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backfiring through tbi

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chevyguy82

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topeka, kansas
just finished tbi conversion. idle was too low no matter how i turn distributor. turned torx screw in tbi to raise idle. if i push the pedal to the floor, in driveway, it will backfire no matter how i turn the distributor. it seems to run it's best with the mark on balancer at very top far away from the timing tab. i have taken the distributor out and put back in several times after finding top dead center. the cap is turned to 2 oclock postion and number one starts above the screw on cap. am i missing something? also how many volts are supposed to go to idle air control. i have 12 on 2 pins.
 
are you disconnecting your EST wire when timing? then reconnecting when done?
 
i have tried timing it with the connector plugged in and not plugged in. i am not running egr, knock sensor, or vss. would that have something to do with my problem? also where does the tan with black stripe wires go that are inside the cab by computer? they both have 5 volts. i can't find them on my diagrams.
 
EGR is one thing but you need the VSS and knock sensor. GM included them for a reason. however VSS will make no difference at idle, I dont know what the computer would think without the knock sensor .
 
Get a knock sensor. An incorrect signal with screw with your timing. Your timing should be 0* with the wire unplugged end of story.
 
i have read that you don't need a knock sensor. fuel pressure runs 13psi all the time. this is primarily a trail truck. i rarely ever drive it on the street
 
it may sound stupid, but are you absolutely, 100% certain that all the plug wires are on the correct cylinders?
 
that does not sound stupid at all. i have taken them off and put them back on at least 10 times. i took the distributor out again tonight and dropped it back in. my plugs on my distributor are at more 4-5 oclock to get it running the best. i have rotated the plug wires back and forth on cap to left and right finally keeping them where #1 is just to the left of the front hold down screw. it does not cut out going down the road. now it seems like it doesn't have much power. it fires right up but i have to give it gas and black smoke comes out and it hesitates. after a minute it will idle but every now and then it has a miss. i also took out all the plugs and cleaned them. i had just replaced them and they were are black. tonight i set up my trailer ramp against my tree and climbed to the top just idling at 60 degree angle. i could never do that before with the carb. this is just driving me crazy. we have a huge wheeling trip this saturday and i don't want it to break down or get embarrased by my jeep and toyota buddies.
 
4xcrazy said:
it may sound stupid, but are you absolutely, 100% certain that all the plug wires are on the correct cylinders?
Click to expand...
I came in here to post this. I did the exact same thing many years ago. Turns out I had been looking at the wrong engine in my manual and had the wires way out of order.

Here's a simple test - unplug one spark plug and then run it. Repeat one at a time, maybe two if you notice a pattern. If the problem goes away ...
 
all non LS chevy's are 18436572


couple things... do you have a timing light? technically, the dizzy can be facing any direction... don't confuse yourself with post 1 at 2 o'clock, etc... just be certain you have the motor at TDC and the corresponding post is used as #1... lace from there...

disconnect est, start motor, it should be with 5 degrees or so, adjust to zero... shut down, hook up est wire, restart...
 
forgive this, i dont mean to speak down to you, but youd be surprised how many times this has happened to me... missing the basics that is. The only time ive had a TBI backfire through the throttle body (fireballs through the butterfly) the dizzy was either 180 out, or the two wire connections for the control module were swapped.

Find TDC - Cylinder 1 is the piston farthest to the front of the vehicle, on the drivers side of the engine. get a piston stop, or put your finger over the spark plug, and rotate engine over until you feel the compression pushing your finger away from the hole. keep turning until that pressure stops building. BOOM, TDC on Cylinder 1.

drop in dizzy, and install cap, and wires, with the rotor pointing to the number 1 plug wire.

check all connections, as stated above, on mine the dizzy was correctly done, timing was right, but the two wire connector for the control module broke, and i attached the wires backwards. correcting them fixed the issue.

when all else fails, buy a vacuum guage, and set the timing by vacuum, adjust till the vacuum reading is the highest you can get, and rpm is around 700rpm, and youre done.
 
thanks, i will give that a try tomorrow and see what happens. i can only get the distributor to go in 2 ways. one way it will start and the other way it won't. so i'm assuming i have it in right.
 
chevyguy82 said:
thanks, i will give that a try tomorrow and see what happens. i can only get the distributor to go in 2 ways. one way it will start and the other way it won't. so i'm assuming i have it in right.
Click to expand...

It won't start for sure 180 deg out but backfiring can be because the timing is off.
 
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