RallyRed701/2

So I recently started havng some issues with my 96 Z28/SS. I'll be driving along and the tach will intermittently stop working. Most times the car will still drive, but the tach will just be showing 0rpms. Other times however it will spit/backfire and want to stall out. I'm usually cruising when this happens, so I just push in the clutch and rev it and it usually stays running. I made a short vid on YouTube that shows what it's doing:

I'm trying to figure out what might be causing this. I just replaced the icm today and it still seems to have the issue. Thoughts?

Injuneer

The tach is driven by the PCM. The PCM determines RPM based on the pulse signals from the optical cam position sensor in the Opti. Scan it for codes to see if you are getting either of the codes for the Opti pulse signals. P0372 , P1371

What brand is the Opti?

Fatdog2

I ran MSD wires on my old bird and the coil wire kept slipping off so I ran a stock coil wire with the MSD wires and had no problem.Might check that.MyO2.

Fatdog2

As far as the tach thing I dunno.

Injuneer

Your 93 drives the tach off the coil, through a filter. 94 and up they changed that to a feed from the PCM.

JakeRobb

I see the Service Engine Soon light is on. Have you scanned it for codes?

In addition to all of the other possibilities, it's possible that the gauge is bad. A decent scan tool can show live RPM data; if that's reading zero when the tach is, that would rule out the gauge.

Also, Fred, doesn't the PCM rely heavily on RPM to choose cells in all the various maps? If the motor weren't getting a good signal from the opti, seems like fueling would be all over the place if it worked at all.

Injuneer

It uses the RPM for everything, including timing the ignition and the sequential injectors. And sets the limits for the long term fuel correction cells, is one axis of other tables.

MY first thought was faulty tach. If the PCM was getting zero RPM from the Opti, the engine would shut down. But also mentioned "stutters and stalls" so it made me think erratic Opti signals.

I'm working with another member who had crazy high idle RPM. After looking at a scan log, I saw crazy RPM numbers, like 7,000 RPM for a fraction of a second. In response, the PCM would open up the IAC valve to max, and the engine would idle at 2,000+ RPM. Through a member on another site who works for MSD, I was able to send the data log showing the crazy RPM readings and my analysis to MSD engineering. They eventually asked the owner to send the MSD Opti back to them for testing. Still waiting for the results. That was the 3rd MSD that I had found with the same problem. I managed to find the data log from the second MSD and that was also sent to them.

Point is the pulse signals from the cam position sensor can affect numerous things within the PCM. But it still seems to be a contradiction with the tach reading zero and not affecting the way the engine runs sometimes, and other times causing stutters and stalls. Not something I have seen before, and still no accurate explanation of why it is happening or what is causing it.

shoebox

That is interesting, Fred. Why? Because the OP has an MSD opti (he had emailed me about the same problem). I was also trying to tie the tach error to the running symptoms (seemed logical).

Injuneer

At this point, may not matter, because the OP hasn't come back and signed in to this site since the instant he posted above, and immediately signed off.

RallyRed701/2

Hey guys. Sorry it's been crazy busy for me the last couple weeks. So much so that I haven't had a chance to tear about the car and get a good look at the wiring at what not that connects all these components. I truly appreciate all the help, and will be looking into all these potential problematic components. I'll keep everyone posted!

Thanks again!

aaberg

Old post but I give it a try. I have same problem with my 1997 Z28. Did you solve the problem and that set it?

Injuneer

Your 97 does not operate the tachometer the same as the original poster's 93. May not be the same problem.

So far this thread would appear to be the possibility of the problem being the same, and getting help::

https://www.camaroz28.com/forums/lt1...-drive-889752/

chevykid

Injuneer,
the thread starter stated this

"I recently started havng some issues with my 96 Z28/SS"

later in the thread someone with '93 said they had same problem, you told them '93 tach runs off the coil.

Injuneer

The whole thing, trying to sort through his 5 different posts on the same topic was confusing enough. The fact that at the time I posted above, I was still groggy 18 hours after hip replacement surgery might have clouded my thought process.

One of the worst ways to look for help is to tack on to someone else's thread, and say “I have the exact same problem”, because you don’t know which post(s) they are referencing. And usually it's not the exact problem. Starting a new thread is the way to go.

shoebox

Agree and hope you are recovering well!