon is on), power steering pressure switch (idles up when the power steering is working hard), battery voltage (I assume so it can idle up if the alty needs more RPM to hit its voltage target), and the vehicle speed sensor (I assume this is so it can work out if you're coasting or engine braking).

 

If the valves themselves check out, I would go to the EL section, work out which fuse link and ground point the headlights use, and check them both. If any of those points are shared with other equipment, and are poorly connected, power could be backfeeding all over the place. Make sure the connections are clean and tight and the fuse links are in good shape. If that doesn't get you anywhere, I would check the sensors that tell the computer to raise the idle (EF&EC should have test info for each) and their connectors. Failing that, check the battery voltage at idle (with the headlights on and off), check the engine grounding (voltage from the engine to the negative post on the battery should be pretty low), check the other fuse links, and check whatever other ground points you can find. Look for any obvious harness damage. Wiggle stuff with it running, see if you can find something that changes the idle. Hell, check the vacuum lines, if you haven't, though I doubt they have anything to do with this.

 

The '95 manual also has some basic troubleshooting, including various idle issues, starting on EF&EC-50.

 

When it's at low idle (with the headlights on), does engaging the aircon or the steering switch (turn full against the stop and hold it) bump the idle back up? Does it idle down for just the parking lights, only low beams, only high beams, some combination thereof? And finally, does loading the electrical system with something else (rear defroster, blower motor, something like that) have the same effect?

[Teesetz]

I’ll have to test some more of those theories. It does seem to be most significant change with turning head lights on and off. Air con and other systems do affect it but seem in the expected range but nothing as much as the headlights. .

Cold start systems seem to be fine. Starts high and lowers as it warms. Foot on the brakes in drive and reverse it’s in normal 800-750 range. It’s not rough when it’s high or low. Runs smooth. I forgot something in my house today and heard it idle up randomly while I was in the house with nothing on (fans, headlights etc) to the 1100 range I’m talking about and stay there. I know in park and neutral it’s supposed to idle a little higher, but not this high.

It sounds like a very light weight ghost is lightly pressing the accelerator and keeping it there lol.

I’ll start with inspection and wiggle test tomorrow, and also see if I can get a video.

Thanks for all the info!

[Frenchy]

Regardless of gear it should want to idle around 700(+-50) RPM's. 

 

Out of curiosity have you done any modifications to the vehicle recently? Also what alternator do you have?

[Teesetz]

Nothing in the motor. Just suspension.

Alternator id have to look at but it could very well be from 1994 lol

[Frenchy]

Just trying to rule a few things out if anything. I doubt a suspension lift would bother with it. My biggest suggestion is to get ahold of the factory service manual. I know that Nico Club has the available to use. I would utilize it as it will give you all the information you need.

[Slartibartfast]

If it's not isolated to the headlights, then tracing the headlight circuit might be less useful than I hoped. I would expect the alt to struggle more with the lights on than it does with them off, but I'd be curious to see battery voltage at idle, loaded and unloaded, when it's acting up vs when it's not.

 

Was your defroster on when you heard it idle up on its own? The auto HVAC system kicks in the aircon when the defroster's running to dehumidify the air. Could be the compressor cycled and that's what made it idle up. It's been a while since I had working aircon in mine, so I don't remember how much it's supposed to idle up for that, but I don't think it was that high. I don't think the manual HVAC does that.

[onespiritbrain]

i know its a different engine but slart solved my high idle issue long ago.. 

 

look here: 

 

[Teesetz]

Bird Brain moment… Vacuum line by the worst spark plug known to the automotive man. It was undone. . I can remember a lot of swearing replacing that spark plug and amidst the blunder I must have pulled it off it’s seat. Slipped it back on, normal idle headlights on or off thank you guys for the vast array of info and knowledge though!

[Frenchy]

So you are aware that particular vacuum line goes straight to the fuel pressure regulator. Besides the vacuum leak it was also causing high fuel pressure at idle.

 

Glad you were able to fix it

[Teesetz]

Aware now!

[Slartibartfast]

I would never have called that being the problem! I'm guessing the incorrect fuel pressure was throwing off the idle mixture, which threw off the idle speed, and the computer ran out of adjustment trying to fix this and went to high idle. Still not sure why electrical loads calmed it down. In any case, good work finding it! Hopefully the issue stays gone.

[Frenchy]

This was most likely a combination of the fuel pressure and vacuum leak when you get down to it

[Slartibartfast]

Wrong fuel pressure, unmetered air, and 30-year-old EFI that doesn't monitor the mixture at idle. Yeah, that makes sense.

[Teesetz]

There is definitely some sort of electrical gremlin tied into the headlights, but the idle is much better and no random surge anymore. Consistently idling much lower.

[Frenchy]

3 hours ago, Slartibartfast said:

Wrong fuel pressure, unmetered air, and 30-year-old EFI that doesn't monitor the mixture at idle. Yeah, that makes sense.

Not sure what makes you think that, but it does monitor the Air Fuel ration at idle with no problems. Depending on what is going on it may go into a fail safe mode and use a preset value for said issue(like the engine coolant temp for example). Now when in open loop the Air Fuel Ratio may not be monitored due to the programming trying to warm the engine up to get to closed loop operation.

[Slartibartfast]

Frenchy, I believe you are confusing our early fuel injection with more sophisticated modern systems.

Modern vehicles use wideband oxygen sensors, which allow their computers to dial in any air/fuel mixture they want, so they can stay in closed loop all the time (once the sensors are warm, anyway). These older rigs use narrowband oxygen sensors, which are useless* for any mixture but stoich (around 14.7:1). The narrowband can't even see stoich--it can only report rich or lean, so the computer bounces the mixture back and forth between them in a zigzag that averages out to stoich. This means these rigs can only run in closed loop for light acceleration and cruising. In any other conditions, they fall back to open loop, and run on stored values.

 

A while back I added LEDs to my dash that mirror those on the back of the computer, and running the computer in one of the first two test modes lets me monitor the operation of the oxygen sensor while I'm driving. The green light flashes with the oxygen sensor when the system is in closed loop. I only ever see the green light flash under light-to-moderate throttle. Under heavy throttle or engine braking, the light does not flash. The light does not flash at idle, either, though I have seen it continue to flash briefly after the engine has returned to idle following closed-loop operation. I assume idle (even warm idle) is set a little on the rich side, and as such cannot be monitored with a narrowband sensor.

 

(*There may be a little more going on here than I'm giving the system credit for. One of the test modes is supposed to tell you if the mixture is more than a certain percentage rich or lean in closed loop, and hell if I know how it's working that out from a sensor that only reads 0 or 1. Nissan may have done something clever to squeeze a little more data out of these primitive sensors, but clearly they were not able to get wideband performance out of a narrowband sensor.)

 

Anyway, Teesetz, good to hear it's back on good behavior, and good luck tracking down any residual gremlins.

[adamzan]

This hose bit me too years ago with my first wd21. I put a zip tie on it which helped it from slipping off when "servicing" things back there. I've got big hands so it doesn't take much to knock things loose.