1992 Ford 4.9 FI stumbles...

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Old 06-30-12, 01:38 PM
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1992 Ford 4.9 FI stumbles...

Okay, here's another one. My cable guy just stopped by so I could take a look at his personal truck, a '92 F150 with the fuel-injected 4.9 and a 5-speed transmission. After finding a couple of vacuum leaks, we thought we had the off-idle stumble taken care of. Manifold vacuum shows a pretty steady 18-19 inches and the computer gives no codes and no Check Engine light. But there's the slightest miss at idle and a little stumble when you swat the throttle with the transmission in neutral. Put it in gear and give it some throttle to take off, and the truck falls on its face. Owner tells me of new spark plug wires, TPS, fuel tank and pump that he installed; TPS and wires do appear new. Could the MAP sensor be giving an erroneous signal when vacuum drops/throttle is opened, causing the stumble? If so, wouldn't it set a code? by the way, my Genisys scan tool, vacuum gauge and multi-meter are about the extent of my diagnostic tools. Unless you count my stick-o-scope for listening to bearing and valvetrain noises... As this is my cable guy's truck, I would really like to make him happy with his toy. Any ideas where to look next? All input is greatly appreciated. Thanks in advance again. - Chris
 
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Old 06-30-12, 01:49 PM
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Sorry - I just saw where I should have posted this in Trucks, SUV's, RV's and stuff! - Chris
 
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Old 06-30-12, 02:49 PM
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Do you know how old the spark plugs themselves are, and whether they were replaced with OEM Autolites when they were last done?
 
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Old 06-30-12, 03:06 PM
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No info on age or condition of plugs, cap or rotor. Today was a quick drive-by to look and see if anything showed up. Did find the same vacuum leak as on my truck - A/C door motor plastic line under the hood had gotten heated, brittle and turned to dust. Plugged the connector in the cab and a few other vacuum leaks fixed, but the problem persists. If I get more time with the vehicle next visit, I will look at ignition components more closely. Thanks for the input - Chris
 
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