OUTATIME GTA

iTrader: (2)

Read a few posts regarding a bouncy tach, but seems most of those accompanied a bad idle or new wiring, etc.

My GTA is in fine running order, no aftermarket guages, cluster, etc.
While at the dragstrip this past week, the tach was bouncing erratically after the first shift or so.
Normal everyday driving, cruising etc, you wouldn't notice it. .. it rises and drops at the proper points, but if you get on it WOT it will jump around sporadically.
Any similar experiences?
I do have a full msd ignition; cap, rotor, coil, 6al, over a year old.

Air_Adam

Check the plug wires... one might not be snapped on fully. Check both ends too. That happened to me once, and caused a bouncy tach.

OUTATIME GTA

iTrader: (2)

Will do, anything else that might cause this?

84z28350

Does it sound like the engines misfiring when the tach starts bouncing?

The 6al's have a rev limiter maybe thats whats causing it...

OUTATIME GTA

iTrader: (2)

no misfiring, runs perfectly normal, except impossible to shift the auto manually if it's bouncing around.

i thought about the 6al, but not sure how the rev limiter would affect it. read about the tach filter, but it seems that's pre-89.

also read the capacitor replacing posts, but it seems those problems were the tach was just off, reading too high or maxing out then dropping to 0.
nobody with a solution for a bouncing tach at WOT.

89gta383

iTrader: (5)

Get the tach filter. I have one on my 89 gta, it will solve your problem.

OUTATIME GTA

iTrader: (2)

Did you have the same exact problem?
Just kinda strikes me as odd all of a sudden it's so whacky.
Is it part # 8910, white wire, for the adapter?

TiMz89TA

Almost the same thing happend to me in my 305 tpi. The only difference was that the tach wasnt bouncing just at WOT, it also bounced under any acceleration. It turned out that my ignition module was bad. It is located under the cap and rotor and is kinda hard to get at. When i took mine out, the contacts were corroded.

88305tpiT/A

iTrader: (2)

Ive had this problem for about 5 years and cant seem to fix it.

Sometimes when this happens I also get engine stumbling.

I have my entire dash apart and harness on the floor to see if I can figure this out.

Apeiron

The tach bounces when the spark doesn't fire on a cylinder.

OUTATIME GTA

iTrader: (2)

Sounds like you guys have a diff. problem.
My engine doesn't stumble, runs like a champ, just don't know what the rpm's are really at!

After some research I think I've narrowed it down to my MSD 6AL box.
There is a tach adapter to buy from MSD that is necessary for certain year Firebirds that don't have a tach filter to remove.
I'm going to give it a shot within the next month hopefully.
Any other ideas are welcome though!


What's the easiest way to check if there is spark to all cylinders?
I already made sure all the plugs are in.

Apeiron

I was commenting on 88305tpiT/A's problem. Your MSD box probably won't have that symptom of a dead plug.

Air_Adam

Whats worked for me in the past is to run the engine (idle) and pull one off... if the engine speed drops or it runs rough, its good. Try this with all the plugs/wires. If you unplug one, and the engine speed/running conditions don't change, then you found one thats not firing.

88305tpiT/A

iTrader: (2)

But wear HEAVY gloves cause being shocked by the secondary voltage HURTS!!! Makes half your body tense up something fierce.

To respond to the solution above of a dead cylinder -- No I dont have one. Runs smooth except when tach jumps around. Sometimes It causes engine stumbling, other times I can be cruising around and the tach will go nuts and the engine will run fine.