severe miss under load
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severe miss under load
ok i did a swap in my 96 f250 from a 460/at. to a 4.9l/ 5 speed now i have a severe miss under load i used the whole wiring harness/sensors/comp. from a running driving 95 f150. so this is my issue if i time it to 10* btdc(with the spout pulled) and put the spout back in it advances the timing like it should until you put a load on it then it retards the timing back to base and misses to the point of not being able to drive it. also if i open rev it it will miss at full throttle but if i reduce it to half throttle it climbs to 5k just fine. it misses worst at 2500-3500 rpm. its stumped everybody i've asked including a freind of mine that was head ford tech for 30 yrs at the local dealer. any help would be appreciated thanks. nate
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Joined: May 2010
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Have a look at your spark plugs.
A bad wire or bad plugs can cause misfires under load.
Possibly rotor, cap or coil, but less likely.
How is the fuel pressure when it does this? Low fuel pressure/volume can cause a miss under load as well.
There is a fair chance that the misfiring is triggering the knock sensor and the computer is retarding the timing. Making it a symptom rather than a cause.
But you could try unplugging the spout to see if it makes a difference.
A bad wire or bad plugs can cause misfires under load.
Possibly rotor, cap or coil, but less likely.
How is the fuel pressure when it does this? Low fuel pressure/volume can cause a miss under load as well.
There is a fair chance that the misfiring is triggering the knock sensor and the computer is retarding the timing. Making it a symptom rather than a cause.
But you could try unplugging the spout to see if it makes a difference.
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Joined: Oct 2008
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From: Davis, CA
Replace your sparkplug wires. Check to see if there is a ground around the exhaust manifold/headers and keep the wires seperated. I installed heat resistant insulators around the spark plugs to ensure there was no grounding.
Fleet Mechanic
Joined: Nov 2011
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From: New York
I try to have road-side swap-able spares in the truck for some of the common parts that fail. They can also be good diagnostic tools. The fact that you're loosing ignition advance, two things come to mind... TFI and PIP. TFI is easy to replace, especially since it's remote on the '92 - '96 trucks. I'd try that first, if only because it's so easy to swap.
Most just swap in another distributor rather than replace just the PIP. This is because the gear needs to be pressed off the shaft to swap the PIP.
Certainly all the other ignition components are suspect, as others have suggested.
Most just swap in another distributor rather than replace just the PIP. This is because the gear needs to be pressed off the shaft to swap the PIP.
Certainly all the other ignition components are suspect, as others have suggested.
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i recently rebuilt the motor so plugs/wires/cap and rotor are all new. and before anybody says it i DID time the cam to the crank correctly i had this same problem before i rebuilt it. now before i forgot to mention that if i manually advance the timing to more than 30* and leave the spout out it runs as it should. as far as the fuel pumps those are the same betwenn the two its the fuel pressure regulator thats different. all ignition parts are good i installed them on my brothers 95 f150 and it ran perfect
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Joined: Nov 2011
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From: New York
Try another PCM. These things are increasingly failing due to capacitor leakage and the symptoms are all over the map. I won't travel without a spare so it's a good investment even if it turns out not to be the source of your problem.
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pcm is good. i'm gonna order a new harmonic balancer i was just told its a common problem that they spin on the hub.
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Fleet Mechanic
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From: New York
Please report back if that solves it. In the mean time, I'll throw my extra in the bag for long trip travel spares.
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well i got the harmonic yesterday, pulled it all apart and compared the timing marks my old one is off by 10* compared to the chinese one i got so thats not the issue. BTW dont by a dorman harmonic balancer unless you buy a speedi sleeve with it the one i got came with a very rough seal surface
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