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high idle on 86 corvette

 
96k1500
I love my Chevy Chevette!
Posts: 5
Joined: 06/12
Posted: 08/26/12 03:41 PM

hi i have 1986 vette the problem i am having is after it gets up to operating temp the idle just keeps climbing. until it hits about 1800rpm. when i unplug the IAC valve it does not affect it. when i unplug the EST wire the idle goes back to fairly normal. now keep in mind i;ve had this thing for a week. any ideas? please help  

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waynep71222
I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 08/26/12 04:32 PM

this kind of issue usually requires a scan tool to look at the live data stream... so you can see what the computer is seeing.. and see what the computer is requesting the devices to ***

i along with a bunch of others are very familiar with the L98 5.7 TPI motors..

a few known failures...

the Coolant temp sensor and its pigtail connector..   easily replaced.. under 20 bucks if you know the right part number for the pair.. SU102   at AZ stores..

your idle air control motor could also be going bad...  but i would bet that you have already replaced it..   the pigtail connector could be intermittent or the wires fatigued..  the computer might also need to be resoldered... as the idle air control motor is a stepper motor..  i have had other cars mostly 87 to 90 caddys with similar issues that the idle speed control motor lost its ability to back off do to the bad solder joints in the computer..   not the job for a first timer with a solder iron.. and you have to use lead, tin and 2 or 3 percent silver solder to get a good joint.. not LEAD FREE..   some of the NOID Light test kits you can borrow from the auto parts stores have a idle air control motor noid light.. to see if the signals are actually coming through to the connector..
the computer uses the idle air control motor like a dash pot..  as you bring the engine rpms up.. it closes it for a second. then backs it way out to around 110 counts.. so when you close the throttle.. the computer can close the idle air control valve bypass to avoid excessive high intake manifold vacuum that pulls excessive fuel from the very short pulses of the injectors during this close throttle high rpms


how are the computer ground connections on the engine.. if they are loose.. you will have issues..

can you also.. engine off.. grab the throttle lever... open it slightly... look carefully.. see if the throttle body shaft wiggles around in the bore... this can cause the throttle blades to contact the sides of the bore. causing the throttle lever to close to a slightly different position each time...
the TPS voltage should be 0.53 volts .. each time you close the throttle.. without any variations..  if the throttle shaft bore is worn..  you will get higher and lower voltages when the throttle is closed...  when they are higher..  the computer things you have your foot on the gas pedal like you are at the starting line with the staging lights on..  foot on the brake and throttle  ready to go.. it will back the idle air control out and richen up the fuel injector pulse..


if you don't have an OBD1 scan tool.. depending on where you live.. you might want to check your local craigs list for OTC..  and then scantool   there are really cheep right now.. OTC monitor 2000s that only have 2 lines of data on the display.. but can be purchased for 50 to 70 bucks...  they only work on cars up to 1989...   OTC monitor 4000E and a bunch of other variations can be used on cars up through 94 and with proper cables (very expensive) up through about 2002.. but OBD2 cars are easily scanned with OBD2 scan tools..

here is a monitor 2000

OTC2000

here is a 4000E.. there are a lot of variations on these..

Otcmonitor4000e

i have a lot more info... but like i said.. its easier to look at the live data stream...

warning. the innova OBD1 scan tools will NOT read live data.. only codes and its very hard to fix these without live data...  

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waynep71222
I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 08/26/12 04:40 PM

you will want to print this chart

TPISCANDATA87

these numbers are close..

if you can get at a scan tool... get some live data with the engine cold..  

RPMs.
coolant temp
manifold air temp
air flow/MAF..
injector pulse width.
TPS voltage.
IAC..
integrator
block learn
oxygen sensor cross counts.
open loop .. closed loop.

do this for cold before start up..

warm

and hot idle...

please.. take the time and post the numbers .. i can decipher them..  

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96k1500
I love my Chevy Chevette!
Posts: 5
Joined: 06/12
Posted: 08/26/12 06:39 PM

alright there are a few things that are in question. somewhere along the way somebody put after market gauges in this thing and it got switches to turn the fan on but i think they have it wired into the relay some how. because the fan come on.every know and then with the switch off. plus i realized today this thing had a digital dash in it before so i hope they didn't mess anything up back there  

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waynep71222
I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 08/26/12 07:31 PM

wow....  

back to the question.. do you have a scan tool...   can you access the live data stream..

more wow..

do you have a proper corvette engine control wiring diagram??

do you have the dash board wiring diagram???

the dash works with the ECM and the BCM on most of these..  

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waynep71222
I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 08/26/12 07:56 PM

85 89Pfiwiring


dual radiator fans on the left side..   single which is usually only 84 on the right...


85 89Corvettecoolingfanwiring

Radiatorfranswitch S500sideview

this is the radiator fan switch.. usually located in the right cylinder head between the #6 and #8 spark plugs above the starter motor... this closes at 228F to 235F usually..   this Usually only turns on ONE radiator fan..  the slightly later cars have 3 radiator fan relays.. yours probably only had 2..   this grounds to the block..  causes the relay to activate and powers the fan..

the other relay  is controlled by the ECM.. when the engine coolant temp reached 228 to 235F as seen by the engine coolant temp sensor in the front of the intake manifold with a black and a yellow wire on it..


this is the sender that   fits in the LEFT cylinder head.. between the #1 and #3 spark plugs. its for the TEMP display on the digital dash board...  since that is gone.. i would expect that this part has been replaced with something else to drive the aftermarket temp gauge..

TU66



one more thing...  these TPI motors were designed to idle at about 450 RPM in drive 10 to 12 seconds after coming to a complete stop...   when coming down from speed..  the alternator will stop charging for exactly 10 seconds.. then  come back on.. all by itself.. its programmed into the voltage regulator if it has the CS alternator on it..    

do check for throttle shaft bore wear... it will throw you for loops...  you will also need to set minimum idle on the Throttle body.. with the idle air control motor at 0 steps so no air is bypassed...  minimum idle on that car is probably 400 RPMs.. in park..  maybe 425..

operating temp idle is like i said.. 450 to 500 RPMs..   idle air counts 20 to 50.. coolant temp on a scan tool 215F to 240F..   TPS voltage closed 0.53 volts..


why go through all this..    these cars when perfect.. can knock down as i understand.. close to 30MPG on highway runs..  

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pepsi1
I mow my lawn and find Chevys
Posts: 1718
Joined: 09/11
Posted: 08/26/12 08:27 PM

Look for a vacuum leak with a can of brake clean. Try note to spray much around the intake. That will give you a false positive. Spray all the vacuum connections.Start there!  

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96k1500
I love my Chevy Chevette!
Posts: 5
Joined: 06/12
Posted: 08/28/12 05:40 AM

well i'm back as far as me having a live feed scan tool no i don't.what is a brand name of one i can get? i found the sending unit for the fan it is on the front of the intake. this car only has one fan to.let me run this by you there is this guy in ruma that builds stand alone harness for these motors fairly cheap.do think i should give him a call? since i'm kind of trying to figure out sombody else mistake.  

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waynep71222
I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 08/28/12 09:28 AM

you have one or two small problems that a New harness will NOT CURE...

edit

Piftpiecmvoltagepinout

C4cluster 32Pin  

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waynep71222
I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 08/28/12 09:02 PM

talked to you today.. you mentioned the AFTER market speedo was displaying highway speeds while stopped...  we talked about the idle air control valve backing out at highway speeds to act like a dash pot...  so the closed throttle does not create HIGH intake vacuum that causes excess fuel to be pulled uncommanded through the minimal injector pulse width duration.. causing increased emissions and reduced fuel economy..

on the image above.. the vehicle speed sensor in the tail housing is connected to pins C15 and *** the cluster converts the signals somehow into a pulse the ECM can handle and sends that out to the ECM VIA pin D12..

so the cluster pin D12 goes to ECM pin A10... probably a brown wire.. warning. there are several brown wires.. use care....

i wonder if the prior shoe makers who removed the digital dash.. wired the ECM to the unbuffered speed sensor signal...  i don't know if i can find easily what sort of buffered signal that you need. but GM cars did use mechanical speedos with LED emitters that reflected off the spinning plate on the back of the speedo drive plate .. these buffers were on a circuit board...

camaros and firebirds also used this same set up without The DIGITAL dash board..   so robbing the buffer electronics from one of those cars..  i just don't know at this point in time..

one of the swap specialists probably has something to fix this issue..

it all depends on what was done behind the dash board.. did they hook up the wire from the digital dash connector  pin D12 to the new digital speedo...  

does the digital speedo have a proper terminal to drive the speedo...????

is there a problem with the new digital speedo wiring since they bypassed it at the back of the transmission.. perhaps it was supposed to be shielded twisted pair wire instead of just open wires... i have not seen the car you are working with..

i did try calling. but could not get thru..  this is better..  ring if you need to...  

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pepsi1
I mow my lawn and find Chevys
Posts: 1718
Joined: 09/11
Posted: 08/29/12 01:25 AM

96K1500:

I think you answered your own question. If unplugging the IAC had no effect on the idle pull the valve out and see if there is some carbon holding the valve from doing it's job. Now that valve ratchets so you won't see it move very fast. Clean that whole valve and inside the base of the valve area. Use carb clean,if need be mark the position the valve was in before you removed it.I think what has happened the carbon got under the pintle and as the engine warms up it trying to shout down to give you a normal idle. With carbon or the valve maybe defective which I don't think it is,or it wouldn't be trying to close the air off as the engine warms up.Good Luck...

Bob  

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waynep71222
I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 08/29/12 04:40 AM

some of thats a great idea..

unplug the EST connector to get the  idle speed down...

then unplug the Idle air control valve..    leave it unplugged. and hook the EST connector back up..

if the idle comes back up to crazy out of control with the IAC unplugged then its NOT the IAC being commanded to step back out to act like a dash pot...



i was always warned to not attempt to let the computer command the IAC valve with the valve removed from the intake or throttle body.. as the computer will attempt to reach 0 counts..  since there is nothing firm to stop it.. it will just push the pintle out till it drops out the end of the threads.. since the threaded shaft is prevented from rotating by a keyway. its going to take an idle air control valve to reinstall it..   i only dropped one out... learned my lesson..


also.. i look in your area... saw a snap on scan tool listed for cheep...  found it by searching craigslist   for snap on... it did not show up when i searched scan or scan tool...  

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priya
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Posted: 08/26/14 05:44 AM

Ooo  
Priya
priya

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priya
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Joined: 12/07
Posted: 08/26/14 05:45 AM

Ooo  
Priya
priya

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