High Idle, Low Tach Reading

[SCRunner12]

So I'm looking for some insight into my issue of a high idle and a low tach reading. Right now my 3.4 auto starts at a normal idle, but after a short drive the RPM's jump up high. Only problem is the tach reads lower than the actual RPM's. It will often read around 900 RPM, but the engine is running around 1600-2000. Also it will read at 0 when in gear. Anyone have or had this issue?

I'm going to clean the IAC this weekend and I've checked my vacuum hose multiple times (didn't do a smoke test though).

 

[Konkordmusk]

Hey Rob!
Sounds like your igniter needs a look-over. It's on the passenger side fender inside the engine bay. That is what sends the signal to the tachometer, I believe. It has a ground on the inside of the fender; make sure that's healthy.

 

[snow junky]

Any mil codes? I'm guessing whatever sensor the rpm gauge uses is bad. Maybe crankshaft position sensor.


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[TheDurk]

Hey Rob!
Sounds like your igniter needs a look-over. It's on the passenger side fender inside the engine bay. That is what sends the signal to the tachometer, I believe. It has a ground on the inside of the fender; make sure that's healthy.

Good call on the ground, bad call on the source of the RPM signal. The igniter TAKES a signal from the ECU, it doesn't send one. From the factory EWD manual:

RPM signal system
Camshaft position and crankshaft position are detected by the camshaft position sensor and crankshaft position sensor,
camshaft position is input as a control signal to TERMINAL G2 of the engine control module, and engine RPM is input to
TERMINAL NE+

So as said above, check your crankshaft position sensor. That signal can also get messed up when the crank bolt is working loose and the crank sprocket moves away from the sensor. Unlikely, but a very bad thing when it happens.