Unusual fast idle condition
 

Hi All,

I have a somewhat unusual fast idle situation. When I start out cold, the idle is up around 1500 rpm. When I drive a few miles and come to a complete stop the idle comes to rest at 1200 rpm. I turn off the motor and restart and the idle, after a couple of seconds, settles down to 650 to 700 rpm. I have run this scenario a number of times and it is always the same. To the best of my knowledge I have no vacuum leaks.

Any ideas ?

Thanks,

Don

Re: Unusual fast idle condition
 

Don, the early cars' original PROM calibration was revised in later years to correct this very situation. You might want to consider talking to one of the chip burner guys out there to have the calibration updated if it hasn't already been.

I think Marc Haibeck's website may have some info about this condition.

Re: Unusual fast idle condition
 

Re: Unusual fast idle condition
 

I goofed and didn't tell you that the ZR is a 90

I guess I didn't make myself too clear on my new problem which is just the reverse of the customary high idle problem I had for the last 2 years.

I now have high idle at hot shutdown but normal idle on hot restart. I can now shut the car off with a 1200 rpm idle and restart immediatly with a normal idle. I drive the car, bring it to a stop with a 1200 rpm idle, turn off the motor, restart the engine with a normal 650 to 700 rpm idle.

This condition just started yesterday.

Kinda makes ya wonder

This might take Mike H. to solve the problem.

Thanks

Don:fahne:

Re: Unusual fast idle condition
 

marks chip is so worth it

Re: Unusual fast idle condition
 

Since it sounds like you have normal idle at "open loop" time I'd start at the IAC. It's relatively easy to get of and clean.

Alternatively you could begin to isolate things at fast idle. (throttle plates sticking etc)
I take it blipping the throttle make no difference when it's at fast idle?

Re: Unusual fast idle condition
 

Hi Scott,

I had to wait for my wife while she was at Zumba class and the same thought occurred to me. I will follow the FSM instructions on trouble shooting the IAC and go from there. The IAC that is in the car is OEM and therefore 23 years old

I suppose that is could also be a ECM issue but I doubt it. I will see if I can borrow a Tech1 from work and go with it as well.

Thanks for jogging my memory.

BTW, I was born and raised in Portland, OR. Worked and retired from Tektronix. Worked part time for Northwest Investment Cars in West Slope and the old Motorsports International (Used Porsche store) located also in West Slope. I am on my 27 Vette but included in all the Vette fun was owning a Pantera, a real Cobra 427 SC and many Porsche 928's

Have a great ZR1 summer.

Don:handshak:

Re: Unusual fast idle condition
 

I think Scottfab has a good suggestion. Sticking throttle. Once motor is shut off, throttle plate returns to stop because vacuum no longer applied to it keeping it slightly open. Check out the TB.

Re: Unusual fast idle condition
 

Hi XfireZ51,

Thanks for the tip. I will do that tomorrow as well as checking and maybe cleaning the IAC valve.

Cheers,

Don:flag2:

Re: Unusual fast idle condition
 

I had the sticking throttle plate problem (secondary) and had Marc rebuild the TB for me (works great now). I notice he adds an extra return spring during his rebuild

Re: Unusual fast idle condition
 

Yes that's what we did to mine as well. Marc said the return spring gets weaker and he adds a small one to help w the closing. Mine works fine now and frankly I like the additional resistance. There's a more direct proportional feel to the pedal.

Re: Unusual fast idle condition
 

What a small world. I too worked at Tek but then who didn't back then. One of my pensions is coming from Danaher the parent of Tek. I was in the Oscilloscopes, Logic Analyzers and later Printer group for a total of 19 before moving on. Bet we know some folks in common.

Re: Unusual fast idle condition
 

Most initial failure on the TB is the plates sticking to the sides and/or top. From the factory they come with a special grease at and around the plates (some stuff used on the IAC) . People tend to over clean these. Later on as the bushings wear there is too much sloop and sticking becomes more frequent. A good way to tell is "blip" the throttle. If it slows down you got it.
Yearly cleaning is required. (or 60mo depending on use) I use axle grease on the edges of the plates (including the small idle plate in the middle at the bottom).
My personal preference is not to use an extra spring. It stiffens the peddle too much for me.