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Old 02-08-2013, 08:41 PM
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Chevy 4.3 vortec, diagnosing backfire through intake

Hey guys. Been working on this 1994 Chevy S10 with the 4.3 liter vortec with CPFI. The truck started developing some major driveability issues that were getting worst and worst to the point that it would take a little bit of cranking to get it started and would idle like a bag of ****. I started by replacing the spark plugs and wires, the plugs were looking extremely fouled with carbon. I also removed the dist. cap and gave it a good clean with electrical contact cleaner as well as the distributor rotor and reluctor. The engine was still running as it did before, hard to start, idle up and down and misfiring....... would eventually stall on its own. I then removed the upper plenum on the intake manifold to take a look inside to ensure the "spider" injection unit or fuel lines were not leaking. Well, sure enough there were pools and pools of gasoline sitting in the intake manifold underneath the spider assembly. Turn the fuel pump on and I see a stream of fuel coming from the fuel pressure regualtor. Perfect, I have found the problem, right? Just clean up the intake of fuel, replace the regulator, button it all back up and it should run like normal again............ not the case

I got it all back together figuring I would hit the starter and hear it fire up and idle with no problems now that the internal fuel leak causing an extremely rich A/F ratio was taken care of. This would ofcourse only be the case in a perfect world. Anyways I finally got it to fire up after lots and lots of cranking and it is still running extremely rough, misfiring just like before. Bring it up to 2000 rpm and it smooths out but if you blip the throttle it backfires something fierce like a popcorn machine out the throttle body. Vacuum is very good, no vacuum leaks. Firing order is correct. Timing has not been touched, distributor has not been removed. I continued to test sensors and all sorts of things with no change. If you open the throttle quickly it just bogs with NO raise in engine rpm and then POPS out through the throttle body like a friggen rifle going off. I figured maybe it wasn't injecting fuel to all the 6 poppet nozzles so i proceeded to remove the upper plenum again and check to ensure fuel is being delivered. With the engine cranking over all 6 poppet nozzles are spraying. What I then discovered after placing the nozzles back in place with the engine being cranked were FLAMES shooting out of each of the 6 intake runners in the plenum!! I couldn't believe this! The engine is now backfiring up through each of the intake runners. All I did was replace the plugs, wires, clean cap and distributor with contact cleaner, replace leaking fuel pressure regulator and now it is backfiring! I didnt rotate the distributor at all so timing is dead on and firing order is good (1-6-5-4-3-2). I am now at an absolute loss of what could be causing this. Only obvious thing would be spark cross firing where the plug would be igniting on an intake stroke. What else could cause this? Anybody have backfiring like this on one of these engines before? All the help and expertise here is greatlly appreciated because I am now beating my head against the wall.

Thank you all very much,
keith
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Old 02-09-2013, 12:51 PM
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Anybody?
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Old 02-09-2013, 12:55 PM
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Popping through the intake is usually a sign of a lean issue not a rich issue. Your spider may be plugged up. I believe there is an upgrade kit for this. Maybe someone else will chime in with the part #.
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Old 02-10-2013, 11:14 AM
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Thanks for the reply Tbucket. I verified it was getting fuel by removing the poppet nozzles and visually ensuring they were spraying fuel while cranking. What is very strange here is that at around 2000rpm the engine is smooth and sounds good, but just to get it started requires 10 seconds or more of cranking while it is backfiring and once it does get going it idles like crap and will eventually die. I am beginning to think this is more than likely an ignition problem rather than a fuel problem. Given that everything is mechanically sound engine wise the only thing that I can see causing flames to shoot out of the intake is an ignition fault. Its just really hard to diagnose because like I say it smooths out and runs normal at 2000rpm.
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Old 02-10-2013, 12:21 PM
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Do you have access to a scan tool, if so take a peek at the coolant temperature and intake air temperature and see what they say. I would also like to know what the fuel trims read.
hard starting is generally fuel related.
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Old 02-10-2013, 12:30 PM
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You absolutley should verify correct fuel pressure. Just because you replaced the regulator doesnt mean it is correct. The line back at the tank may have started leaking as you now have a good regulator in place, which would cause a rise in fuel pressure, long enough to blow the hose off in the tank. Just a thought.
Also...are you absolutley positive you have the plug wires on correctly?
Double check your work.
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Old 02-10-2013, 05:05 PM
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Thanks for the replies/suggestions. I do not have access to an OBD1 scan tool, I would have to purchase one to check for codes etc. which is last case scenario for me. I also have not verified the actual fuel pressure with the new regulator installed, I just verified the nozzles were spraying fuel during cranking. I may have to bite the bullet and buy a fuel pressure tester to confirm I am getting the 60psi or whatever it is. My question then would be it is definately getting fuel since it is shooting flames out of all 6 intake runners so lack of fuel would seem unlikely. I have confirmed the firing order multiple times (1-6-5-4-3-2) as per the manual but I know it is correct since the engine does run and sounds good at 2000rpm........ so that can't be it. This is a really strange one guys and I am loosing sleep over it because I really want to/ need to figure it out! I just dont understand how changing the spark plugs, wires, fuel pressure regulator and cleaning the distributor cap and rotor could result in this. Timing hasn't been touched. I guess I will continue troubleshooting and see if I can make some progress. Any more suggestions are greatly appreciated
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Old 02-12-2013, 11:28 PM
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Crappy Fuel......

Alright guys, got an update for all those that helped me with this thread. After a lot of thinking and troubleshooting I decided to throw a new distributor cap and rotor on it today to completely rule out the spark "crossfiring" possibility. I also bought a new fuel filter and replaced that today as well. After changing those things I hit the key and.......... same old thing, no start. I proceeded to crank and crank until the battery died then got out the jumpers. When replacing the fuel filter I captured some of the gas and to get a glimpse of the condition of it. Well the gas was very brown, likely from rust formation inside of the steel gas tank, especially since the truck had been sitting for over 3 months. I then figured that the gas in the lines and rest of the fuel system may be so bad that it is the cause of these problems. To further rule out the ignition system (which is basically all new now) I poured some gas straight into each of the 6 intake runners and then hit the starter. Well sure enough the engine span up to 5000rpm on its own for a mere 2 seconds then shut down. This further supported the idea of poor fuel in the system. I continued to turn over the engine in hopes of clearing all the old fuel from the fuel system, a lot of it had already been removed when replacing the filter. Sure enough after much cranking, she fired up!! It finally ran and wasn't misfiring NOR backfiring out the intake anymore! Success!!

So, with this in mind it appears the root of the cause of this problem was simply old/rusted gasoline. The fuel pressure regulator was leaking and did need replacing ofcourse but really all I had to do was evacuate the system of all the poor fuel and it would have eventually ran proper! In the meantime I've spent sooooo much time replacing ignition components and troubleshooting a multitude of things. It still baffles me WHY the engine was shooting flames out of the intake all due to poor gasoline? It is no longer backfiring meaning the gas in the fuel system now is much better. I just can't seem to understand how poor gas could cause backfiring out of the intake, its not like the timing has changed or anything. Anyhow, I am thankful I finally figured it out and thankful to those on this board who chimed in. I hope my findings can help somebody in the future if they encounter a similar problem, step one....... CHANGE THE FUEL!

P.S.- If anybody would like to explain to me WHY poor gasoline was creating this backfiring condition, I am all ears.

Thanks,
Keith
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Old 02-13-2013, 03:18 PM
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Anytime the combustion process is not working correctly there is the chance of backfire.
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Old 02-13-2013, 05:20 PM
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You mentioned a distributor cap. That could also have been causing a crossfire.
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