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Old August 24th, 2011, 05:40 PM   #1
in2deep
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Default 454 backfire

this thing is driving me nuts it backfires out tbi under load.if you go from idle to wot it back fires at around 5k rpm if you pull a hill loaded and ease into it at about 1/3 throttle it backfires, so the harder it has to work the lower the rpms it will backfire at. its a 88 tbi
ive tested,looked at or swapped wiht know good parts the fallowing
spark plus
spark plug wires
cap&rotor
good distrubutor
new intake,tbi adptor,tbi gaskets
compression is between 145-155 psi in all 8
no leaking valves
no broken valve springs none felt weak
cam is good no flat spots
good timing set and in time
had is a few times on scanner no codes, all sensors working as they should
fule pressure 12.5 psi
new fuel filter
new cts
i have noticed it runs rich
any ideas would be helpful no matter how dumb or unlikely

Last edited by in2deep; August 25th, 2011 at 11:23 AM.
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Old August 24th, 2011, 05:47 PM   #2
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you have egr on it yet? how close to spec is the fuel pressure?
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Old August 24th, 2011, 05:51 PM   #3
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yes it has egr it works and closes, fuel pressure is on the high side but still with in spec
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Old August 24th, 2011, 05:53 PM   #4
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ok have tried pulling the vacuum line off the egr and running it, as it may be opening and closing at improper times.
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Old August 24th, 2011, 06:24 PM   #5
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Did you chang coolant temp sender or sensor, I think they had both that year, Di you change the one for the gauge instead of the computer.
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Old August 24th, 2011, 06:40 PM   #6
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is it running rich at idle or while driiving? coolant temp and o2 sensor control that end of your problems. it sounds like you have a vaccuum leak on the backfiring bit.
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Old August 24th, 2011, 06:40 PM   #7
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Is the exhaust leeking?

When it back fires is it one at a time or multiple.

Years ago my brothers car back fired it was from a leek in the exhaust introducing air. Wierd shit did not make sence, once the leek was kixed, no more back firing.
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Old August 24th, 2011, 07:53 PM   #8
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Quote:
Originally Posted by MiNi Beast View Post
ok have tried pulling the vacuum line off the egr and running it, as it may be opening and closing at improper times.
i tryed it with no vac lines to to change

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Originally Posted by booblinker View Post
Did you chang coolant temp sender or sensor, I think they had both that year, Di you change the one for the gauge instead of the computer.
change sensor for shits and giggles both worked could see it on scanner
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Old August 24th, 2011, 08:14 PM   #9
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run rich all the time

it does have a exhaust leak at y pipe on both sides
ive run a sb tbi with out exhaust before with out problems but ill try to see if it fixes it

will backfire many times till u let off
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Old August 24th, 2011, 08:28 PM   #10
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couple things. you need to run the 195* T stat. it makes a huge diff on fuel mix. if it thinks its cold then it will dump fuel.
secondly carbon build up. if you have a layer of carbon on the pistons it will cause detonation. its a lot like a back fire but it can go to the intake side or exhaust side. flush with GM top engine cleaner right down the TBI.
Run the engine till hot, then pour into the TBI while running and drown the engine out with the bottom 1/3 of the can. let sit for at least an hour. then restart. it will smoke like hell. run till it cleans out then take for a drive.
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Old August 24th, 2011, 09:00 PM   #11
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it does have a 165 stat but it still hits 200 with a trailer which it always has on it
can u explain more on the carbon causing the backfireing
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Old August 24th, 2011, 09:17 PM   #12
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165 is too low of a t-stat for a FI motor, odds are the ECM is thinking it is in warm up mode most of the time.
if the the exhaust leak is before the O2 sensor, the O2 sensor wouldnt get a correct reading and try to dump extra fuel in the mix because it is thinking it is running lean.

also carbon buildup on the O2 sensor could also cause a poor reading and dump extra fuel in the mix. i am a firm believer that an O2 sensor has 3 levels of working...working, not working, and somewhere in between, an area that wouldnt throw a code to the ECM because its range isnt in left field but off enough to make it run poor but not throw a code.

carbon buildup will get hot enough till it is glowing, thus it could be causing the mixture to fire before it is supposed to. on carbed motors you would also notice the engine dieseling when you turn the engine off, but since it is FI, there would be no fuel to keep going in the engine.
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Old August 24th, 2011, 09:41 PM   #13
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i could be wrong but when its in open loop (cold) o2 has no effect on anything than why would it still backfire
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Old August 25th, 2011, 08:00 PM   #14
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tryed the engine flush changed nothing, also 195 t stat and tryed a good set of injectors
and fixed exhaust leak for the most part no change any new ideas
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Old August 25th, 2011, 08:38 PM   #15
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just some ideas here:

remove the fan, find a cigar or some cigarettes, check for vacuum leaks. pull the tbi unit and look at the gasket, they crack pretty frequently.

whats the mileage on the motor? you could have a bad valve. other than that, timing could be off - worn distributor.
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Old August 25th, 2011, 08:48 PM   #16
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ive have already changed the tbi,adaptor and intake manafold gaskets
not sure on mileage its a gm reman if i had to guess 80k or so timeing chain and distrutor grear are fine starting to think its a bad vavle
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Old August 25th, 2011, 08:56 PM   #17
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do a leak down test in each cylinder. I'll bet you have a cracked cylinder head
in the intake valve seat
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Old August 25th, 2011, 08:59 PM   #18
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Weak valve springs could cause the valves to float and make it back fire.
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Old August 25th, 2011, 11:09 PM   #19
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i really dont see it being springs i noticed today it seems to get worse that hotter it gets
i cant think of anything else it could be other that a burnt intake valve but i did a leak test and did find any leaking. maybe ill take the heads off for scrap it
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Old August 26th, 2011, 07:44 AM   #20
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I guess everyone else missed this question to. does it back fire out the exhaust or up through the intake when it does it? if its up into the intake then its a cam problem or valve/ push rod/lifter problem
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