Hey everybody. I have some major issues with my 99 Jimmy 4.3 4X4. Ill start at the beginning:
It all started when the fuel pump finally quit. I knew it was going, so I decided to order a bunch of parts while it was in the garage anyways.
Heres a list of all the parts i changed:
Fuel tank & sending unit
Fuel filter
front wheel hubs
Changed ex. manifolds for some headers
Coolant temp sensor (broke off installing headers)
Plugs, wires, cap and rotor
battery
And a couple things in the steering but it shouldn't affect what the truck is doing.

I get the truck all back together, and it was running rough at first, but seemed to clear up. I took if for a road test and i noticed it didn't have all the power that it had before, and gradually it started to get worse and worse. Now its at the point where it will stumble horrible on take off, runs extremely rough on the highway, and backfires/misfires under hard acceleration. It also shifts extremely early (1400RPM 1-2) when its acting up at its worse.

I got a bunch of dtcs(i cant remember what specific numbers they are) but i do have a P0300 and the other dominant one is TPS Sensor Intermitant low voltage. I have gotten O2 codes, MAF codes, and IAT codes but they are more random. I also have an erratic wheel speed sensor code as well. I have checked:
Plugs, wires, cap, and rotor- All new
swapped out tps, maf, o2's and iat-no change
checked fuel pressure - 59 psi

I suspect that it may be a wiring issue since it just happened after i did all this work. If anyone has any info or guidelines I can use to try and fix this, then I'm all ears!

 

Proper diagnosis starts with all of the present & pending DTCs. Please get an accurate list of the DTCs that are present and pending then post it in this thread and we'll go from there.

What brand of cap & rotor did you install? These trucks prefer AC Delco caps & rotors.

 

I only use AC Delco ignition parts because I have had problems before with other brands. I will try to get a list of codes as soon as I can.

 

Ok, sorry about the wait. I had 3 codes thrown (po122, po452 and p1122). Also before i had a p1111, but it didn't come up again. In effort to try and cure it, i borrowed a TPS, IAT, and mass airflow off my dads jimmy just to see if any were the problem, but no cure.
It seems like the problem gets worse with temperature. When the engine is at running temperature, if i floor it on the highway it will backfire and miss. Earlier i had it on the Tech 2 and it showed #2 and #3 cylinder almost dead, and #1 starting to miss at one point under really hard acceleration.
I had to stop working on it today because i was about to try and limp it over to the wreckers. I love my truck but its just getting really frustrating. Any one have any ideas on where to start?

 

If your truck still has a catalytic converter have it checked out, I had very similar symptoms with my 99 blazer. I took it into the shop because I was stumped, I was sceptical when the mechanic told me what it was but it ran excellent after having it replaced.

 

I already had to punch out the catalytic because it plugged up about a year ago. Thats the first thing i would suspect if i hadn't already done it. Im wondering if the fuel pump just letting go can cause debris to end up in the injection system or would the fuel filter be enough to stop it? I changed it but the old one wasn't anywhere near being plugged. ANd even so, i guess that wouldn't cause my electrical issues that i have been having would it? Ive never had such a multitude of symptoms all happen at once.