Muzzletones

Everything seems to be functioning normally, but at night when I start my car and turn my gauge lights on the idle surges to 1000, and will consistently go back down and up when I turn the lights off and on. Is this normal operation or is there something wrong with a fuse or resistor or something of that nature?

Darcy Bekkema

XenonZcar.com Z31 Idle-up solenoid (Non-Turbo only)

rogerz

if you would really like answers that would help you and avoid flames give the following info when you post: year of car, n/a or turbo engine, stick or slushbox. Most of us can tell by the year what model you have ie 85T is a turbo z31. Z cars come in many flavors and they are all different. example 84,85,86 Z31's pretty comparable. there were changes in 87,88,89. in addition 300ZX can refer to two completly different cars the Z31 and the Z32. No mindreaders here the more info you can post the more likely you get a cogent useful answer. The cars with L engine (straight six) changed almost yearly from their introduction. A 240Z and and a 280Z have similarities but they can also have wide variance.

when you say you turn on the gage lights what do you mean. the stock gage lights come on with the headlight switch. there is no separate switch. if you have a separate switch someone has modified something and perhaps they wired up the idle up air circuit to that thus energizing it. far fetched yes but after what I have seen morons do to z cars anything is possible. if you don't like get rid all that idle junk hanging off the left side of the plenum.

Muzzletones

I'm sorry I'm usually good about that I zoned out and forgot to put info about my car in the post.

It's a '87 Gl non-turbo, automatic. everything is completely stock and unmodified, the car has 122,000 miles.

I changed the spark plugs, the dist. rotor, and all the vacuum lines.

When I say turn on the Gauge lights I mean the switch on the headlight stick, the gauge lights come on first, then one more turn and the headlights pop up and come on. As soon as I turn the switch for the headlights/Gauge lights on the idle jumps up very consistently to 1050.

Muzzletones

The car runs super great, no misfires or backfires, no hesitation at all.

zxguy1986

OK, here's my theory. If the Z31s were smarter - with computers that told you what they were actually doing instead of throwing you hints about this or that 'area', it would be telling you that the headlight load (power demand) isn't being met by your alternator/battery set up and so the computer raises the idle to make up for the power drain from the lights. So alternator puts out more charge. But Z31s aren't that smart, I think. '87 was still early in the history of ECUs.

For example, what happens if you switch on the lights, your idle goes 'up' to 1050 -and then you crank on the AC and the blower motor to full? Higher idle than 1050? 1400?

1050 rpm isn't that high for an idle. In fact, it's low for warm-up. My original '86 n/a, from day one, idled at 1400 until warm and then never went below 900. You don't say what your hot idle is so I'm guessing it's at the factory-stated 600-700.

I have to go with the earlier comment that your idle-up solenoid is weird - unless you have flaky alternator/battery circuits. Any weak link in those could send you to high-idle territory. You can get your battery and alt tested for free at places that sell batteries (Sears?) It's all hand-held computerized now and they give you a print out. If all is OK (lots of alt output above 14v and lots of good batt storage), check out that solenoid shown in that cool Xenon diagram located above - and see what happens.

Please report back so we get the feedback. Thanks.

Muzzletones

When I turn my A/C on to full blast the idle does jump up to 1400 in park and when I turn the A/C off the idle doesn't go back down until I turn the car off and back on.

I also think the oil pump is starting to go out if that's relevant at all. It does that thing on startup where the oil light comes on for a second and it knocks or makes noises after sitting for a few hours but it goes away quickly. I read to change to 10w40 oil.

I checked the timing and it's at 20 degrees BTDC.

I've been keeping my eye on the volt gauge and it's looked normal, I have a brand new battery from oreilly in the car, so if there's a problem with the electrical system it's in the alternator or something else.

zxguy1986

OK, now you need to know your alternator voltage output.