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Username Post: miss fire under load        (Topic#225581)
kartracer46 
Forum Newbie
Posts: 35
kartracer46
Loc: Norfolk, VA
Reg: 09-09-09
09-09-09 07:03 PM - Post#1769212    

I have a 1989 GMC Suburban, 5.7L, TBI, 700r4 trans

I have an issue similar to "88lowbowtie". lets say i'm cruisin 45mph and its in OD already, if i try to speed up without letting it kick down, it feels like a horrible missfire and runs rough. if i put it to the floor it clears up and runs ok but the SES light is on. When I check the codes a 43 pops up. I tried disconnecting the knock sensor, no change. I don't know how to bypass the ESC module to test without it.

I have done the following:

new cap,rotor and wires
new plugs
new map sensor
verified egr valve and solenoid are operating
new fuel pump and intank filter
new fuel filter (inline)
new fuel pressure regulator
flushed supply and return lines from throttle body connection to tank connection

any help would be greatly appreciated.
Motorsport Junkie

My rides:
1989 GMC Suburban
1994 Chevrolet Suburban
1985 Chevy Suburban (Went to Bowtie Heaven)
2003 Nemesis Racing Kart


 
Low priced Genuine GM Auto Parts
88lowbowtie 
Forum Newbie
Posts: 57

Loc: kentucky
Reg: 07-26-09
09-11-09 01:02 PM - Post#1770125    
    In response to kartracer46

The ECM did not detect a knock signal during near wide open throttle operation with coolant temperature above 194� F or the knock signal was present for 4 seconds or more during normal engine operation.

thats your code 43, seeing you tried the knock censor i guess you knew that already. lol try the ignition module under your distributer cap. its what the 2 plugs plug into on the distributor on the side. has 2 screws in it, same head as your distributer cap screws. then the 2 plugs on the outside and one on the inside. that is what everyone told me mine was, but it ended up being a broken spark plug.
1991 chevrolet s10 short bed ext. cab
4.3tbi, 700r4
camaro irc-z wheels.


 
someotherguy 
Moderator
Posts: 25747
someotherguy
Age: 44
Loc: Texas
Reg: 08-01-03
09-11-09 02:22 PM - Post#1770165    
    In response to 88lowbowtie

Check base timing with the timing bypass wire (tan w/black stripe) disconnected. Should be zero for factory, up to maybe a max of 4 degrees advanced.

Either the system is producing a code 43 because the self-test for the knock sensor can't produce a knock (it momentarily advances the timing to force a knock) because the base timing is too retarded, or it's knocking constantly because it's too far advanced.

Richard
94 C2500LD / 97 Suburban LS1 / 06 300C SRT8
Check out my truck shop projects


 
boogie 
Senior Member
Posts: 2143
boogie
Loc: Louisiana
Reg: 07-11-02
09-11-09 05:16 PM - Post#1770259    
    In response to kartracer46

I bet your problem is distributor related. As problematic as these are for this era truck,I allways buy a new one to replace a high mileage one...even if I know it's just a module.
'85 GMC C1500 SWB 355 sbc


 
kartracer46 
Forum Newbie
Posts: 35
kartracer46
Loc: Norfolk, VA
Reg: 09-09-09
09-11-09 08:27 PM - Post#1770376    
    In response to boogie

thank yall' for the input. since the post i replaced the knock sensor and now the SES light stays off, but i still have the miss fire and no code. i think boogie may be right. i am going to try a new/rebuilt distributor with all new components and see if that cures it. i checked the timing like "Someotherguy" said to and it is setting on 0 just as it was set from the factory.

thanks again, more heads are always better than my one.
Motorsport Junkie

My rides:
1989 GMC Suburban
1994 Chevrolet Suburban
1985 Chevy Suburban (Went to Bowtie Heaven)
2003 Nemesis Racing Kart


 
kartracer46 
Forum Newbie
Posts: 35
kartracer46
Loc: Norfolk, VA
Reg: 09-09-09
09-26-09 07:23 PM - Post#1778804    
    In response to kartracer46

found the problem. it was a vacuum leak. the throttle body was either oblonged or there was a bushing missing where the rod for the butterflies pivoted. it appears it was leaving just enough air by to foul up and cause a lean condition under load. it runs good now, i just have to adjust the timing back to 0. 4 degrees seems to be a bit much since I installed the new distributor and replaced the throttle body. it pulls like an ox, it is just lacking a bit on idle after setting at lights in gear. it seems to load up just a bit, then clears after you take off.
Motorsport Junkie

My rides:
1989 GMC Suburban
1994 Chevrolet Suburban
1985 Chevy Suburban (Went to Bowtie Heaven)
2003 Nemesis Racing Kart


 
brucemc777 
Forum Newbie
Posts: 99

Reg: 06-14-10
06-14-10 02:59 PM - Post#1930151    
    In response to kartracer46

kartracer46 - are you still out there? Have a near-identical problem with a 1994 CK2500. Not anywhere as knowledgeable as you - would like some details.

 
Low priced Genuine GM Auto Parts
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