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Hello,

I have a 77 280z powering a 76 280z cali chassis.  This is my real car project and I feel I'm getting close to having this thing as a reliable daily driver.  

My issues is this:  When starting my car, it idles around 800-900rpm and once I get going it starts to float up around 1200rpm.  Once the car is warm I can easily find myself at a stoplight idling at 2k rpm.  Now, if I leave the car in gear and basically bog it down (let the clutch partway out with the brake on), I can bring the idle down to anywhere I want between 1k-2k rpm.  It is strange that it stabilizes wherever I "set" it.  Besides this, the car seems to have plenty of power and no other noticeable faults in engine function.  This is what seems to set my issues apart (i think) from the hundreds of high idle thread most people are having.  Is there something related to the heating up of the motor or FI system that is causing this?  

 

FYI - Car lives at 6k feet.  The timing is set correctly.  Proper AFM function (checked all resistance levels).  I've looked over the vacuum lines to the best of my knowledge - my latest breakthrough was moving the vac advance line from intake mani to the throttle body which fixed numerous idle/power issues.  Did a vacuum "delete" to just run the basics.  Fuel and oil pressure are good. 

 

Does this sound like faulty BCDD function or faulty cold start (stuck open)?  I have the FSM and FI Bible for those that suggest it - there just may be something I'm not getting.

 

Thanks in advance.  

 

Rock the Dashpot

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It's not the CSV.  That valve just adds fuel when starting.  The BCDD can do odd things like that.  It has a diaphragm that can leak and bypass passages that could get stuck open.  Could also be a sticking throttle blade.  Either linkage or gummed up in the throttle bore.  All of these things have caused a high idle for many people in the past.  Try pulling up on the throttle pedal with your toe to see if it's the linkage or blade.  The BCDD is harder to diagnose.

 

If the vacuum advance hose was on the wrong port then other things may have been messed with.  The 800-900 RPM right after starting indicates your AAR isn't doing its job.

 

"my latest breakthrough was moving the vac advance line from intake mani to the throttle body which fixed numerous idle/power issues. "

 

Some background on who installed the engine and when would help give some context.  Problems from nowhere are tough to figure out.

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Thanks for the response.  I lubed up all the linkage bits and ran the car.  I have the same symptoms.  I did notice, like you said, if I pull back on the throttle pedal that it budges maybe 2-3mm and my revs drop around 100rpm.  I don't think it's the main issue, but it's a good point and one I'll watch out for.  I might bend that retracting spring behind the pedal a little bit out so it has more umph in restoring the pedal to the fully "up" position.  

 

Regarding the 800-900rpm idle - I should have mentioned I had my idle screw turned waaay in to compensate for the high idle while driving.  With the idle screw turned out to 6 turns from fully in, my revs are around 1300rpm on start-up cold.  I don't know if that's normal or not.  This, in turn, means when the car was warm today and I was at a stop light, I was steady at 2k rpm idle and could only bring it down to maybe 1700-1800 by semi-stalling it. 

 

Some more background - I actually installed the engine with some help from a couple of friends.  I pulled this engine from a 77 2+2 after confirming the motor was good and basic function was fine after replacing some old parts and cleaning out the fuel system.  Since the body was rusted out I swapped it and a 5spd into this new body.  I admit, the list of "what to consider next time" is pretty long, as this was my first project.  I can say though that at this stage, I have had a lot of help from non-Z familiar folks to get the basics pretty nailed down on this car.

 

The AAR is actually one of the few things left that I have not considered/researched.  I have the original L28 from this car I could try pulling the AAR off of and swapping to see if anything improves.  Would you recommend this before digging into the throttle body or BCDD?  Thanks

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The comment about the AAR was just a point about things not working right.  But it would be worthwhile to make sure it's not stuck open.  Anything that lets air get by the throttle blade.  Don't forget the EGR system.  They've been known to leak.  Intake manifold gaskets too.

 

If the engine is running rich, any vacuum leak at all will raise idle speed.  You should be able to drop the idle low enough with the idle adjustment screw that the engine dies.

 

The fact that it stays down after stalling the engine seems to point at the BCDD.  It's designed to use intake vacuum to determine when to open the bypass port, I think, along with engine speed and a solenoid.  Could be that your high idle is self-reinforcing by creating high vacuum.  Maybe.  The system is described in the Emissions chapter.

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