Cadillac Owners Forum banner

tachometer like a rollercoaster...

1195 Views 11 Replies 5 Participants Last post by  rockybalboa11
hello pals. :-D

i have a '94 N* STS. i've had:

plugs & wires
failed ignition coils
& a vacuum-port (3 hoses that sit on top of the intake)


however, i (or my dealer:p) have been unable to fix the problem of this rpm needle going up and down during a coast towards a red light. the only suggestion i got was "the IPC, which costs $1700 to replace. deal with it."

i want to believe its an injector sticking causing a misfire. because the instant miles per gallon go up and down in sync with the tach.

any suggestions?
1 - 12 of 12 Posts
Does the engine change in response, or the tach moves without an engine speed change?

If the engine is surging: Throttle kicker motor.

Check it, adjust the clearance between the throttle and pintle. There should be some threads on how to do that here.

If the engine is not surging: Likely ignition problem. Probably ignition module or bad connection somewhere.
nah, the car isn't surging.

i've had two coil packs replaced already. no rough idle or anything aside from this up and down needle when coasting.
coil packs don't affect the tach.

The tachometer reading is reported from the ignition module. Unplug and reseat all connectors going to it. The major parts suppliers should be able to test the ignition module for free.

Another thing to check is a loose or corroded battery cable. Those can mess with all sorts of systems.
oh thought you meant ignition coils

ignition module, hm i shall have to ask the dealership about that
argh. intermittent rough idle returned. feels like another set of coils is failing. although they must be attempting to stay alive, as it was a quick 5 second roughness. no surging either. has only happened once.

are we all in agreement that the ignition module is the first place to check for this tach gone crazy? :-D i'll mention when i take the car in whenever the coil pack completely dies.
I was thinking it might be a transmission thing -- maybe the TCC going in and out as you coast. I guess it depends on the reported RPM variation. Is it a few hundred RPM, or is the tach swinging from 1000-4000 RPM or something obviosuly erroreous like that?
a drop & add of 100-200rpms until idle for the most part.
Has the ISC motor and/or vacuum leaks been positively ruled out? Are you sure that this is just an indication problem and the engine is definitely not surging 100-200 RPM?
If it's only varying 100 RPM or so, I wouldn't really consider that usual. That's going to happen as the A/C cuts on and off, as the PS pump varies its load, as the transmission downshifts, etc.
I'm also thinking it could be the ICM (Ignition control Module) Have them test the module .. If your coil packs are going bad one right after another, a short in the ICM can cause the Coil Paks to fail...
Has the ISC motor and/or vacuum leaks been positively ruled out? Are you sure that this is just an indication problem and the engine is definitely not surging 100-200 RPM?
not 100% sure. it is hard to feel a few hundred rpms.

all i know is that when i am coasting, after the initial drop, the tach needle will raise and lower, raise and lower, until i come to a very slow roll. usually at about 1000rpm.

edit: also, gearhead, 100% positive? because that is definitely the case. feels as though a pack wants to fail again. an intermittent surging/rough idle, usually within the first couple minutes of the car being started, as i have experienced in the recent past.
1 - 12 of 12 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.