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351w EFI misfire above 3k


kunar

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So, i bought a 91 f250 with a 351w. It's got the same tfi ignition they used on the rangers. it didnt run when i bought it, no spark. i put an unknown condition used tfi module in it and it fired right up, now the problem is that it misfires above 3,000 rpm. the misfire shows on the tach (universal, aftermarket tach) so i have to believe that the issue is before the coil, not in the plugs or wires, correct? either TFI or PIP? just dont wanna throw money at it :icon_thumby:
 
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kunar

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I would put crank on TDC with rotor pointed at #1, then pull out distributor.
Check it for wobble and check the CMP sensor distance in the distributor.

Also read here for testing TFI module: http://www.therangerstation.com/tech_library/TFI_Diagnostic.shtml
thanks for the info, Ron. i had been using that tfi diagnostic page already, to figure out what the initial issue was. didnt really think about using it for this misfire issue though. ill read through it again.
 

RonD

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High RPM or load issues can also be low fuel pressure at high RPM or restricted exhaust cause back pressure at high RPM.
Although these generally wouldn't be described as misfires more a lack of power at high RPM.
 

kunar

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High RPM or load issues can also be low fuel pressure at high RPM or restricted exhaust cause back pressure at high RPM.
Although these generally wouldn't be described as misfires more a lack of power at high RPM.
it definitely feels ignition related, you can see it in the tach, and it pops and sputters. its bad enough it won't continue accelerating at all at that point. it's also got an occasional miss at lower rpm. i suspect it's the hall sensor in the distributor. i might just get one on order from rockauto tonight. i cant imagine what else it would be.
 

Earl43P

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I don't know diddly-squat about a 91 F250, but if the automatic transmission has a DTRS instead of a NSS, it COULD be misadjusted or bad and the PCM thinks it's in Neutral instead of Drive. Ford purposely limits engine rpm to 3000 in Park and Neutral.

I've seen several bad DTRS cause this 3000 rpm limiter to be applied in Drive.
 

kunar

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I don't know diddly-squat about a 91 F250, but if the automatic transmission has a DTRS instead of a NSS, it COULD be misadjusted or bad and the PCM thinks it's in Neutral instead of Drive. Ford purposely limits engine rpm to 3000 in Park and Neutral.

I've seen several bad DTRS cause this 3000 rpm limiter to be applied in Drive.
Thats a good thought Earl, but it's a 5 speed. It's also way too violent of a misfire to be a rev limiter. I took it out tonight and got it up to about 4k just to be sure. Thanks for the input.
 

kunar

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got my distributor a couple days ago, and finally had a chance to drop it in tonight. all my misfire issues are gone. the new distributor came with a cam position sensor and a tfi module, so im sure it was one or the other causing my issues.
 


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