Originally Posted by JG33
When accelerating needle will peg out , back out or use cruise control and it starts working fine touch the gas and and it will jump right back up . RPM's are normal just needle pegging out . Any ideas what to look for ?
Hi and Welcome JG! About the tach pegging out the first thought I had was a bad distributor, or mis-routed wiring. 153stars would have a good idea if it was a diesel as older automotive diesels used a pickup mounted on the alternator for the tach, but no gas engines that I know of. Your 8.1 should generate it's tachometer signal by the Crankshaft Position Sensor to the PCM. AvFord and Mudfrog could both be partially right, a bad plug or wire should not cause the problem, however if the wiring that goes tachometer is routed close to the coils or spark plug wires the increased load on the coil from a bad plug or wire increases EMF exponentially and can cause induction in nearby wiring and this could cause the "pegging out" of the tach. Ford had an issue with the V6 Taurus family vehicles inducting into the speed sensor wiring for the transmissions that would cause the speedo to freak out similar to what you describe. It also caused the transmission to do wild crap too because it also fed the transmission controller.
Check your wiring looms on the engine and in close proximity to the ignition areas of the engine, you may find one that has become loose and is laying on or next to a coil or plug wire. Inspect the plug wires for damage and replace any that are damaged, (squirrels, rats, mice and other rodents seem to think all wiring is tasty, the more expensive the tastier!). And third, if you are close to or over 25,000 miles on the spark plugs replace them. Th 8.1l is know to burn oil as a characteristic of the engine and it wears out the plugs about 4 times faster than GM's 100k mile life expectancy. If all that fails get back to me and I'll see if I can trace the tach wiring on paper to see where the signal actually comes from.
I know someone will ask this question so I will answer it ahead of time. Why does it only happen when accelerating manually? When the engine is idling or under very little load there is also less load on the secondary ignition system, (the spark), fuel mixtures and ignition timing are near optimal. As soon as the human controller initiates demand for more power the combustion triangle is set out of balance. The Engine control module changes timing and injector phase width (on/off time) and the throttle plate opens allowing more air into the engine. All of these changes alter the plasma state of the air fuel mixture at firing requiring more voltage and higher discharge amperage, this increases the Electromotive Field around the plug wires and the saturation of other nearby conductors causing electrical induction pulses in the nearby conductors. These pulses we see as the tachometer "pegging".