Aftermarket tach needle spiking while driving?

[Chevy3+3]

I have had something going on with my truck for a little while now. I can't seem to track down the cause. When driving my truck ('88 V30 454) I notice that if I try and hold a constant rpm around 1800-2200 rpm that my tach needle will spike a few hundred rpms higher.

So basically if I try and hold 2000 rpms running down highway my needle will bounce erratically up to around 2300-2400 but the motor speed doesn't change. When it does this I can stop it by slightly accelerating. Its almost as if it only does it when I'm slightly coasting or when engine isn't under load.

I've replaced the entire distributer (including all new internals) and plug wires. The spark plugs only have around 5K miles on them. I pulled them and they appear fine. I have not replaced the coil itself but not sure how it could be causing my problem anyway.

I'm a little stumped because it wasn't a problem when I bought the truck a couple of years ago. I am able to duplicate the problem while in neutral also. If I just hold the throttle steady around 1800-2200 the needle will start bouncing. When I duplicate the problem in neutral and the needle starts bouncing the engine does stumble very slightly.

Any ideas? Thanks

[tommy217xxx]

My guess would be a bad tach.

[TrailerTrash]

I had this problem a few years ago in my truck that had been sitting for some time. it would bounce all over the place all the time. i tried 2 other tachs with the same problem.

it turned out to be a bad ground wire. i had it grounded to the dash down near the ebrake pedal. it was a bad connector at the end of the wire that i hadnt crimped well. i redid the connector, used a better bolt, then grounded the dash to the metal on the Ebrake pedal mount. there should be a star kinda thing with a bunch of tabs for grounding.

I havent had a problem since. I did alot of searching the internet some people get interference from other electrical sources and the make things to counteract that affect. hopefully your issue is a simple grounding problem

[Chevy3+3]

Thanks for the input. I will check the ground to the tach. But would this necessarily effect the way truck runs? I guess if a tach was bad internally then maybe it could effect it. Like the signal wire shorting to ground?

[Chevy3+3]

Originally Posted by tommy217xxx

My guess would be a bad tach.

After googling this topic more in depth it appears that an internal short in the tach itself will and has caused problems like what I'm seeing. So I plan on unplugging the tach and seeing if I can tell a difference. I may even get a different tach and see if a different one shows spikes also. If it doesn't then the I will know it was the tach.

[tommy217xxx]

My aftermarket tach quit working but I dropped it a few times. It still lights up but no needle movement. I think I'm just gonna remove it.

[Chevy3+3]

Well I unplugged the tach signal wire. Couldn't really tell any difference. Today I decided to double check my initial timing and when I disconnect the bypass wire my tach does not jump around at any RPM. When I plug the wire back in my tach needle starts jumping again.

Could it be my ECM?