GMT400. Distributor, TBI, Fuel. What could it be? Feed for multiple common issues!

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Dillhart541

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New here. 4.3 Vortec (V6).

I seen it get brought up that it's not as common for the v6 dizzy caps/dizzy' to take a dump. Well, here I am to tell you first hand what's going on.

I have bought my gmt400 about 3 months ago and she was running about as good as a garbage can. Would start right up but as you drive, 1st gear mint, 2nd gear you start getting up in RPM and it would just fall on it's face, 3rd was a joke. Which led me to looking into my new trucks common issues.

Without even taking anything apart to check for warn out, defective parts. I went and bought a new set of plugs (NGK), wires, distributor and cap (Delco). Got home and just got right to the money.... I changed plugs and wires with ease. Then....

Got comfy cozy sitting in my engine and started on my dizzy. Unscrewed the cap, took it off, and the ENTIRE dizzy was rusted over wires and all. I'm almost 100% certain this is the original manufactured distributior made in 1989. It was amazing that this truck even started let alone drove. Anyways...

Popped my brand new one in and wired n fired. Never had a issue. runs perfect, drives perfectly, until 2 weeks ago.

2 weeks ago-- was off roading and hitting some puddles, was fine and fun until I got back on the main road. It started to give me a sputter again (not as bad as before) but highly noticeable in performance drop. About a week of driving it, through the week, I noticed if I floored it the truck would do a rapid backfire with no acceleration. But would do fine if I drove it like a normal citizen. So I just drove it with a Lil more care than I normally drive my vehicles.

During this week I was coasting down a hill and turned the truck off popped it in neutral and glided down. Come end of the hill, I turn key on, and slap her in second and give it a bump start. BAD IDEA, truck went ahead and said nope on the fuel. Dies, pull over, cranks but won't start. It had spark forsure because I has a lil butane as starting fluid that I used to confirm. I flat tow it to buddies and am wondering what to do while doing minimal things to maybe fix the no start issue . So just to move it forward a bit I pour gas in the TBI, and start it. Shoved it in gear and popped her forward a bit. STAYS RUNNING. Cool called it good. Couple days go by and.......

then yesterday happened. I was driving to the corner store I got there fine and as I'm pulling out of the parking lot I hit a pothole, Albeit a huge one. Truck gets shook forsure, then truck just has a stroke. It wouldn't idle for more than 10 seconds and without a Lil bit of throttle play it would stall, I couldn't get to any kind of speed without toasting my clutch a lil, and if I floored it the truck would backfire into the TBI. I couldn't get up a massive hill because of the lack of power and backfire if I were to give it a good throttle. WTH could a bump do to cause the engine to fall on it's face....

Well I got it home and just let it sit over night. Decided to start it in the morning and see if it sitting would suffice. Nope. Started it up and the idle was way worse, the full throttle was even more restricting, and the back fires were even louder and more out the TBI. So I got to work.

Checked my fuel supply, had good pressure and spray out my injectors on initial start But is kind of intermittent due to the lag in idle and almost dying, if I gave it full throttle it would spray great and then truck would flood itself out, die. So I know fuel is good.

Royally sprayed my TBI with carb cleaner to the point of shine and brand new. Still no fix same issues.

So as of the most recent tinker, I got to my distributor. Brand new 3 month old dizzy. I haven't took the cap off, I just simply unbolted the clamp bolt and while a friend started the truck, I adjusted the timing here and there with no improvement on the issues. So I went back to the TBI. While my friend starts the truck I wiggled the injector connections, played with the throttle, the spark plug wires, the vacume hoses. NOTHING to help the issues. At this point my truck is dead in the dirt. So I went back to the distributor. Instead of twisting the timing at what not, I pulled it towards the front and pushed it towards the back while keeping the advance and reta*d plum. AND IT HELPED, ONLY when I pulled it towards the front tho. So the gave the dizzy a tug whilst I tightly fastened it with the clamp and bolt I gave it a test drive and it drove great as long as I didn't go over half throttle. If I did it would just rapid backfire (It sounds like a Tommy gun fully automatic kinda backfire).

So tomorrow I will be taking the cap off and the dizzy out to investigate further.

Please let me know what you think and thanks for taking the time to read. I will be posting this in it's own forum.

Thank you!
 

CumminsFever

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My mind immediately goes to grounding issue. Did you check/clean/tighten the grounds on the engine and body? They're known failure points.
Also, could you tell us the year of vehicle? You call it "vortec", then mention being able to see injector spray, as though it's tbi. Just seeking clarification for better analysis!
Also, if you could get access to a scan tool, it would be extremely helpful to you and us.
 

Schurkey

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Don't tell us the fuel system "is good". Tell us the fuel pressure at prime, and when the engine is running at idle, and when the engine is running under heavy load--going uphill, for example.

How old is the fuel filter?

How old is the O2 sensor?

Verify EVERY sensor with a scan tool. Same for the computer outputs--electronic spark advance, EGR, IAC, and so forth.

What is the cranking compression pressure of all six cylinders?
Pull the valve covers and assure all valves are opening properly. Rapid-fire backfires make me think of intake valves that aren't sealing any more, or exhaust valves that won't open.
 
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