stumbling '94 F-150

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  #1  
Old 07-16-04, 11:10 PM
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stumbling '94 F-150

This is ancient history, but the problem still exists. I just have some new information to go along with it. My '94 F-150 (4.9l straight 6, 4-speed auto trans, 106k miles) has an annoying intermittent stumble on acceleration. If I catch the stumble before engine the dies, I can floor the accelerator a couple of times and usually clear the problem right up, until the next time it decides to act up. This can be anywhere from a few seconds later to a few days later. The first stumble can occur at first start of the day, or after a few minutes of driving, or after a few hours of running errands. There seems to be no pattern.
Many helpful suggestions have been made by members of this forum as to a cause, from fuel pressure to TPS/MAP/EGO/IAT sensors. Some of these suggestions I have tried; others require test equipment I do not yet possess.
The new information is this: when the stumble occurs, the voltage gauge on the dashboard reads VERY low, like out-of-normal-range low, and lower than standing battery voltage. And when I floor the accelerator, it takes a bit for the voltage to return to where it normally runs (a little to the right of the middle of normal range). Could a sketchy charging/starting system be responsible for all this? It would kind of make sense if the PCM and sensors are voltage-sensitive. I have not yet seen what order things happen in, stumble then voltage drop or the other way around, but I will try to keep a closer eye on this. Any ideas would sure be appreciated. Thanks in advance - Chris
 
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  #2  
Old 07-18-04, 06:41 AM
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If the base electrical system is faulty all types of problems occur.The computer in the truck needs a minimum voltage to operate properly.The computer sends a reference voltage to the sensors and gets a signal return voltage in most cases.So if the voltage to the computer is low the ref volts will be low and the signal return volts will be low and the computer doesn't change the strategy properly for engine operation to react normally.
 
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Old 07-18-04, 07:03 AM
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Thanks, davo! I suspected that was the case. I'm going to watch for a while and see whether the voltage drop happens before the stumble or after. That way I'll know which of two areas to address. Thanks again - Chris
 
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Old 07-18-04, 02:03 PM
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If your engine is stumbling bad enough to drop the idle below normal and almost stall, a voltage drop at that point would be normal.
 
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Old 07-18-04, 07:31 PM
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I may have found something, guys... While poking around under the hood today, I found a white vacuum hose that goes to one of the climate control door actuators that has been destroyed by heat - the upper part of the hose was missing about 2 inches, and the rest crumbles when touched. The hose gets vacuum when the selector is on MAX A/C and OFF or VENT ( I don't remember which). I've got it disconnected and plugged until I can replace it. Think I'll drive it like it is for a day or two and see what happens. I'll let you know how it turns out. Thanks again - Chris
 
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Old 07-19-04, 07:44 PM
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Looks like a vacuum leak was the problem. I drove the truck A LOT today, and gave it plenty of opportunity to act up, and it never did. Another couple of days should really tell the tale, but as for now, it seems to like the golf tee in the vacuum hose. As always, thanks to all for the input and advice. Back soon - Chris
 
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