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It doesn't happen when a/c is on just when all accessories are turned off and the idle drops down, what possibly could be causing this, installed new spark plug wires, rotor and cap, pcv, platinum 4 spark plugs and K&N filter and passed smog test with flying colors, removed thermostat.......car does have 197k miles on it. It is a 1987 n/a
 

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never heard of something like that happening before. when all your accessories are off, what is your idle speed at? maybe you need to put a thermostat back in... I don't know where you live but if its cold right now you're engine may not be getting up to proper temperature...
 

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well when you turn on any accessory, there is a valve that diverts more air into the intake manifold, and as far as I know that is the only device that affects the idle speed. So, given that it runs fine with more air being diverted and then starts having issues when that solenoid is closed, perhaps the idle control valve is faulty... I really am not sure though i'm kinda perplexed.

does it idle pretty stabily when accessories are turned on? the idle doesn't vary or vibrate does it?
 

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Discussion Starter #6
When accessories are on like fan or lights the engine speed steps up above 1000 rpm...I thought this is what the car was supposed to do...the problem is the missing at lower rpms
 

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no, it is supposed to step up when you turn use an accessory. there is the Idle-up solenoid valve which controls this. It is attached to the Idle valve, which has a screw that can be turned to adjust the slow idle speed. I'm saying that if it runs fine with the lights on and the idle up, but not with them off and the idle down, the problem may be caused by the idle control valve. I would try raising your slow idle speed by 100RPMs and see if that solves your problem. just a heads up, doing this will cause your fast idle speed to go up by 100RPMs as well.
 

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on the driver side, right in front of the throttle body. there is a device that has two screws and an electrical connector. the connector goes to the Fast Idle solenoid which is what causes the engine to jump up when you turn something on. There are two screws, the one you will be using is the one closest to the back of the car. disconnect that wire to the fast idle solenoid, and let your car idle until its warm. now use a flathead screwdriver to turn that screw until your idle is at like 850 or so, see if that fixes your problem.
 

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Auxiliary Air Valve... If your idle is bad until you turn on the AC then your engine isnt getting enough air, and it's most likely a bung auxiliary air valve. I've heard of people using degreaser to get it working better again but if you want to make sure it's perfect then buy a new one. They're cheap, like $80 new.
 

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My Z is an AUS build so Im not certain it'll be in the same location on yours so I'll have to leave that question to someone in the states to answer for you. On mine it's just behind the radiator cap...
 

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"Auxiliary Air Valve... If your idle is bad until you turn on the AC then your engine isnt getting enough air, and it's most likely a bung auxiliary air valve. I've heard of people using degreaser to get it working better again but if you want to make sure it's perfect then buy a new one. They're cheap, like $80 new."

hmm..
When I was trying to fix the cold start lean mixture problems, I asked one of the mechanics at my dealership about that valve. He told me that it shouldn't cause the engine much trouble at all. According to him, the valve is composed of basically two parts. One is a spring which changes shape depending on temperature, much like the spring which controls the actuating of the thermostat. The spring is connected to a plate with a hole in it. When the engine is cold, the hole in the plate should line up with the vacuum tube and allowing air to be diverted into the intake manifold. As the engine warms up, the spring twists, and the plate rotates until the hold no longer lines up and the air is blocked. He told me that the car should be able to function entirely without this valve, as long as it is removed and the hole in the manifold is sealed.
I have the valve unplugged, so obviously its operation doesn't effect the operation of the ECU in any way. I know my valve works though, so I can't really comment on whether the engine can actually function totally without it. This is just what I was told by a Nissan mechanic, but I would assume he wasn't just blabbing non-sense at me.
 

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Dealership mechanic? Well there you go... I think it's common knowledge that dealership mechanics aren't exactly the best to say the least :p Keeping in mind that the more your car screws up, the more money they'll get out of you.

"Sure just rip it out; it doesn't do anything" I can see the guy now devilishly rubbing his hands together haha... Joking.

Seriously though I dont think it's a good idea to listen to anybody who says "just pull it off, the car wont care" lol Im sure there's lots of stuff you can just tear out and the car will still "work" but I think the manufacturers who built the car might know a little bit more about what the engine needs to function properly rather than some nissan dealership mechanic who works on primarily late model nissans.

If something breaks, fix it. A cold starting car that's idling low can knock and rattle like a s.o.b which obviously cant be good for the engine, neither can plugging up holes. Considering it's not even a $100 part, backyard fixes like that are silly in my opinion.

Personally I'd never take my Z to a nissan dealership mechanic...

To conclude, all I know is that the problems BAADZ describes are very similar, if not identical to the issues I was having a while back which was corrected for me when the AAV was replaced.
 

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I think we may be calling the same valve a different name, and possibly arguing over a misunderstanding, but essentially saying the same thing. The valve that controls the idle speed depending on electrical load, an important piece that will cause problems when not working properly. The cold-start idle valve, on the other side of the manifold on US models, shouldn't cause any issues, unless the large vacuum tube to it is torn.

The mechanic didn't say "rip it out its useless", he said it can be removed and the car won't have a problem. Mechanics at the Nissan/Infinity dealership here must be certified by Nissan to work. they may be way too expensive, but they know what they're talking about, 95% of the time. This guy is a good friend of mine and wouldn't be screwing with me.

either way, a malfunctioning valve may cause problems that the complete absence of the same valve wont cause. who knows. whats important is that we've gotten off course. BAADZ issues have nothing to do with the engine being cold or warm.

"It doesn't happen when a/c is on just when all accessories are turned off and the idle drops down, what possibly could be causing this, installed new spark plug wires, rotor and cap, pcv, platinum 4 spark plugs and K&N filter and passed smog test with flying colors, removed thermostat.......car does have 197k miles on it. It is a 1987 n/a"

Based on this, i'd say the engine is starving for air with all the accessories off, which would lead me to think that his slow idle valve gummed up or something. it works fine when the fast idle kicks in. both valves are supplied by the same vacuum line, so a leak isn't likely the problem. I'd say remove and clean the valve and put it back on.
 

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The dealerships mechanics anywhere have to be nissan certified; but they're still dodgy because they're not trained to work on older nissan model cars, at least not in most cases. Although I must admit if they have worked on many z31s before they'd be trust worthy, problem I had here was that z31s are rare and not a single nissan mechanic had worked on one before and had no clue wtf they were doing.

Back on topic, I dont know of any other name it goes by other than Auxiliary Air Valve and it looks like this:

http://www.bavauto.com/assets/product_images/hardpart/0_280_140_124.jpg

The only function it serves as we've mentioned earlier is raising the idle so the engine won't stall when cold.

When mine went bad; turning on the AC did infact raise the idle speed up a little which is same thing he's describing. That's what made me believe that his problem was the same.
 

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85zx:
That part is often referred to as the "air regulator", because even Z31 FSMs call it that. In fiche-speak it is also a IACV.
E.g.:



The part that phxZ31 is talking about can be called the AAC (turbo) or IAA (NA), and even these names can vary a bit depending on where you look. Sometimes the parts fiche uses names completely different to what the FSM does.

The air reg tends to fail by not raising idle when cold i.e. the bimetal strip that controls the shutter fails. Warm idle is generally not be affected unless the shutter is stuck part way open, in which case the idle speed is raised.
AAC failures can be more complicated. If the motor is adjusted completely to FSM spec, disconnecting the AAC should only result in a drop in warm idle revs of about 50rpm (no headlights on, no aircon etc).
A dubious AAC should only cause variations in the idle speed, not misfiring, since the AAC/IAA and indeed the air reg are supplied with metered air.

All sorts of things can cause idle misfire, like unmetered air leaks or even a leaking EGR valve.

BAADZ:
did the engine miss at idle before the "platinum 4 spark plugs" were fitted?


Edit: Even the factory is not consistent with naming parts in the fiche.
The NA idle-up solenoid and FICD unit is called a "IAA UNIT", the air reg is called an "IACV - AIR REGULATOR", but the gasket between the IAA unit and the plenum is called the "IACV - IAA UNIT GASKET", even though it is on the opposite side of the plenum to the IACV air regualtor.
The IACV air reg does not even use a gasket, it has an O-ring.



Post Edited (Oct 11, 10:12pm)
 
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