[SOLVED] extream high idle problem on 1990 chevy 1500, 4.3 v6 TBI

NID_FTL · 02-26-2008, 08:44 PM

I bought a 1990 Chevy 1500 pickup several months back. it idles around 1500 to 1800 rpm and it is tearing the transmission apart. I have checked all vacum lines and replaced the oxygen sensor on the exaust, also replaced the intake manafold gasket and the Idle air control IAC, I have run OB1 diagnostic and only code 12 present. yet i still have an extream high idle problem, any sujestions, thanks in advanced Jim S...

cardoc · 02-27-2008, 03:58 PM

Hi NID FTL
Two things come to mind very quickly.
1. Leaky throttle body base gasket.Very comon on this engine.I don't know if you serviced this when you pulled the intake.
2.Misadjusted or bent throttle linkage.You don't know what was done before you got to it.
If these ck ok there are other more complicated issues you would need a scan tool to ck.
IAC count, EGR percent,TPS etc.They all have an effect on idle speed and may not set codes in an OBD 1 system.Don't forget a leaking pcv valve (Too obvious).Also a bad coolant temp sensor or connecter.Try wiggle test on 2 wire connecter near thermostat housing with it running.You may get lucky.Harness has been known to go bad at sensor connecter.
Good luck,Cardoc.

NID_FTL · 02-27-2008, 09:16 PM

Hey dude, thanks for the info, will take me some time to get thru all the test. However i just went out and checked the pcv valve and it is in good shape, however there is only one on one side, the otherside is vented str8 into the TB, is this normal or should there be a valve on both sides? What is a TPS? I replaced the IAC with a brand new one. Can an EGR valve cause this extream of an idle condition? when I press on the EGR diaphram the idle slows and I can actualy slow the idle to what seems normal to my ear. To change the TBI gasket do I have to do the intake manafold again?

to do the code reads I have been simply jumping pin 1 to pin 2 in the ALDL and counting "check engine" blinks.
Thanks again, Jim S...

cardoc · 02-28-2008, 04:37 PM

Good afternoon.
Yes the pcv valve circuit you decribe is correct.You might call it flow thru filtered crankcase ventilation.TPS is throttle position sensor-probably 3 or 4 wire sensor at throttle linkage. EGR valve is exhaust gas recirculation valve.Valve diaphram should only lift above idle with road speed.Could cause high unsteady idle if internal leak but not likely on that type.When you lift it,it delutes air fuel mixture with exhaust.My money is on throttle body base gasket,have seen it many times and is cheap to fix.You could very carefully(!!!) try spraying a little carb cleaner at base while running to confirm this-rpm should change noticably.And yes I do mean very carefully.The IAC is run by the computer and may be fully closed but engine is getting air thru vacuum leak such as base gasket.To change base gasket simply lift throttle body not entire manifold.If bad it will look like it's been sucked into one of the ports.
Good luck Jim
Cardoc.

NID_FTL · 02-28-2008, 07:58 PM

Dude you have been awesome. I will let you know how it goes, soon. Thanks a million. Jim s...

NID_FTL · 02-29-2008, 04:39 PM

Thank you so much. I whiped that out in less than an hour, was obivious as soo as I got the TBI loose that the thing has been sucking in air for a long time. With the new $5 gasket it purrs like a kitten. I am amazed how nice it idles now. I was only getting 12 to 14 miles a gallon, hope to get more like 16 to 18 now. I can not thank you e-nuff...
Jim s....

**qldit** · 02-29-2008, 04:53 PM

Good Morning Gentlemen, I was following this thread with interest, and must congratulate you both on an excellent solution very well executed.

Very well done!

Cheers qldit.

cardoc · 02-29-2008, 05:10 PM

Good evening Jim.
Very happy to help.
Cardoc.

02-26-2008, 08:44 PM	#1 (permalink)
NID_FTL Registered User Join Date: Feb 2008 Posts: 5 OS: xp home sp2	[SOLVED] extream high idle problem on 1990 chevy 1500, 4.3 v6 TBI I bought a 1990 Chevy 1500 pickup several months back. it idles around 1500 to 1800 rpm and it is tearing the transmission apart. I have checked all vacum lines and replaced the oxygen sensor on the exaust, also replaced the intake manafold gasket and the Idle air control IAC, I have run OB1 diagnostic and only code 12 present. yet i still have an extream high idle problem, any sujestions, thanks in advanced Jim S...

02-27-2008, 03:58 PM	#2 (permalink)
cardoc Registered User Join Date: Apr 2007 Location: Ct. Posts: 198 OS: win xp sp2	Re: extream high idle problem on 1990 chevy 1500, 4.3 v6 TBI Hi NID FTL Two things come to mind very quickly. 1. Leaky throttle body base gasket.Very comon on this engine.I don't know if you serviced this when you pulled the intake. 2.Misadjusted or bent throttle linkage.You don't know what was done before you got to it. If these ck ok there are other more complicated issues you would need a scan tool to ck. IAC count, EGR percent,TPS etc.They all have an effect on idle speed and may not set codes in an OBD 1 system.Don't forget a leaking pcv valve (Too obvious).Also a bad coolant temp sensor or connecter.Try wiggle test on 2 wire connecter near thermostat housing with it running.You may get lucky.Harness has been known to go bad at sensor connecter. Good luck,Cardoc.

02-27-2008, 09:16 PM	#3 (permalink)
NID_FTL Registered User Join Date: Feb 2008 Posts: 5 OS: xp home sp2	Re: extream high idle problem on 1990 chevy 1500, 4.3 v6 TBI Hey dude, thanks for the info, will take me some time to get thru all the test. However i just went out and checked the pcv valve and it is in good shape, however there is only one on one side, the otherside is vented str8 into the TB, is this normal or should there be a valve on both sides? What is a TPS? I replaced the IAC with a brand new one. Can an EGR valve cause this extream of an idle condition? when I press on the EGR diaphram the idle slows and I can actualy slow the idle to what seems normal to my ear. To change the TBI gasket do I have to do the intake manafold again? to do the code reads I have been simply jumping pin 1 to pin 2 in the ALDL and counting "check engine" blinks. Thanks again, Jim S...

02-28-2008, 04:37 PM	#4 (permalink)
cardoc Registered User Join Date: Apr 2007 Location: Ct. Posts: 198 OS: win xp sp2	Re: extream high idle problem on 1990 chevy 1500, 4.3 v6 TBI Good afternoon. Yes the pcv valve circuit you decribe is correct.You might call it flow thru filtered crankcase ventilation.TPS is throttle position sensor-probably 3 or 4 wire sensor at throttle linkage. EGR valve is exhaust gas recirculation valve.Valve diaphram should only lift above idle with road speed.Could cause high unsteady idle if internal leak but not likely on that type.When you lift it,it delutes air fuel mixture with exhaust.My money is on throttle body base gasket,have seen it many times and is cheap to fix.You could very carefully(!!!) try spraying a little carb cleaner at base while running to confirm this-rpm should change noticably.And yes I do mean very carefully.The IAC is run by the computer and may be fully closed but engine is getting air thru vacuum leak such as base gasket.To change base gasket simply lift throttle body not entire manifold.If bad it will look like it's been sucked into one of the ports. Good luck Jim Cardoc.

02-28-2008, 07:58 PM	#5 (permalink)
NID_FTL Registered User Join Date: Feb 2008 Posts: 5 OS: xp home sp2	Re: extream high idle problem on 1990 chevy 1500, 4.3 v6 TBI Dude you have been awesome. I will let you know how it goes, soon. Thanks a million. Jim s...

02-29-2008, 04:39 PM	#6 (permalink)
NID_FTL Registered User Join Date: Feb 2008 Posts: 5 OS: xp home sp2	Re: extream high idle problem on 1990 chevy 1500, 4.3 v6 TBI Thank you so much. I whiped that out in less than an hour, was obivious as soo as I got the TBI loose that the thing has been sucking in air for a long time. With the new $5 gasket it purrs like a kitten. I am amazed how nice it idles now. I was only getting 12 to 14 miles a gallon, hope to get more like 16 to 18 now. I can not thank you e-nuff... Jim s....

02-29-2008, 04:53 PM	#7 (permalink)
qldit TSF Gearhead Join Date: Mar 2007 Location: Brisbane Australia. Posts: 592 OS: All Systems, mainly Linux	Re: [SOLVED] extream high idle problem on 1990 chevy 1500, 4.3 v6 TBI Good Morning Gentlemen, I was following this thread with interest, and must congratulate you both on an excellent solution very well executed. Very well done! Cheers qldit.

02-29-2008, 05:10 PM	#8 (permalink)
cardoc Registered User Join Date: Apr 2007 Location: Ct. Posts: 198 OS: win xp sp2	Re: [SOLVED] extream high idle problem on 1990 chevy 1500, 4.3 v6 TBI Good evening Jim. Very happy to help. Cardoc.