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91 "J" body check engine light

 
netwarrior36
V-6 Camaros rule!
Posts: 100
Joined: 10/08
Posted: 09/25/13 12:34 PM

In my 91 J body car with a 2.0L 5 speed, the check engine light will come on...a lot but then go right back off as soon as i hit highway speeds but any slower and it likes to come on a lot. I've flashed the codes but all i get is the 12 code and not an actual code pointing me to the reason. any help on getting the proper code flashed would be greatly appreciated.

Almost forgot, when the light is coming on or turning off there is absolutely no change in the way the vehicle is behaving.  
--------------------
"If it breaks it was good, and if it doesn't break it's bad."

1978 Chevrolet Nova two-door hatchback

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waynep712222
I have an SS396 tatoo
Posts: 251
Joined: 03/13
Posted: 09/25/13 09:43 PM

go back and check all the

battery to engine block grounds..

battery to body ground cable..

engine to body ground cable..

and... engine to computer harness ground connections...

if any of those are loose or marginal.. you will have an issue...

this is the first place to check...

does the speedo work????

did you hook up the vehicle speed sensor? or wiring to it..

you might need to borrow an OBD1 scan tool..

or leave the jumper in the ALDL and take it for a test drive.. see if it will flash codes then...  

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waynep712222
I have an SS396 tatoo
Posts: 251
Joined: 03/13
Posted: 09/25/13 09:46 PM

i don't know if this is the exact chart for your body style...

Tbipinoutchart

TBISCANDATE87  

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netwarrior36
V-6 Camaros rule!
Posts: 100
Joined: 10/08
Posted: 09/26/13 09:51 PM

well i'll have to get around to checking the grounds but i think it has something more to do with temperature. when i started it up this morning it died almost as soon as it started, in order to keep it running i had to give it gas...iac maybe? then while driving down the driveway it died and had to be restarted.  all the while the engine light was on. when the vehicle finally reached operating temp the light was off and i had no other issues.  
--------------------
"If it breaks it was good, and if it doesn't break it's bad."

1978 Chevrolet Nova two-door hatchback

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waynep712222
I have an SS396 tatoo
Posts: 251
Joined: 03/13
Posted: 09/26/13 11:48 PM

a few things..

change the engine coolant temp sensor and its 2 wire pigtail..  at AZ stores is a SU102 and under 20 bucks... ask for it by part number.. not application.. trust me it fits.. and is the cause of MANY many issues...

looks like this..

Th Su102

one wire should have 5.0 volts from the computer.. the other goes back to the computer.. if you disconnect the coolant temp sensor.. it should set a code 15 when you start and run it...

if you short one to the other with a short jumper pin.. you should get a code 14 when you have run the engine... this proves the wiring is intact..



C4codes1

8322E570 2049 4052 828B 35Ed2ef25dab Zps9f74f5c8  

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waynep712222
I have an SS396 tatoo
Posts: 251
Joined: 03/13
Posted: 09/26/13 11:59 PM

did you attempt to adjust the idle speed by turning the idle stop screw???

this really requires some forethought..

please... engine off.. open the throttle part way with your hand.. wiggle the lever to see if there is play between the shaft and the shaft bore...

(o) is good..  (O ) ( O) is bad.  if you have play. you will have idle issues..

the far end of the throttle shaft has the Throttle position sensor.. this sends from 0.5 volts to 4.5 volts back to the computer to let it know what you are doing..

there is a minimum  idle voltage. that the computer relearns each time you turn the key on..  this is always less than 1.0 volt.. usually 0.53 to 0.80.. this varies between cars...

if the throttle shaft bores are worn.. the throttle can close to slightly different angles each time  this confuses the computer as it never knows when you actually have the foot off the gas pedal...

this excessive amount of play can change the amount of air bypassing the throttle when you allow it to close..

this air bypassing the throttle blades and the air thru the idle air control valve circuit is critical to a stable idle..

the idle air control motor..  is a stepper motor.. 2 sets of windings.. it can move a fraction of a turn in either direction depending on which way the voltage pulses are applied..

when the throttle is closed idle where the computer expects it.... engine at operating temp.. TPS voltage below 1.0 volts.. the idle air counts should be between 20 and 50 .. if the IAC (idle air counts) are less than 20.. you need to close the throttle farther.. if the IAC (idle air counts)  are above 50.. you need to open the throttle farther..  this give the computer enough range of motion on the idle air motor to handle most situations..


how the IDLE air control motor handles engine operation..

at cold start up.. the idle air control motor is back about 70 to 110 counts.. so you get fast idle at start up..

when you are driving down the road.. the idle air control motor bottoms out to zero for a split second to relearn where zero is..  it then backs out to 80 counts while you are cruising.. when you take your foot off the throttle allowing the throttle to close fully..  the manifold vacuum is not sent way high because the idle air control motor is backed out to around 80.. as you slow down. the computer moves it in.. so you don't stall the motor coming to a stop..


this is all complicated.. but its how ALL CURRENT electronic fuel injection systems work... some asian models use a shutter valve instead of an idle air control motor.. as it can react faster. but they tend to carbon up and cause stalling or lack of fast idle issues..  

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netwarrior36
V-6 Camaros rule!
Posts: 100
Joined: 10/08
Posted: 10/01/13 07:42 PM

I cleaned my IAC and it improved idle a bit but i've ran into bigger issues...Today I changed my plugs and wired, didn't mess with anything, started the car it ran smoothly. After I got off of work I started driving and in the mid rpm range the engine was jerking really hard but in low rpm and high it was smooth.  last time this issue happened i changed the ICM and the issue resolved itself but in doing so the retainers for the wire spades on the ICM crumbled from age and heat so I had to remember which order they went in, well no issue then to get it done. so the jerking continued so i stopped into my work (autozone) and pulled the ICM and ran in and tested it but it tested good...so i had to put it back in...i got the wiring wrong i guess and it won't start then i changed around some wiring, only to discover the on the 3 pin side there is a ground with an external ground running off of it...well that external broke off the distributor so i've gotta figure how to wire that but overall the biggest issue is getting the wiring straight...anyone got a diagram? i checked a haynes/chilton and i couldn't find a single matching diagram to get it back together so now it's stuck at my work...before i got picked up i checked the codes again for the heck of it and it went the flash...flash,flash ok 12 then a few moments later it was flash...flash...flash,flash,flash,flash,flash ok a 25 well that's a manifold air temp sensor but i really don't know why all this happened so suddenly let alone why it acted as it did when i changed the plugs and wires.  
--------------------
"If it breaks it was good, and if it doesn't break it's bad."

1978 Chevrolet Nova two-door hatchback

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waynep712222
I have an SS396 tatoo
Posts: 251
Joined: 03/13
Posted: 10/02/13 12:35 AM

hmm.. the wiring harness for the distributor is not cheep..

011 **486027 TERMINAL BLOCK (INCLUDES CAPACITOR)

this should be the module pinout.

http://www.wellsve.com/custom_searches/showpinoutdiagram.php?partno=DR134#zoom=100


B9b46c18 D67d 49E6 A0a1 E44d3ce338a2 Zpsf55f54ba  

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waynep712222
I have an SS396 tatoo
Posts: 251
Joined: 03/13
Posted: 10/02/13 12:36 AM

hmm.. the wiring harness for the distributor is not cheep.

011 **486027 TERMINAL BLOCK (INCLUDES CAPACITOR)


B9b46c18 D67d 49E6 A0a1 E44d3ce338a2 Zpsf55f54ba

91 Jbody 20 Enginecontroldiagram Zps50b529b7


i actually have the exact book that shows what goes where.. its a mitchell manual for ignition.. but its packed away in a massive stack of boxes right now..  

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waynep712222
I have an SS396 tatoo
Posts: 251
Joined: 03/13
Posted: 10/02/13 01:11 AM

part #11 is the harness and this is the wrong diagram.. its for a non electronic spark timing control motor..


91 Jbody 20 Distributordiagram Zpsb60b4381  

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waynep712222
I have an SS396 tatoo
Posts: 251
Joined: 03/13
Posted: 10/02/13 01:25 AM

if the magnet in the pickup coil is cracked.. it will create ghost patterns ..

what.. yep... drive people crazy.. timing jumps all over the place...

what happens is the AC wave form created by the reluctor spinning inside the pickup coil is used to trigger the ignition module..  when the voltage raises above 0.3 above ZERO it turns on the power transistor.  as the reluctor poles pass the pick up coil poles. the voltage drops thru ZERO into the negative range as the poles move apart.. this causes the power transistor to open and discharges the coil..

but with electronic spark timing.. EST..  this signal is sent to the ECM and it sends a control voltage back that varies a delay count down.. this  varies the delay. if the computer shortens the delay time. the timing advances.. if it lengthens the delay.. the timing retards..  this gives the computer full authority of the ignition spark timing..

when the pick up coil magnet or on some the reluctor magnet are cracked.. what happens is you get Ghost patterns in the nice AC sine wave. this confuses the ECM and random delay pulses are sent to the module.. so the ignition creates sparks out of the normal spark pattern.. so you get a weird random misfire..  

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waynep712222
I have an SS396 tatoo
Posts: 251
Joined: 03/13
Posted: 10/02/13 01:39 AM

this is the proper code chart..

1991 20L VIN K DTC1 Zpsaf5f3c49  

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Celina Wakley
I love my Chevy Chevette!
Posts: 1
Joined: 11/13
Posted: 11/26/13 11:25 AM

Hey there is a website from where you can decode that code...
Find code detailed info here.

http://checkenginecode.com/  

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dlacher
I love my Chevy Chevette!
Posts: 3
Joined: 02/14
Posted: 02/13/14 07:57 PM

My speedo doesn't work correctly.. I am having the code 12... Is this as simple as changing the speedo out? I haven't checked the grounds out as I just bought the truck.  I have codes 12 and 32... Any help is awesome, I know its an old post.  

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dlacher
I love my Chevy Chevette!
Posts: 3
Joined: 02/14
Posted: 02/13/14 07:57 PM

My speedo doesn't work correctly.. I am having the code 12... Is this as simple as changing the speedo out? I haven't checked the grounds out as I just bought the truck.  I have codes 12 and 32... Any help is awesome, I know its an old post.  

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