1994 GMC Sonoma random misfire - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 10-23-2014, 06:51 PM   #1
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Unhappy 1994 GMC Sonoma random misfire

so my truck has been the hardest vehicle ive ever had to troubleshoot. ill try to keep this as short as i can.

Originally before i got the truck it had fouled the plugs and set for a long time. well i did a tune up and got it running. i noticed it was runnin super rich so i did me research and found out the most likely cause was knock sensors. so i replaced them. it helped but didnt cure it. so because the way these trucks are set up without a thrown code they are really hard to diagnose. OBD1. anyhow because of this i eventually replaced all sensors. map,tps,pick up coil, icm, knock, ignition coil, coolant temp. well this all helped but didnt completely cure it. so i finally replaced the spider injector. problem solved. almost. still getting horrible gas mileage. like 14 on a good day and i have noticed that now all is right that when you drive at a steady speed at any rpm that randomly its like it just quits firing like u turn the key off last for a half a second and continues on but then throws a code. code wont stay on and only says random misfire anyhow. i have had this truck fry 2 ignition control modules for no appearnt reason. it has a autostart on it i did not install it was there when i got it. from what i can tell it was kind of a sloppy job and i am begining to think this could be part of it i have unhooked some of its connections. any thoughts would be great. im outta ideals.

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Old 10-24-2014, 08:14 AM   #2
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how old is the plugs wires cap rotor fuel filter? check fuel pressure?
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Old 10-27-2014, 07:33 AM   #3
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like i said in my post earlier everything is new, fuel pressure is good, fuel filter is new. the only things that hadnt been replaced in the last few months is O2 sensor about 18 months ago, and pick up coil in the distributor was replaced about 12 months ago. every other sensor including plugs, wires, cap, and rotor have been replaced in the last 6 months. i live in alaska so pulling the distributor at this time of the year is not very fun. last time i had to do it it was in febuary and a balmy 10F degress
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Old 10-27-2014, 08:18 AM   #4
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does it sit alot? the cap and rotor like to corrode in areas like that. i used to have to clean my terminals on my jeep every couple months
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Old 10-27-2014, 02:27 PM   #5
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I had one of the early vortech V6 have a rich condition and I found the fuel pressure regulator was bad. on that engine they were under the intake so it didn't have a vacuum line. To check it your going to need to pull the upper intake and get a vacuum pump and see if the diaphragm is blown in it.
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Old 10-27-2014, 06:18 PM   #6
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yeah i just replaced the spider injector/regulator. regulator was pouring fuel in the intake. also replaced the fuel line nut kit while i was in there. didnt wanna have to tear it back down again. took a drive the other night about 110 miles round trip seem to get about 17 or 18 mpg on the hwy at 65 to 70 mph on slick roads in 4x4 HI part of the time. in town sucks about 13 or 14 mpg. i have also checked and cleaned most connectors. it only set for about 10 months and has been a daily driver for about a year now even with all the mechanical problems. been tinkering with cars most of my life so most of the time i can diagnose and repair any problem with reliteve ease. but a misfire that just wont go away no matter what steps i take has really got me stumpped. it always starts first try even at 20 degree weather. runs a lil rough til it gets up to temp then about 85% of the time or more it runs perfect a small shutter at idle is all. plenty of power even from a stand still. 0-100 in less than a mile (found out by accident... oops). i have been really impressed with the preformace of this truck. it doesnt smoke, rattle, or knock and has about 178,000 miles on it. very tight motor.
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bad gas mileage, misfire, rich condition, rough idle, surging

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