Tachometer Jumps Around
 

I did a search, but didn't find exactly my problem. First off, I have a 98 Civic LX with a standard transmission. Every so often (becoming more frequent) my RPMs jump around. The video I linked to below shows it jumping up, but it also drops down to nothing too. It's not just the tachometer malfunctioning because the car surges and coasts according to what the tach is doing. When it drops down to zero, the car coasts and pressing on the pedal has no affect. Typically it happens at higher speeds (between 60 and 75 ish), but it's so random that I wouldn't count out other speeds also. And it doesn't matter if it's raining, dry, cold, or hot outside. So far it only happens after driving for a little while. This morning it was after about 40 minutes, but usually it's closer to an hour. It also only lasts a few of seconds, then everything is back to normal. It might happen once during an hour+ trip, or it might happen several times. I mostly drive with my cruise set and when it drops down it doesn't even kick the cruise off (which is weird). But it also happens when the cruise is not set. Recent repairs since summer: Tune up with plugs, wires, and distributor cap, timing belt and water pump, new (used) transmission. But it was happening before all those repairs too. Had the check engine codes read last night and got: misfires on all four cylinders and catalytic converter (Can only assume they are related to this problem, but I don't know for sure.) Took it to my mechanic when it first happened, but he couldn't reproduce the problem so he couldn't do anything. Not sure what else you might need to know. If anyone can help, it would be much appreciated.

https://vimeo.com/145991836

Thanks,

Scott

I was going to say it could be your crankshaft fluctuation sensor. If you look down by your serpentine belt sometimes the belt will actually cut into the wires going to the fluctuation sensor. But if you pulled the codes and that wasn't one of the codes that came up then it probably isn't that. I think the code for that is a P 3316. And when you pull it up on a code reader that comes across as crankshaft fluctuation sensor and then in parentheses it says rpms. But I would look into it open your hood look down by your belts and see if the belt driving the alternator is rubbing into that wire. If it's not then it could be your crankshaft sensor or camshaft sensor going bad. The bummer is if it is your crankshaft sensor or fluctuation sensor . It's behind your harmonic balancer so you would have to remove everything you would to do a timing belt. But I would look into that wire first before assuming anything else.