I drove the long way home from work and the tach didn't bounce at all (except once when I killed it).

 

I guess you mean it did not bounce at idle? With respect to the trailing coil, unbolt the bracket to the fender and sand down the part of the bracket which mates w/the fender, sand down the fender mating place as well and clean the bolt threads. Then remove the jumper and see if the tach still behaves erratically.

 

I cleaned both surfaces and used two new stainless bolts to bolt the bracket back on. Removed the jumper, no tach.

 

It would appear your trailing coil is the culprit.

 

I'll second the motion. There are three pins at the ECU for the trailing coil. Pin M (Blue/Red wire) reads below 2 volts at idle (mine reads 1.3 volts). Pin U (Brown/Yellow wire) reads 2.2 volts at idle and 4.4 volts w/key to on. Pin X (Blue/Yellow wire) reads .8 volts while idling and 0 volts w/key to on. If these check out then your either the coil, coil wires, plugs are the source of your problem.

 

I'll check the pins at the ecu as soon as I get time. I'm 99% sure it's the coil though.

I flooded the car today, and de flood wasn't working on my rtek, so I pulled the trailing plugs and the INJ fuse and deflooded. I forgot to reconnect the trailing plug wires after this, but the car ran exactly the same. I only noticed my mistake when I changed the oil afterward. With the plug wires reconnected, there is no change in how the car drives.

 

the trailing plugs are like a placebo for power, you can hardly tell they are there when the leadings are working properly. the trailing is merely to help complete combustion and improve emissions and slightly aid fuel mileage. however the tach uses the trailing coil for its source which also happens to be the most problematic coil, mostly because they are overworked by having to drive the plugs that do not self clean readily and are shrouded. running old plugs that are semi fouled can kill the coil, especially when combined with poor grounding of the coil, which is why some people keep running into the problem time and time again.

in other words, it's a good time to verify your car is due for a tune up. are the plugs and wires 10 years old? if yes or you aren't sure you may want to address them first or possibly kill a replacement coil. 8 years/60k on a wire set, 15-20k on plugs is what i recommend in changing intervals.

 

Plugs and wires were replaced when the engine was rebuilt, which was 3 weeks/500 miles ago.

 

Then replace the coil and be done with it.

 

My tachometer stopped working after a morning start, but idled nicely. After turning off the car and turning on, the tachometer worked, and drove for about 30 miles. Started again today and it seems normal. I went ahead and cleaned the grounding area under the trailing coil. I made an attempt to clean the other grounding area on the shock tower, but the most I could do was loosen the bolt and spray Deoxit on the area. Darn, AC pump. The difference I noticed after starting was an unusually cold start idle close to 2000 RPM. I don't remember it ever being that high during a cold start. It eventually dropped to 500 after warming up to normal operating temperature. I'm guessing this is normal. It is fair to assume cleaning the ground may have fixed something?

Thanks.