I have a 2000 accord 2.3 v tec 5 speed. It happened once or twice before winter. Today it happened again . The car was parked. I started it and when idling I saw the rpm Gage jumping or bouncing up and down. Maybe from 1500 rpm's to 2000 rpm's ( more or less) I drove for a few minutes and it drives ok. There is no check engine light. It drives good but gas milage could be better. What can be the problem? Thanks

 

You say you noticed the tach bouncing up & down...
What about the engine itself? Is it actually revving up & down or is the tach flaky?

 

+1 It sounds like a sensor problem. If the engine speed insn't "bouncing" with the tach, then it's prolly your VSS. If it is, then it's prolly the idle air valve/sensor.

 

No, the VSS is for the speedometer. The tach signal comes from the ECU.

If the engine is actually revving up & down (sorta common), that could be a problem with the IACV. Or maybe it just needs the BASE idle set properly.

 

Right, but won't a bad sensor signal still cause the tach to bounce? (correct me if I'm wrong here.)

 

The VSS measures the road-speed of the car, so if it were flaky, that would ultimately feed to the speedometer (not the tach).

The engine RPM comes from the ignition timing signals (crank & cam sensors) first to the ECU. Then the ECU generates a tach-signal to drive the tachometer needle. (On older Accords the tach signal is generated directly by the ignitor inside the distributor.)

 

to answer your question: The engine is actually reving up and down not just the tach meter.

 

So I should check the iacv? Any other suggestions?

 

First verify all of these things:
- no vacuum leaks
- no air in cooling system (really!)
- throttle plate & throttlebody bore are clean
- throttle closes completely
- accelerator cable goes slack, at least a little bit

BASE idle setting:
- find the idle-air screw, recessed in a hole above the throttlebody bore
- engine all warmed up, all loads (electrical & AC) turned OFF
- unplug wire from IACV (engine wants to stall - nurse it with throttle)
- adjust idle-air screw for 550rpm
- - (if the radiator fan runs, wait for it to stop)
- turn it off & plug everything back in

Force it to re-learn the idle control behavior:
- reset ECU by pulling fuse #13 in fusebox near right-front door hinges for a minute
- let the engine get cold
- start the engine without touching gas pedal
- let it warm up completely without touching gas pedal

 

Well Jim, it appears to me that you are obviously the man, so thanks for your input!
Just to clarify, even if the ECU is getting bad information from a sensor like the VSS, that WILL NOT affect the signal the ECU sends to something else, like the tach????