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Discussion Starter · #1 ·
89 w100 4x4 318 np435. no codes

This TBI system has been quite buggy and the PO and his backyard mechanic were hacks. When i first the truck it ran terribly. it backfire, stall when coming to a stop, running filthy rich, and the engine was missing. they swapped out 3 throttle bodies in an attempt to cure the drivablilty woes

During my typical diagnostic routine i found a disconnected power wire for the TBI temp sensor and 3 spark plugs with an almost closed gap. that cured my overly rich condition and dead cylinders.

not too long after that, the  fuel pump was replaced, wiring terminals tightened up, hose split repaired.

I have 2 current issues. First, the idle is low and the IAC hunts a little until it finds a sweet spot. On a cold start, it fires right up but the idle creeps up into normal rpm (i dont have a tach but it sounds close to normal idle rpm) then it hunts a little and stabilizes. when coming to a stop the rpms will drop and the engine stalls. this isnt consistant but it happens often about 3 out 5 times. i tried to adjust the IAC rod but that doesnt help.

the second issue is the lag or hesitiation on acceleration that often results in a backfire through the throttle body. full throttle, half throttle feels the same. It feels like the igntion cuts out, backfires with one big puff, comes back and can spin some tires of i was into enough throttle.

timing is set @ 10deg, connections appear good and clean, dizzy shaft has very little play (no more than .005), however the cap is corroded and i will be replacing it today.

one thing i did notice is that my timing light does not have steady flash so i know the igntion has a miss. i can only feel this miss if im in too high of a gear with light throttle (not lugging but dang near close to it)

i currently suspect the pick-up coil, ECM, and igntion coil. is there anything else i should consider before i start throwing parts at it?
 

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Have you checked the EGR valve, all the fuel lines (feed & return) you may have a partially blocked feed or a weak (collapsing) return line. my 89 is doin prettymuch the same thing. replacing all rubber lines today an blowing out metal lines with compressed air also checking pressure reg, will let you know if that helps.
 

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Discussion Starter · #4 ·
thanks dodgeboys, that album is amazing!! tons of great info!!

just throwing this out there; what is the ASD relay is getting weak or has a bad connection?? my best description would be a backfire, no power/ shutdown, then it fires with tires chirpping
 

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Discussion Starter · #6 ·
stalling was corrected after a proper IAC adjustment. I'm still getting that backfire though. i did change my dizzy pick up coil with a spare i had but it didnt solve the backfire. i suspect 3 things; the EGR, TPS, and MAP sensor. i havent had the time to to do any further diagnosis.

I do know that my injectors have questionable patterns which would explain my lack of power but its doesnt explain this backfire.
 

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a couple of things will cause back fire , but first , what do you mean by back fire ? out tail pipe or  out intake ? it matters .  :)
 

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Discussion Starter · #8 ·
right up out of the intake. i have not been able to duplicate it on demand. its somewhat random during acceleration under part throttle
 

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most times from intake backfire is electrical , a cylinder is firing at the wrong time  . I would check the distributor , make sure the wires are in the right places in the cap , the 89's computer WILL try to correct a mis-wired cap . It acts like ing has cut off and turned back on ... BUT if it were only that it would build up fuel , and the backfire would NOT flame out the intake , it would come from the exhaust . Disconnect the 2 wire temp sensor , pull NO 1 plug turn engine until 1 is on compression stroke , line up the timing mark  0' pull dizzy cap , install NO 1plug wire where rotor points on cap carefully work your way around in correct order and direction . install cap start and time to 10 btdc , shut off reinstall 2 wire temp sender . carbon tracing inside or outside of cap on rotor , bad plug wires shorting to each other , these type things first then .... try plugging vacume at egr or ...  burnt valve worn guide ?
 
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