yesterday and today my 1988 Pontiac Fiero base coupe 5 speed had an electrical issue regarding the engine.
The tachometer would jump a few thousand rpm for a split second and almost bog-out when accelerating....

any thoughts

Sounds like an ignition control module.

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Better filling of the cylinders with air (lower RPM and open throttle) causes more air to be in the cylinder. More air means more voltage is needed to jump the spark plug gap. Higher voltage in the secondary means weak insulation can be jumped. That is why this trouble occurs at low RPM high throttle times. Secondary voltage is jumping through the coil from the secondary to the primary. Check the resistance of your plug wires. They should be less than 10K per foot. Look at your distributor posts and rotor for corrosion. Make sure your plugs are gaped correctly. Then replace your ignition coil.

Wait long enough and your ICM will fail also.

[This message has been edited by phonedawgz (edited 06-08-2014).]

quote Originally posted by phonedawgz: Better filling of the cylinders with air (lower RPM and open throttle) causes more air to be in the cylinder. More air means more voltage is needed to jump the spark plug gap. Higher voltage in the secondary means weak insulation can be jumped. That is why this trouble occurs at low RPM high throttle times. Secondary voltage is jumping through the coil from the secondary to the primary. Check the resistance of your plug wires. They should be less than 10K per foot. Look at your distributor posts and rotor for corrosion. Make sure your plugs are gaped correctly. Then replace your ignition coil. Wait long enough and your ICM will fail also.

How would this effect the tach? He didn't say the engine jumped, just the tach..

Correct. Just the tach jumped and the engine kinda cut out.

The tach jumped because the ignition module was firing the coil too fast, causing the stumble. This is definitely an ignition fault. Likely the module, possibly caused by plugs/wires/cap and rotor.

I have DIS ignition as it is an 88 2.5 coupe.

The coils are new as off today. The wires are fine and Ill check the plugs again but they will probably be fine as they are less than a year old.

quote Originally posted by Custom2M4: How would this effect the tach? He didn't say the engine jumped, just the tach..

The ICM generates an ignition pulse about 10 deg btdc. The ICM passes that pulse onto the ECM on the Purple/White wire. It is and also out sent out of the ICM on the tach port. Once then engine is rotating over 400 RPM the ECM puts a voltage on the Tan/Black that the ICM should be firing now on the ECM's generated signal. That signal will have the spark timing occurring perhaps 40 deg BTDC. The ECM sends that signal on the White wire and that signal is what is used to actually fire the coil. (Go figure that how does the ECM generate a signal that occurs BEFORE the reference signal from the ICM. It does)

Anyways that signal is ran into the coil and the coil produces that high voltage output. If that high voltage output instead of jumping the spark plug gap instead shorts back to the ICM, there will be a pulse at about 40 BTDC and a second pulse at 10 BTDC.