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Old 02-08-2013, 08:41 PM
383SBC 383SBC is offline
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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,
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