1991 RPMs
Zen Vet Master Level VII
Joined: Mar 2016
Posts: 5,218
Likes: 1,174
From: Southlake, TX
Yeah, that is pretty high. You should be 11-1200ish when cold and once warm around 750 +,- 50.
You may have some air leaks. Double check your hoses for vacuum leaks. If that some not do the trick we may have to check the IAC.
You may have some air leaks. Double check your hoses for vacuum leaks. If that some not do the trick we may have to check the IAC.
Team Owner
Joined: Sep 2003
Posts: 25,387
Likes: 2,741
If you have C68 check the AC control for RPM that the ECM is reporting. If you don't have C68 get your hands on a scanner or a hand held tach and compare to your cluster.
Turn key to ON without cranking and the TACH should park at 0 - if the TACH doesn't park at 0 with KEY-ON (NOT RUNNING) then it could be assumed that you've got a cluster that's needing attention/calibration.
Turn key to ON without cranking and the TACH should park at 0 - if the TACH doesn't park at 0 with KEY-ON (NOT RUNNING) then it could be assumed that you've got a cluster that's needing attention/calibration.
Team Owner
Joined: Sep 2003
Posts: 25,387
Likes: 2,741
Do you have the FSM (Factory Service Manual) for your '91?
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Team Owner
Joined: Sep 2003
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The feature the OP is interested in can be calibrated to be quite accurate. Sure the analog gauge pack isn't to be considered "gospel" but TACH and SPEEDO can be calibrated to be considered quite accurate.
Drifting
Joined: Jul 2014
Posts: 1,426
Likes: 46
From: Lynchburg, VA
FYI, the procedure for checking RPM with the C68 interface is:
Press and hold the fan up + fan down buttons until the fan speed display changes.
Using the up and down arrows, select (I think) number 5 and press "auto" to confirm.
You will then have a readout of RPM divided by 25. My tach is a bit off, but this will be accurate.
I believe autos are supposed to idle around 700-800 RPM; if yours is significantly above that I'd suggest looking for vacuum leaks, although another possibility is that your IAC valve is malfunctioning.
Press and hold the fan up + fan down buttons until the fan speed display changes.
Using the up and down arrows, select (I think) number 5 and press "auto" to confirm.
You will then have a readout of RPM divided by 25. My tach is a bit off, but this will be accurate.
I believe autos are supposed to idle around 700-800 RPM; if yours is significantly above that I'd suggest looking for vacuum leaks, although another possibility is that your IAC valve is malfunctioning.
Team Owner
Joined: Jul 2004
Posts: 28,431
Likes: 3,284
From: Hartford WI
Depending on the ECM programming and engine temperature. My command idle is 900 at stable temperature. To use it in a pinch, sure. I'd get a scanner since you will need it sooner or later so why wait?
Drifting
Joined: Jul 2014
Posts: 1,426
Likes: 46
From: Lynchburg, VA
Based on my limited understanding of how the tach works, if your tach reads high, couldn't you just add resistance to the tach wire coming from the distributor to reduce voltage enough to fix the error?
I agree with you here. Specifically, an ALDL cable that hooks up to your computer is a must-have for troubleshooting. The USB ones are pretty cheap, and TunerPro is free.
That said, it's still nice to know about the C68 functionality. For this it works just as well, and unlike a ALDL cable it's immediately available. Even though I have a Bluetooth ALDL adapter, I've used the C68 readout a couple times when I didn't feel like getting out my laptop.
That said, it's still nice to know about the C68 functionality. For this it works just as well, and unlike a ALDL cable it's immediately available. Even though I have a Bluetooth ALDL adapter, I've used the C68 readout a couple times when I didn't feel like getting out my laptop.
Team Owner
Joined: Jul 2004
Posts: 28,431
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From: Hartford WI
Drifting
Joined: Jul 2014
Posts: 1,426
Likes: 46
From: Lynchburg, VA
I don't think so. It isn't resistance only. http://batee.com/corvette/dcrg/9096_analog_ic/