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exhaust bacfire

chevyuser74

chevyuser74

1/2 ton status
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Jun 19, 2006
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Mukwonago, Wi
recently built my motor up and now when u give it anymore than slight throttle, it backfires out the exhaust and falls on itself, use to only do it once in a while. then my fuel pump failed and it does it way more consistent now. but is still somewhat intermident, im on my 6th custom burning, im starting to think that its not a chip issue.

opinions appreciated, here is what i have done, or swapped with new stock at work to test.

played with base timing moved from 0-15 just to see if any difference
02 sensor
fuel pump-because it failed new one seems to be holding steady pressure.
tps sensor
map sensor
ecm
dizzy includes icm
cap rotor wires plugs
iac
adjusted valves
vacuum leaks
ecm ground
maybe some other crap im forgetting.
no codes, and havnt really seen oddities in the datastream. but brian keeps adjusting the fuel curve in the chips. im just really wishing i would have never dabbled in performance work.

everything i built the engine with is tbi compliant, most everything i did too it was approved or recommended to me by brian at tbi chips.
 
my gut feeling is its a timing issue like the knock sensor was retarding it but i would think i would be able to see that in the datastream. or an issue with the esc, but these are two thing i we dont have at work, so i would have to buy em to see if it made a difference and dont know if they really fail.
 
Have you changed the plugs recently or was it earlier? If you foul some spark plugs it can pop through the exhaust when the unburnt fuel hits the hot exhaust from the other cylinders. If they foul again then you are too rich or something else is wrong to make them foul. If it was too rich a new fuel pump with higher pressure would definitely make it worse.
 
Kind of sounds like a fuel delivery problem to me. You may be going way lean on acceleration.
Fuel filter?
Checked EGR for proper function?
Coolant tenp sensor?
Check your injector wires and injectors. Maybe there in an intermittent short or loss of ground.
Do you have the ground strap from the engine block to the firewall connected?
Any blocks or kinks in the fuel return line?

On the electrical side.. maybe a bad coil? or wire?
 
fuel filter is new, tank, lines, pump, sending unit all new have a sensor monitoring the pressure never drops below 16 usually around 18( have an 18psi regulator spring)

no egr so no problems there

temp sensor reading right

new injectors(flow tested) wiring harness not that old, but besides that is still in great shape

coil is new but threw in one we stock at work to make sure it was ok no problems there, no problems with the wires, spark plugs are new as well, pulled and still look good.

ground strap non existent but computer grounds are on the engine and ground to the engine good.

little back info
swapped tbi engine into my 74 a year ago or so, 3 months ago or so pulled out to build to make more power, truck ran fine stock. truck ran fine for about a week after build then started having issues.
 
Last edited:
chevyuser74 said:
ground strap non existent but computer grounds are on the engine and ground to the engine good.

little back info
swapped tbi engine into my 74 a year ago or so, 3 months ago or so pulled out to build to make more power, truck ran fine stock. truck ran fine for about a week after build then started having issues.
Click to expand...

So, what does your ground path from the battery look like? And does wherever your ECM is mounted have a good ground that isnt through the computer? In PLC world, that could cause lotsa problems
 
ground cables mount to the body, frame, and one of the intake bolts connection is clean mainly because, cable, battery, intake and bolts are all new. the computer is grounded to back of intake(thats where it said to go in the painless manual i believe), this connection is also good, ive also messed with it a couple of times because ive had issues with computer grounds on previous trucks. but in short 4 gauge ground wire going to intake, computer ground on intake.

my mess of an engine bay( taken right after motor was dropped in been cleaned up a bit since then.
picture.php
 
yea this is a big time headache, its like its gotta be the prom but im on like my 7th one and it doesnt make a difference in the running. so that leads me to believe its something else. i so wish i would have just left it alone now
 
i just know when i find the issue its going to be a doh! moment. maybe it will be something cool like the altenator is putting out to much ac voltage, and now that i say that im going to have to check it.
 
after looking at a wiring diagram it really cant be the knock or esc because the esc reports to the ecm then the ecm retards the timing so if the esc was acting up i should see it in the datastream which i dont. now im pretty much out of ideas. think im going to start back at base 1 and see if i missed something, this is really making me hate my truck.
 
if timing jumped a tooth would it be an intermitent problem? because sometimes it will run just fine. problem got worse after changing fuel pump, it was acceptable before that, only did it once in a blue moon. only reason i can think there is that i was having proms burned while the fuel pump was failing so it was compensating, now that the fuel pump is good again it flooding it self, but brian says its running a bit rich but shouldnt be enough for driveability issues
 
i was looking at my pictures and the timing marks were lined up and i remember spinning it three times to make sure they ended up togther each time. but it would be something to check, i would think it would run crappy all the time and not just at mid/hard accel then.
 
readjusted the valve again tonight, this time with it running, make it idle better and game me a lil more grunt, and moved the falling on itself up to heavy acceleration. also tryed a different coil no difference. thinking about maybe warrantying my dizzy. although i still think its flooding itself, but according to the data its running rich on accel but not flooding according to ...
 
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