Unusual fast idle condition - ZR-1 Net Registry Forums

Don in VT · 05-19-2012

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

VetteMed · 05-19-2012

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.

VetteMed · 05-19-2012

http://www.zr1specialist.com/HAT%20W...librations.pdf

Don in VT · 05-19-2012

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

Kevin · 05-19-2012

marks chip is so worth it

scottfab · 05-20-2012

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?

Don in VT · 05-20-2012

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

XfireZ51 · 05-20-2012

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.

Don in VT · 05-20-2012

Hi XfireZ51,

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

Cheers,

Don

gbmidyear66 · 05-20-2012

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

05-19-2012	#1
Don in VT Join Date: Jul 2010 Location: Fairfax, VT Posts: 203	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

05-19-2012	#2
VetteMed Join Date: Mar 2009 Location: Orlando, FL Posts: 1,828	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. __________________ 2004 Z06/Z16 LeMans Commemorative Edition 1991 ZR-1 #302 White/Gray (sold) 1991 ZR-1 #1147 Red/Saddle (sold)

05-19-2012	#3
VetteMed Join Date: Mar 2009 Location: Orlando, FL Posts: 1,828	Re: Unusual fast idle condition http://www.zr1specialist.com/HAT%20W...librations.pdf __________________ 2004 Z06/Z16 LeMans Commemorative Edition 1991 ZR-1 #302 White/Gray (sold) 1991 ZR-1 #1147 Red/Saddle (sold)

05-19-2012	#4
Don in VT Join Date: Jul 2010 Location: Fairfax, VT Posts: 203	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

05-19-2012	#5
Kevin Join Date: Jun 2005 Location: pittsburgh Posts: 4,632	Re: Unusual fast idle condition marks chip is so worth it __________________ It's not the car, it's the people - Doug Johnson 90 r/r "KEYS ON" nick named "T.L.B"

05-20-2012	#6
scottfab Join Date: Jan 2009 Location: Portland Oregon metro area (Washington side) Posts: 3,207	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? __________________ Scott Vett owner since 1979._It's about the car and the people

05-20-2012	#7
Don in VT Join Date: Jul 2010 Location: Fairfax, VT Posts: 203	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

05-20-2012	#8
XfireZ51 Join Date: May 2007 Location: Chicagoland, IL Posts: 9,709	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. __________________ [SIGPIC][/SIGPIC]Former Membership Chairman Former ZR-1 Registry - BOD 1972 Corvette 4speed base Coupe SOLD long time ago 1984 Corvette Z-51/4+3 SOLD 1992 Corvette ZR-1 Aqua/Gray #474 SOLD 1992 Corvette ZR-1 Black Rose/Cognac #458 2014 Honda VFR Interceptor DX

05-20-2012	#10
gbmidyear66 Join Date: Feb 2008 Location: Vancouver, BC Posts: 252	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 __________________ [SIGPIC][/SIGPIC] __________________ 1990 Corvette ZR-1 #2824 4.10's, Custom PROM, Flowmaster, Top End Port NCRS BC Chapter