IAC valve?? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Jason Spears · 01-15-2003, 07:54 PM

I think my IAC valve is not working correctly on my truck ('87 GMC 4x4, 305 TBI) but I'm not quite sure. When I start the truck cold it idles fast for about 45 seconds like it is supposed to and then slows down. I'm pretty sure this is normal operation. After driveing and I put the truck in park the engine starts to idle really fast. (Sorry, no tach yet, I bought one last week end but its been way too cold outside to hook it up.) If the IAC was shot why would the truck slow down after a cold start? Could this be a bad coolant temp sensor? I want to figure this out before I start buying parts. The temp sensor is $32 and the IAC valve is $92 from GM. Thanks for the help.

Jason

*mike reeh* · 01-15-2003, 10:28 PM

could possibly be a vacuum leak that only comes out when the engine warms up, maybe the throttle isnt returning all the way to idle, could be IAC motor.. last time I diagnosed the IAC, I did this:

start the truck and let it come down to the "slow" idle after 45 seconds or whatever.. then unplug the harness from the IAC motor and drive around and see if the fast idle comes back.

mike

87gmc · 01-15-2003, 10:36 PM

DO NOT BUY THE IACV!!!!! That is not your problem. i have the same problem. I replaced that thinking that that was the problem. But its not that i am still at a wonder to find out what it is. I have gone through everything and reaplaced it. EGR and all off the emissions stuff. Checked and no vaccuum leak. But dont buy the iacv becasue that not your problem.

**swervin ervin** · 01-15-2003, 10:42 PM

I don't know, since I'm not a TBI expert, but, why not pull the codes and see what they say?

May be a start.

Jason Spears · 01-15-2003, 11:21 PM

Thanks for the suggestions. I recently checked for fault codes and found a fault with the egr system so I replaced the valve and the solenoid. There have been no fault codes since but I still have the problem.

I think I'm going to try replacing the coolant temperature sensor and the thermostat first to make sure the the engine is running at the right temperature. Then if that doesn't work I'll replace the IAC and the gastket between the throttle body and the manifold.

Thanks,

Jason

01-15-2003, 07:54 PM	#1
Jason Spears Registered User Join Date: Dec 2002 Location: Stoney Creek, ON Posts: 89	IAC valve?? I think my IAC valve is not working correctly on my truck ('87 GMC 4x4, 305 TBI) but I'm not quite sure. When I start the truck cold it idles fast for about 45 seconds like it is supposed to and then slows down. I'm pretty sure this is normal operation. After driveing and I put the truck in park the engine starts to idle really fast. (Sorry, no tach yet, I bought one last week end but its been way too cold outside to hook it up.) If the IAC was shot why would the truck slow down after a cold start? Could this be a bad coolant temp sensor? I want to figure this out before I start buying parts. The temp sensor is $32 and the IAC valve is $92 from GM. Thanks for the help. Jason

01-15-2003, 10:36 PM	#3
87gmc I Got My Engine!!!!! Join Date: Oct 2002 Location: Palmer, AK Posts: 254	DO NOT BUY THE IACV!!!!! That is not your problem. i have the same problem. I replaced that thinking that that was the problem. But its not that i am still at a wonder to find out what it is. I have gone through everything and reaplaced it. EGR and all off the emissions stuff. Checked and no vaccuum leak. But dont buy the iacv becasue that not your problem. __________________ F.O.R.D. "F*cked Over Rebuilt Dodge" -+-+-+-+Cameron+-+-+-+- 1987 GMC HIGH SIERRA LWB 1500 32X11.50 383 TBI/Vortec Heads/Intake MSD Ignition Dual Exhaust Custom Pinstriping Sweet interior & stereo http://www.cgalvin29.freehomepage.com

01-15-2003, 10:42 PM	#4
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	I don't know, since I'm not a TBI expert, but, why not pull the codes and see what they say? May be a start. __________________ Mike 1985 Chevy C-10

01-15-2003, 10:28 PM	#2
mike reeh 5 day ban, learn to behave. Join Date: Nov 2002 Location: san diego, ca Posts: 684	could possibly be a vacuum leak that only comes out when the engine warms up, maybe the throttle isnt returning all the way to idle, could be IAC motor.. last time I diagnosed the IAC, I did this: start the truck and let it come down to the "slow" idle after 45 seconds or whatever.. then unplug the harness from the IAC motor and drive around and see if the fast idle comes back. mike

01-15-2003, 11:21 PM	#5
Jason Spears Registered User Join Date: Dec 2002 Location: Stoney Creek, ON Posts: 89	Thanks for the suggestions. I recently checked for fault codes and found a fault with the egr system so I replaced the valve and the solenoid. There have been no fault codes since but I still have the problem. I think I'm going to try replacing the coolant temperature sensor and the thermostat first to make sure the the engine is running at the right temperature. Then if that doesn't work I'll replace the IAC and the gastket between the throttle body and the manifold. Thanks, Jason



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