hawk55

my car just started having some weird electrical issues. when i first start car it idles very rough and the tach jumps all over the place, 1200 to 2000 to 1300 etc etc when i drive it it is rough and backfires, tach all over and no power or pull below 3000 rpm's, after it revs higher it smooths a bit but still has smaller coughs. now whats weird, i think, is as it drives more and warms up it idles better doesn't back fire, the tach still jumps around but much much less and from 3500 rpm's and up it runs somewhat smooth but the minute the idle drops it idles rough and tach jumps around. it has an MSD unit, new spark plug wires, i have checked battery connections and cables, fuses thus far. so any thoughts and suggestions from the experts would be great. how can i check coil? it is also an msd unit. this is a 78 3.0 911.
thanks

Ed Hughes

Start with the engine ground and the ground from your battery negative terminal to the body. Also check that all of your connections look good. That's a start.

AVDCAV

Did you check for vacuum leaks ? What makes you
Convinced its electrical" check the simple things first.

Torque up the entire intake and snug every hose clamp
I had the same issue in the summer.

Ed Hughes

I'd think the tach behavior and vacuum leaks would not be intertwined.

Ed Hughes

Question to the OP: Did you do any work and then have these symptoms? Or, did they just come from out of the blue?

hawk55

thanks, will do. no it was weird, car was running good, a little rough here and there. i changed plugs and it ran much better, then out of the blue it started running rough in mornings and the tach issues started. also every now and then it will run fine for a moment, as if there is nothing wrong. then it goes back to the tach jumping all over the place and the misfiring.

Ed Hughes

Yes, I'd sure look for a ground connection that is going intermittently bad. You should have a bundle of wires grounded to the motor. Or, as I mentioned, the ground strap from the neg on the battery is known to fail over time.

hawk55

i will go through it all. thanks, also i measured resistance on the rotor and it was 4.9, i am thinking that is high. could that be a issue.

theiceman

a couple of things.

1. get a voltmeter on the syten m and mo nitor while idling and running. Make surr your alternator isnt spikin like crazy. keep in mind it your tach senses an over rev condition it will cut motor spark. So it could be something sending oyur tac h into over rev condition.


Also check the ground that runs from the transmision mount bracket to the body. These can corrode. You will have to get right under there but it is worth checking out.

dshepp806

as noted,....simple things first: grounds/connections and voltage levels.

When did the new plug wires go on?

Doyle

Shannon123

ED
No intent to hijack the thread, but have been reading a bit about engine ground straps etc. Is this a thing to be proactive or reactive on ? It seems like many issues can be resolved by having a fresh engine ground (I guess based on Pelican parts, this would be called the transmission ground?) on your car....which begs the question from me, is this an easily replaced strap ? I was trying to find some DIY stuff but did not see too many visuals on it.

Ed Hughes

I'd say proactive. It's a 1 or 2 on the difficulty scale. I'd say less than $10 at your local auto parts store for the new strap.

wwest

The factory CDI has something of a desight flaw in that the SCR, Silicon Controlled Rectifier, is subject to "self-firing" if the battery supply voltage, or voltage spiking, gets too high, >15 volts. Aftermarket CDI's also make use of an SCR so....

It's more important to check the voltage somewhere in the engine compartment rather than all the way upfront where the battery will act as a "sink" for any high voltage, >15 volts, excursions a failing alternator/regulator might generate.

A simple test for this problem is to turn on ALL of the electrical loads and see if the engine RPM smooths out.

wwest

Light green metallic '78 Targa purchased in Fresno?

wwest

E-brake light "sticks" on..?