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454 tbi idels to fast

 
zimmerbeme
I love my Chevy Chevette!
Posts: 7
Joined: 02/08
Posted: 02/04/08 10:51 PM

i have an 87 gmc 3500 with a 454 that is tbi. engine idles high replaced map sensor but dosn't seem to help. however when i disconect the vac line to the map it idles down but runs poorly lacks power and will misfire back through the top end when excelereating. before i go and start replacing sensors....dose anyone have an idea as to wich it may be? i was thinking throttle position sensor....? any help would be great thanks.  

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waynep712
I have an SS396 tatoo
Posts: 264
Joined: 01/08
Posted: 02/05/08 12:08 PM

several things could cause this...  but you really need a scan tool to see exactally what is happening...

the wrong thermostat.. other that 192/195 or no stat may cause hi idle problems as the computer slows the idle down when it comes up to operating temp... (190 to 220F)

the coolant temp sensor or wiring to it...  black and yellow twisted pair to the sensor on the front if the intake manifold.. this tells the conputer how warm the motor is so it can set the proper mixture..   sometimes the sensors go bad.. sometimes the pigtails go bad.. and i have found the twisted pairs in the harness with insulation falling off and shorting out...  a truly intermitant problem...

these throttle bodys also suffer from throttle shaft wear..   with the engine off.. and the return spring unhooked... open the throttle slightly... then see how much play there is in the shaft/bore...  

what happens is they leak air... they also let the throttle close to slightly differnt positions... this throws off the throttle position sensor voltage readings to the ecm...
when the key is turned on the voltage from the throttle position sensor is read by the ecm.. this is closed throttle voltage... if the bore is worn and it goes below this voltage it acts strange...and may set a trouble code ...

there are two types of idle control possible.. an idle speed control motor on the front side of the throttle body.. this works the throttle lever...

and an idle air control motor... this is a big hex with wires plugged into the top on the rear side of the throttle body...    idle air counts (IAC) show up on a scan tool.. 20 to 50 is normal at idle...




the scan tool is required to try to fix these with out just replacing parts....

 i will come back and edit and add more info later tonight..  

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zimmerbeme
I love my Chevy Chevette!
Posts: 7
Joined: 02/08
Posted: 02/05/08 04:52 PM

well i have since replaced all of the spark plugs and i have never seen plugs in such bad condition. neglect from the previous owner. i also replaced the pcv and it now seems to idle where it should, however when trying to start the engine i observed the fuel coming from the injectors and it seems inconsistant in that i have to stop cranking the engine and then re crank it several times until it starts. not sure if there is sposed to be a steady spray of fuel while cranking the engine over to get it to start or what. when atempting to drive it seems to bog and has no real exeleration. it is no longer missing nor backfires, i belive that was due to the plugs. so one thing fixed but not entirely. thanks for all advise.  

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55Guy
Big Block power for the win
Posts: 895
Joined: 08/07
Posted: 02/06/08 06:47 AM

I'd take a look at the throttle position sensor. Some of the sysmptoms you describe could be releated to a bad TPS. This could also be affecting your idle.  

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zimmerbeme
I love my Chevy Chevette!
Posts: 7
Joined: 02/08
Posted: 02/06/08 09:54 AM

i am thinking about buying an obd 1 code reader off e-bay. will this help me to diagnose the bad sensor or sensors including the tps? i just want to make sure i replace only what is needed cause they aren't all that cheap but i am. Grin  

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waynep712
I have an SS396 tatoo
Posts: 264
Joined: 01/08
Posted: 02/06/08 10:43 AM

code readers only give you codes.....if it sets one... .  

scan tools read live data..... and codes... lets you see what the computer is looking at...

you can read the codes on that year with a short legnth of wire... jumping the a to b terminal... and counting the flashes of the check engine light....

there are a lot of scanners out there... and scammers...

actron and other brands have some scanners for slightly under 100 bucks. i have not kept up... kragen stores have a kit that does obd1 and 2 for 2 bills..so it can be used on all 96 and newer cars too.. those you cannot just jump.. it takes a code reader or scanner to turn off the check engine light... and don't do that if it is just going in for smog testing.. it will fail it..96 and newer...


copy and paste this link

http://www.autozone.com/az/cds/en_us/0900823d/80/11/4b/c4/0900823d80114bc4/repairInfoPages.htm

it is the online repair manual for your truck  free from autozone...  

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zimmerbeme
I love my Chevy Chevette!
Posts: 7
Joined: 02/08
Posted: 02/06/08 02:42 PM

hey thanks for the help as i was able to read the codes with the dash light and this is what came up a code 51 and it was all that came up. Code 51—Faulty or improperly installed PROM or calibration chip in the ECM. The ignition must be on for approximately 10 seconds before this code will set.

so what do i do i have no idea where the ecm is located or what to do with it when i find it. is the chip bad or what? i'm clueless at this point Confused  

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55Guy
Big Block power for the win
Posts: 895
Joined: 08/07
Posted: 02/07/08 06:26 AM

ECM in that truck is located behind the glove compartment. It's pretty easy to get to, four screws to pull the glove compartment out, and four clips that hold the ECM in.

That code tells me that the ECM has gone belly up, you're gonna need to get a new one. The good news is, a new computer for your truck is only about $80.  

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zimmerbeme
I love my Chevy Chevette!
Posts: 7
Joined: 02/08
Posted: 02/07/08 09:37 AM

ok tryed to reseat the chip ..... same result so do replace the prom chip, computer, or both a/z has an aftermarket performance chip for around $120.00 the dealer wants $152.00 for just a stock one what would be better? as far as the ecm not sure where toget one of those.....a/z as well?  

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waynep712
I have an SS396 tatoo
Posts: 264
Joined: 01/08
Posted: 02/07/08 08:14 PM

get a complete used ecm from the junk yard... with the exact chip that you need...


mitchell on demand has a prom number section..  where you can enter the applicaton...  and the differnt codes for proms are listed... there are mid year updates...    post the ecm number and the prom codes..  and someone with on demand will answer..  with the updated numbers to save time junkyarding...

when you call  the professional junk yards... say as they answer brain box 88 gmc 3500  454.. number  122??????  just give teh last four numbers... dont worry about the 122 as they dont even bother with that...


A-1 cardone has all the chips that they pulled from the cores they received and are selling them as needed..... i dont have their number.. most parts store managers will know how to contact them.... it is in the back pages of the a1 cardone ecm catalog..

where are you at...  

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zimmerbeme
I love my Chevy Chevette!
Posts: 7
Joined: 02/08
Posted: 02/07/08 09:23 PM

i am located in twin falls idaho.....  

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chevybuilder.
I love my Chevy Chevette!
Posts: 1
Joined: 02/09
Posted: 02/15/09 07:51 AM

you are on the right track with your 454.  REally if your problems are limiited to a code then youare really lucky.  I have one of those trucks and they are riddled with problems.
You should just begin by replacing the cam... it is a smog cam and it makes the truck run HOT.  If your compressiion is within acceptable limits consider yourself very lucky...   replace the cam and then proceed fromthere....  

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