idle speed not constant

I got a tachometer that attaches to my spark plugs wire, and it's showing something very disturbing..

my idle RPMs are fluctuating.. for example, it keeps going from 700 rpms, to 1200 rpms, going up, and down, and up, and down..

When I push the accelerator, I then get a constant reading...

Here's the thing.. When I push the accelerator when the tachometer shows 700 rpm, it sounds like the car wants to stall.. but any other speed it revs up great..

that's when just in Park..

mind you, this car has NEVER stalled..

well does it go rapidly from 700 to 1200 (like in less than half a second) or does it slowly modulate between the two.

not sure about the low idle, but i am pretty sure that most tachs attach at the distributor.

the low idle could simply be that the idle stop is set too low.


does it go away when you disconnect the tach?

well does it go rapidly from 700 to 1200 (like in less than half a second) or does it slowly modulate between the two.

not sure about the low idle, but i am pretty sure that most tachs attach at the distributor.

the low idle could simply be that the idle stop is set too low.


does it go away when you disconnect the tach?
it does this even without the tach.. the tach connects to the spark plug wire, it's an inductor, and it senses the pulse going through the wire, I just tell the tach how many cylinders the car has, so it does the math..

the numbers do seem to corespond to the sound of the engine..

it slowly modules between the two, going from up to down in 3 seconds, and back again..

it does this even without the tach.. the tach connects to the spark plug wire, it's an inductor, and it senses the pulse going through the wire, I just tell the tach how many cylinders the car has, so it does the math..

the numbers do seem to corespond to the sound of the engine..

it slowly modules between the two, going from up to down in 3 seconds, and back again..

If it uses a magnetic pickup (aka inductor) to produce a reading, it won't be reliable. If anything, this may be telling you that it's time for a new distributor cap, rotor and plug wires! A good tach will interface with the tach output of the coil. On coil-in-cap HEI distributors, this is a tab marked "TACH" next to the "BAT" terminal, ahead of the 3-wire pickup-coil interface. On external coil HEI, the TACH output is *usually* a white wire that pairs with the power source (pink) wire which attaches to the coil. Not all cars with these distributors have a TACH lead.

EDIT - nvm this is your Baby LT1, right? I am not sure if these have a TACH lead, but it could be an indication of a failing OptiSpark unit.

If engine RPM seems to be in step with the reading, then it is time to look for a vacuum leak somewhere, or perhaps the IAC motor.

You can pick up the TACH signal on the OptiSpark system right at the coil, just like on any other form of HEI. There is an extra lead in the black coil connector for that purpose - At least, there is on both my LT1s. I can't imagine the L99 is much different.

If the idle RPM is not constant, and the PCM is not changing the target idle, consider removing, disassembling, cleaning, and lubricating the IAC stepper motor and pintle.