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misfire/backfire and loss of power


mechjames

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my friends 89 bronco II 2.9L has an interesting problem. at idle to about 1200 its fine, from 1200 to 3000 it'll run rough, and stutter, and sounds like its backfiring. if u pump the gas a lot and get it above 3000 it'll take off like a bat out of hell.

changed the fuel pump, pressure regulator, fuel filter, distributor cap, rotor, spark plugs, oil and air filter. pressure regulator helped a bit, but it still does it. throttle position sensor, and manifold pressure sensor has also been changed.

any ideas? fuel pressure was checked around 20 lbs before the regulator was changed, so thats what we thought the problem was.

it also seems like after we change something, it'll run perfect for a few minutes, then the symptoms return.
 


IMenriched

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Does it do this more after it is warmed up??? then check out the TFI (aka ignition module).

check to see if all the vacuum ports in the upper intake. make sure that they are not plugged with carbon & gunk. also check the back side of the throttle plate for carbon build up
 

mechjames

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it'll do it totally randomly. sometimes it'll run fine for a few minutes, then start bucking, backfiring, etc. ease off the throttle, and ease back into it, and it'll run a bit better, but not necessairly perfect.

tonight we decided to check the timing, and it was a way off. we set it to 10degrees with the spout connector disconnected, and it runs perfect til the end of the street, but really horrible on the way back. it runs its best when set 25+ on the mark without spout connector, with spout ocnnector, it jumps to 33.

i think we've decided that its a really worn timing chain. the timing light isn't all that steady either, the mark jumps around quite a bit, but then it'll be steady. sound like it?
 

IMenriched

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HAve you done a thourgh check for vacuum leaks?

the distributor maybe worn and or the pick-up coil under the distributor could be and issue too.

yes the timing chain can be the problem....but before you go digging that far, check the harmonic balancer....the rubber cushion the is moulded inbetween the metal halves could have deteareated and has slipped ...then giving you a false timing setting.

Not sure...if this unit would have the 2 fuel pump system, 1 in the tank and 1 on the frame rail. you mentioned that fuel pump was replaced...if your replaced the 1 on the rail and the 1 in the tank is malfuctioning...one could get simular symtoms. was the 20psi after the new fuel pump & FPR...that seem a bit low.
 

mechjames

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yes done a thorough check for vaccum leaks.

i'll swap the distributor out of my working parts truck and see if it helps. it ran fine before I started parting it out.

its only got one fuel pump, and the 20psi was before the regulator was changed. it helped it a bit for power and smoothness when it runs good, but didn't help with the ignition problem. once again the fuel pump was out of my good running parts truck.
 

gotmudd

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i'd be curious to know how many miles are on the motor and i believe you might have a timing belt and not a chain, either way it might be getting ready to puke, especially if your timing was that far off and it only seems to run half way fine at the upper end of the timing spectrum, just went through simular(SP) problem on my older full size. after changing timing set and dist. cap it runs like new
 

mechjames

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its most definitely got a timing chain, and the miles gauge says 55,000. Now honestly its probably rolled over once, maybe even twice. we don't know. most likely got 155,000 miles on it.
 

WildSide

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No mention if codes are present but I would check and clean all grounds and conections andthe O2 sensor.Check the plugs for proper color too.
 

mechjames

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code 14 only showed up with the off test as a memory code. PIP signal? that doesn't show up now that the computer is reset. nothing comes up with koeo test or koer.
 

bradical

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have you checked the Oxygen sensor. my 88 has had similar problems in the past. ive had to put a couple in it.
 

WildSide

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After checking eletrical conections including grounds and plug color (Firing order correct, wires and coil in good shape) TPS adjusted to specs... I would recheck FPR is ok- yes good ones do go bad too, or perhaps return fuel line blockage. Otherwise perhaps swap the computer?
 

jcm

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There is a little white button down under the passenger side dash on the floor, feel around for it and have someone push it while the engine is being funny. Might do something and its an easy check.
 

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