Idle problem...

[rcb280z]

My 77 280 has a strange idle issue when warming up. Start up is perfect, fires right up. Idles around 1200 rpm until it starts to warm up. During it's warm up the idle will slowly drop to around 500 rpm and then, as it warms up more (towards operating temp) it slowly increases to 800-900 rpms. This is every time. Iv'e checked for vacuum leaks at every connection I could think of but I'm not convinced its vacuum related. I sprayed starter fluid around the throttle body and anywhere I think there could be a vacuum leak while its idling low and found nothing. Im not sure where to go next. Any advice?

Thanks

I should also add that I checked for linkage binding and found none.

Edited September 26, 2013 by rcb280z
forgot to add....

[JLPurcell]

I have very little knowledge of the fuel system on the 77, but it sounds like a sensor issue. Possibly a temp. sensor relating to the cold start enrichment system.

[rcb280z]

Jerry, the only time cold start is "working" is on initial start up. Just enough to squirt a little extra fuel for starting. At the same time an Auxiliary Air Valve is adding extra air for a higher idle. All this works. The issue is after it begins to warm up, I get the idle drop that drops too low and then slowly over about 4 minutes time comes up to a normal idle. And stays.

Thanks

[Captain Obvious]

My read is that everything is working as best it can and you're simply at the mercy of mid-seventies control technology.

Once the engine is fully warm, it will idle smooth at 800-900 with no AAR help, but it sounds like the AAR is closing off before the engine has warmed up to the point where it will sustain that idle.

Unless you turn up a hard issue with one of the FI components, your (only?) choice would be to adjust the AAR so that it's open more and takes longer to close. Of course, that will mean that your initial cold idle will be higher, and you might not like that, but that's what I meant about being at the mercy of the old tech.

The AAR's closing speed is based on things that are out of the control of the ECU, and there's no ECU control of the idle speed. It's the best they had back then.

[rcb280z]

Hmmm...that's funny Captain, I was on my out to the garage to do just that, adjust the AAR. I figured if it idles 150-200 rpms higher for 30 seconds longer then I can accept that. Will post results tomorrow.

[Captain Obvious]

Be warned that the AAR adjustment is finicky... A lot of adjustment made through a very small amount of movement of the adjuster nut. And it's spring loaded inside to boot. I suggest you mark it well with a fine point marker for your initial position before you loosen anything so you know where you started and can always go back to that spot in the future if necessary.

Good luck, we're all counting on you.

[rcb280z]

Oh don't I know how finicky. It once took a couple of days, trying off and on, to get it where I wanted it. Of course this was around 20 years ago

[rcb280z]

So, update to the "idle" issue. It seems that I had adjusted my AAR to idle too low during the warm up cycle. I did this last year sometime and never really put 2 and 2 together that this would cause my idle problem during warm up. Now that I drive the Z nearly daily it was beginning to bug me. So I adjusted it back to where it was and it has solved the issue. Obviously Nissan had already discovered this issue and that is why it is at the "factory" setting it is at and glued as to deter any adjusting by "tinkerers" like myself