I have a 1994 Chevrolet C1500 with 5.7L TBI.

The problem shows up when it’s fully warmed and the weather is hot:

On load (uphill only at first, then progressively worse until any load like coming off a stop): backfires through intake (occasionally through exhaust); obviously, no power. Letting off the throttle lets the engine return to idle with no trouble. I can often gently accelerate, but any real throttle and all it does is backfire constantly.

I’ve replaced:

Fuel pump - steady 13 lb. at all times
Fuel filter
Fuel pressure regulator
Vapor canister purge valve
Idle air control valve
Throttle position sensor
Engine temperature sensor
Manifold Absolute Pressure (MAP) sensor
Ignition coil

Any ideas?

 

I don't know much about the pre-1996 5.7l but in my 96 vortec, adjusting the timing on the distributor stopped my backfiring.

 

Did you replace the wires and plugs first? Also check EGR valve. My EGR had failed slowly and was out for years and running weak. swapped on lunch break. truck ran tons better like new again. It would often throw a "Check Engine" light after 15 minutes of running. Also check air intake manifold gasket, not that it's too likely the problem (could be.. hope not) , but it might be getting close...

 

had same problem with my 97 k1500 bad distributor bearing, check by taking off distributor cap hold rotor (dont break)and rock back and forth if it has quite abit of play it will backfire, mine started backfiring more and more because the timing was loose and advancing every time it backfired, eventually it would not start or run at all till i replaced and reset the distributor and timing.

also heard of this when the timing chain got too stretched to work properly, again easy test..take key off and out of ignition ..distributor cap off...rotate crank shaft by hand one way till the rotor starts to move mark distributor rotor on the distributor where it points to...then mark a spot on harmonic balancer that you can see how far it moves now watch rotor and move crank in opposite direction till the rotor starts moving. stop immediately and

check your marks on the harmonic balancer if it has over one inch between marks chances are the timing chain is bad...at that point get your mechanic to verify this

hope this helps...

 

HAS THIS BEEN SOLVED? DOES ANYONE KNOW?
Sorry to bring this old thread back but this is what I am dealing with right now. I'm not 100% sure it's only on hot days but my truck goes from running fine idles fine (not kittens but pretty good). But 10-15 min into a drive and I hit a long hill that requires continuous power input and the truck goes from just fine to slowly reduces power (slowly meaning like you let off the pedal 3/4 not over the course of miles) and slows down and pop pop boom pop. Just as explained above. Only things I'll ad to mine are I'm not sure its a general "backfire". This usually refers to a gun shot from the exhaust. I believe they are more of a deep bass like pop so maybe intake backfire?
the truck ran perfectly and there was a slight knocking noise I asked to have looked at and was told it could be spark knock and the high idle could be timing related. Well he did plugs and wires and then this problem showed up. Then cap and rotor. Problem still. Then I took cap off to look and the rotor and cap had some small plastic and I notice the rotor had rubbed the cap in a few soft. I put another cap snd rotor again. Didn't work and the new cap and rotor had that same wear mark as the first one. So then it was suggested the distributor may be bad cuase the metal ring thats below the rotor was alittle easy to wobble with your hand. Still same problem.! I thought the distributor was it but its not for me. I also checked Fuel filter. I can't check the fuel pressure there's no Schrader valve but I had the line open and turned key on a few times and the Fuel did come out under a decent pressure but I'm not sure what psi. Please if anyone knows anything here let me know what to at least look for / test? I'm really really desperate here and need the truck to move forward. Not sure if it matters but my truck is a 95 gmc w4 with a 5.7l amd a 4l80e.

 

Timing! I bought used 95 in 2001/2 and only discovered a couple of years ago that timing was set incorrectly but never backfired. The manual states to disconnect base timing wire behind glove box. There were 2 wires when I examined. I suspect former owner disconnected incorrect wire. Truck runs great now.

 

It is more likely than not in a vehicle this age the Catalytic Converter. Had this problem in my 1994 K2500 Silverado, replaced the Catalytic Converter and it went away.

 

Check if your plugs are wet