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Old November 19th, 2010, 04:53 PM   #1
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Default tbi 454 back fireing

so i got this truck a while ago ran ok and after doing plugs and wires becuse they were cooked and had a few miss fires now i have the problem of after it gets warm 5 to 8 mins of running it back fires through the throttle body under load no fire just air and fuel,i have looked at vacuum leaks,timing,fuel pressure,map sensor,tps, no codes, any ideas befor i start to look at the intake valves
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Old November 19th, 2010, 06:30 PM   #2
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you may have cracked a plug. I would water down the plugs and wires with a 2 liter pop bottle of water with a small hole punched in the top. that way you can aim it. if you see or hear spark jumping you found the problem. I have found BBC ignitions very hard on cheap wires also. if its not a Delco or better it takes them out fast.
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Old November 19th, 2010, 07:42 PM   #3
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cam

bbc's have been known to eat cams.
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Old November 19th, 2010, 08:10 PM   #4
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cam doesnt really sound right since it doesnt do it till its warm though. usually bbc's just tear up timing sets do to lack of oiling.
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Old November 20th, 2010, 05:52 AM   #5
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Quote:
Originally Posted by mprmn08 View Post
usually bbc's just tear up timing sets do to lack of oiling.
after 11 years working at the busiest Chevy dealer in southern Michigan I can't say I ever saw one eat a "timing set". all I did was fix power train problems & do run-ability of trucks, motor homes, etc.

without throwing code its has to outside the engines ability to see. if it was a cam or timing chain issues you would see a code as a low vacuum signal or your timing would be doing back flips due to not being able to see spark knock due to a chain jump.
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Old November 20th, 2010, 06:41 AM   #6
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you may want to inspect the distributor, i have a 91 k2500 454. i ran into a huge problem of firing our the exhaust and intake. turned out one of the pole pieces in the distributor cracked. it is a $ 8 die cast part. the real problem was corrosion. I found it by pulling the cap and rotor and inspecting with a flashlight. if you see a bunch of corrosion, you may have the same problem. There are screen vents on the bottom of the distributor that plug up and cause the corrosion. Also, these trucks carbon track the distributor cap, you did not mention if this was changed.
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Old November 20th, 2010, 10:16 AM   #7
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well due my boss rebuilding motors for about 30 years now, and knowing bbc's very well, we groove the front cam bearing when installing them from the oil hole to the front to oil the gears a little better since they lack oiling up there. maybe its a problem on the older ones and they fixed it by the now but it is a problem with them.
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Old November 20th, 2010, 10:26 AM   #8
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Maybe the distributer is worn. The shaft could be moving a bit as the oil thins out from temp
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Old November 20th, 2010, 11:38 AM   #9
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ill take the distrubutor out and look will a Sb tbi distubutor work? i checked no cracked plugs, if it was i timeing problem wouldnt i have flames coming out of the throttle body and not just air and fuel like i do now
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Old November 20th, 2010, 12:39 PM   #10
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small block wont work. different heights.
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Old November 20th, 2010, 12:41 PM   #11
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I think the sbc one is the same, but I'm not 100%. As far as I can tell there is only one distributor for small block and big block. Also when you set the timing make sure to unplug the electronic spark advance

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Old November 20th, 2010, 06:44 PM   #12
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just a dumb question. firing order is it correct? 18436572 clock wise


Quote:
Originally Posted by glenns-garage View Post
you may want to inspect the distributor, i have a 91 k2500 454. i ran into a huge problem of firing our the exhaust and intake. turned out one of the pole pieces in the distributor cracked. it is a $ 8 die cast part. the real problem was corrosion. I found it by pulling the cap and rotor and inspecting with a flashlight. if you see a bunch of corrosion, you may have the same problem. There are screen vents on the bottom of the distributor that plug up and cause the corrosion. Also, these trucks carbon track the distributor cap, you did not mention if this was changed.
I have seen both of those cause issues. the cracked reluctor wheel with the 8 points is a odd piece to go bad but it does happen, seen it more on older Mopars.

the carbon tracked rotors is very common. look for tiny pin hole marks under the part that slides on the shaft. the spark seeks the fastest route to ground.



if its a cam lobe gone it would back fire just free revving in neutral. it would sound like string of fire crackers going off if you hold the throttle down with the air cleaner off. that would be an indicator that an exhaust side lobe wiped out.
if its not a cam lobe it could be a rocker arm or push rod also.

so with that I suggest if you get the valve covers off fire it up and see whats moving. if the rockers move normal put them back and wash everything off that go oil on it(sorry). if you look at the rockers and find one barely moving then you found the problem. try and adjust it if possible. if not then you may have bad came for sure.
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Old November 21st, 2010, 03:11 PM   #13
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well i change the cap and rotor to a know good ones still does it. so looks like
im pulling some valve covers tomarrow
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Old November 22nd, 2010, 03:57 PM   #14
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took both valve covers off nothing loose/bent/broken i did notice alot of burnt oil build up around the push rod where the cross over for the exhaust to go into the intake but that was is
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Old November 22nd, 2010, 04:09 PM   #15
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Thats ok, that just means the cross over isn't plugged with carbon build up. as for nothing broken I really didn't expect much as BBC hardly ever wipe a cam. I had one but it was a `72 402 that had 120K on it.

back to the issue at hand. did you spray water on the plugs and wires and check the firing order?
as for distributor this may sound like dumb test but see if the shaft of the distributor has a magnetic field ( try and stick a steel screw driver to it) if it clings it has the field its supposed to. if it doesn't cling to the shaft it or is very weak it may be part of the problem. I have only seen a couple in my life but if the shaft doesn't have a magnetic signal it has a problem seeing the pick up coil. that or the pick up coil is getting an erratic signal from it.
also check the bearing on the distributor shaft if you can wiggle it in the housing thats enough to make it hit the pick up coil and cause a miss.
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Old November 22nd, 2010, 04:21 PM   #16
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something just hit me. since your dealing a 454. does this have a TBI or is this the later direct port injection engine?
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Old November 22nd, 2010, 04:50 PM   #17
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i did spray the wires and plugs got nothing. fireing order is correct, the distrubutor has te normal up and down play but none side to side, there is some rust on the pick up and all the other steel parts but its just surfist rust, i did not look to see if it was magmetic, it is tbi

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