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Old 07-03-2005, 10:00 PM
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Need help with a stubborn 4.3L V6 that won't start

I have a 2002 GMC Sierra with a (gutless) 4.3L V6 that will not start. Here is the story of my bad luck...

I parked the truck for the summer because it needed some minor body work done to it and I thought I could save money by doing it myself when I had spare time. Over the last three months I completed the work, having the truck sit with the exception of a periodic start up every few weeks. Just a couple nights ago I finished the body work, and I went to start the truck to move it into the paint shop to get it painted. The truck hadn't been started for a few weeks prior so I thought it would be a good idea to let it run for a few minutes before I took it out. After about 5 minutes of steady idling, the motor started making thumping sounds and the truck shuddered quite a bit, like the engine was way out of balance. I shut it off and checked under the hood to see if I had stupidly left one of my tools in there from the body work. I found no problems and no leaks so I went to start it again. This time I could not get it to start at all, the motor turns over just fine but it will not start. In fact, it sounds like the timing is like 80 degrees off, because while it is cranking, the motor periodically "jams up" like the mixture is igniting on the compression stroke, and it shakes the motor when it does this. It sometimes sounds like it is struggling to get to an idle but never seems to stabilize itself. After I stop cranking it, it thumps around for a few seconds after, like it is "diesel-ing" or the timing is way retarded. After cranking it for a bit trying to get it started, I smelled raw fuel pretty bad afterwards. After trying to start it several times, I gave up for fear of wrecking the motor.

The wierd part of this is that this motor is a modern, sequential fuel injected engine with only 30,000 miles on it, and it is only three years old. I have owned this truck for a year and it has been nothing but rock-solid reliable, without even the slightest glitch. When I did the body work, the only thing I did in the engine compartment was disconnect the battery to get at a few spot welds. It started up fine after I re-connected it, and a couple of times after that as well. I never touched a thing on the motor. It's like the motor decided to crap out for no particular reason and I just don't have the knowledge or equipment to diagnose the electronics of one of these new engines. Since my truck was bought second hand, I do not have any warranty remaining on it.

So far the only things I have dared to try to resolve this issue was to put gas line antifreeze in the tank, thinking water might have got in the gas somehow. This did not help. I also checked to see if any fuses were burned out, thinking the computer may be malfunctioning somehow. None of the fuses were bad. I have not tried anything else because it would likely involve taking apart a motor that I would have difficulty re-assembling.

Any help you could provide could save a poor young guy a very costly trip to the $$$$$ GM Dealership $$$$$



And I was so close to seeing my brilliant body work job be finished

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Old 07-04-2005, 06:33 PM
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Gee, it looks like everyone is a baffled as I am about this...

Tell me the bad news then, should I tow it to a dealership and get it fixed?
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Old 02-10-2007, 07:30 PM
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help that might make that 4.3 start

My friend had 98 S-10 with that same motor with almost the problem. I watched his dad turn the distributer about 1/4 of a turn and it busted right off. Hope that helps.
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Old 02-10-2007, 07:38 PM
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I am assuming this engine is equipped with ?coil packs? due to its age. If so I would begin checking various sensors that the system depends on to fire the packs. Cam sensor, crank sensor, throttle position sensor, idle air control valve all of those finnicky computer crap parts. Dont have to buy new ones just see if they are plugged in with clean connectors. I don't see how one could come unplugged but I have seen weirder.
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Old 02-10-2007, 07:41 PM
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Sounds like maybe your timing chain has jumped a tooth or two.
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Old 08-04-2011, 08:29 PM
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try the ignition control module i seem to have had the same problem on my 1986 monte carlo
but i changed the ignition module n rotor n coil now it starts at the kick of the key!! hope this helps
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Old 08-04-2011, 11:01 PM
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only thing that ever goes wrong with these trucks is fuel pump. it sounds like it could be your problem. i have replaced many. the truck dies. the truck turns over then goes bang bang bang like timing is out, like you said sounds like an old ford diesel with injection troubles haha. and the final is it idles but ****ty and when you hit the throttle it doesnt pick up or even dies.

the jamming up i have never seen on a 4.8 or 5.3 but when the fuel pump whent on my 4.3 these were the symptoms. im guessing the computer is compensating or getting messed up and messing with timming. the timming on these is all computer controlled to a certain extent.


but if fuel pump is good. 4.3's are notorious for crank sensors and fuel rail/injectors/spider whatever you wanna call it. its all interconnected under the intake manifold.

ignition coil isnt as common but possible same goes for ignition module.
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Old 08-04-2011, 11:18 PM
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yeah while your at it replace the fuel filter too. i had some minor problems with my sisters 4.3 and did a complete tune up, maybe run some sea foam or injector cleaner through it too because all that bang bang nonsense is vewwy dirty for the fuel injection system.
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Old 08-05-2011, 07:51 AM
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I don't know about anyone else but i think the first thing i would try is a little fresh gas not some gas that has been sitting around for months. Preferably non-ethanol.
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Old 08-05-2011, 09:12 AM
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This post is 6 years old.
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Old 08-05-2011, 09:33 AM
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lmfao i didnt even look...........Puts his head down in shame.
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Old 10-06-2011, 09:51 AM
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Good guide for those who still have same issue

The post is old, but helps others who are still having same issue. Keep posting updates! I am having the same issue. We replaced the rear main seal and the oil pan. Now truck won't start as if the fuel pump is gone. Haven't checked yet, but we thought it was timing so we have been working on setting the tdc and rotor. So far, truck won't turn over at all. We were able to start the truck last night by priming the carburator, but today it won't even start. Seems like we just keep making it worse. Any help would be great!
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Old 10-06-2011, 03:55 PM
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Are you sure your vortec 4.3 has carb? If it fires up by putting fuel down teh throttlebody, then there is something up with your fuel pump probabaly. When you turn teh key to run, do you here the pump running?
These engine willrun with 58-66 [si of fuel pressure. you can get gauges for $20.

peace
Hog
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Old 10-06-2011, 05:12 PM
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No sound from fuel pump

We have pretty much narrowed down the issue to the fuel pump. We aren't hearing anything, but I don't understand how it could be fine before we changed out the rear main seal and then bad when we put everything back on. We never touched the fuel tank or any of the fuel lines.
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Old 10-06-2011, 11:48 PM
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Fuel pressure depends on the type. TBI uses about 13 psi. CPI/SFI/MFI uses a lot more.

It will help to know how this engine is set up. Is it stock, modified from EFI to carb? The problem could be as simple as a bad relay.
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