High idle while shifting gears on hot L28 engine

[Labubre]

Hi there,

Looking for ideas to test or for THE solution to my problem.

So far did not find the same topics open and closed.

So at stop, everything is perfect. Stable idle at 800-900rpm. No effect of pressing clutch or brake. When gas pedal is released, rpm comes back to normal.

When engines hot (above 120°) then start the troubles:

1. Gear shifting up or down: from gear 2, rpm goes up to 2000rpm on their own and only come back to lower idel when gear is engaged. During shifting, if i keep clutch down, idle comes back to normal after 1-2 seconds. And so, when engine is cold, no similar issue till temperature reaches 120°. I also remember the first time i drove her smoothly at 40-50 mph in 5th gear, idle at 1500rpm...i felt the car would never stop as if there were no "engine brake" (bad translation i guess).

2. Second test: in a downhill at low speed and all pedals up, no gear engaged, i let the car take speed on its own. When the car reached a certain speed, i saw the rpm moving up to 2500rpm without any action from my side. Rpm went back to normal when i succeed to slow the car and engaged a gear. Same, on a cold engine, nothing similar happens.

So any help would be more than welcome for a dummy mechanic like me that just fell in love with Z and just imported my 280 from US this summer.

Cannot believe anybody faced this and solved it.

But nobody clearly post the solution.

Help me to do it! 

Thanks Seb

 

[gwri8]

Sticky Throttle maybe? Pull your AFM Boot to the throttle and have a look at the throttle blade. It can get gummed up sometimes.  Mine had a problem of idle hang last summer and I wound up cleaning the throttle blade with carb cleaner. 

[Labubre]

Thanks Greg

Look easy enough for me!!!

Is this hypothesis compatible with the fact that issue does not exist when engine is "cold"?

Will try

Love your rims!

[gwri8]

You can download the Fuel Injection Manual here . There is also A factory service manual there also.  Both very much recommended for this and future adventures.

 

Edited September 14, 2016 by gwri8

[siteunseen]

I would think the oily build up in the throttle body when cold wouldn't be sticky.  As it heats up it would get sticky causing the blade not to completely seal off, high idle like a vacuum leak.  Carb cleaner and an old toothbrush works good, be super careful removing that accordian boot it'll crack.  I'd probably start at the AFM and take stuff off until you get to the throttle body, those hoses are expensive. 

There's a lot of stuff out there on high idle or hanging high idle.  The BCDD, dashpot, throttle position switch adjustments are in that EFI manual Greg linked.

 

[MattSaint]

Sounds like the symptoms I have which has to do with the BCDD.  To help, I actually took the retaining screw out and squirted in some PB Blaster.  Seemed to help for a little while but it is back to acting up.  Mine is an early 76 (9/75) so your BCDD might be configured differently than mine.

[240260280z]

I agree with BCDD.  Just turn the screw or nut at the bottom then drive and stop,  If the rpms stay up when stopping, you have turned the wrong way.

 

btw the ECU has a fuel cut mode when gliding and the engine is above ~ 2800 rpms.

 

 

 

 

[240260280z]

from Z Doctor:

 

Unplug wire to BCDD control solenoid (this takes the speedo switch out of the picture, and makes the system active all the time)

Remove rubber plug on bottom and adjust small screw on bottom until the idle speed returns to normal after blipping throttle. Some BCDDs use a 12mm nut and retainer clip instead of rubber plug and screw. If so, remove retainer clip and adjust nut until idle speed returns to normal.

Reattach wire to BCDD control solenoid.

If the BCDD does not respond to adjustment (about half the time), it is trashed and needs to be disabled. Run a wire from the "hot" end of the ballast resistor (BW) to the control solenoid wire so that it stays energized and the system is "off" all the time.

If this doesn't work, remove the AFM/Throttle body boot, locate the 1/2" inch hole in front of the throttle plate that feeds air to the BCDD. Plug up the hole with a paper towel/RTV glue amalgam to permanently shut off air.

[siteunseen]

Something else you can do is pull the black cover off the AFM and make sure there's no spider webs or anything on the vane or wipe pad. Make sure it moves freely. If the cover is still stuck on with a clean look it probably has never been off so maybe do that when all else fails.   

[Zed Head]

Saw your thread on zcar.com.  The BCDD was suggested there also.  But you never followed up on THE possible solution.  You might have to address it if you want to solve your problem.

[Labubre]

Great!

Sounds like we're getting closer

BCDD / AFM / Throttle valve

As soon as i can drive my car out there, i'll get it to a local experts with all your ideas!

Thanks to all

Be sure i will post the solution to my problem

 

[Labubre]

Hi there

BCDD removed! Problem solved and i can enjoy now cruising with my Z. Niiice

Thanks for support!

Seb