I have A 1991 5.7 Z with a replacement GM Longblock freshly Installed. The only Upgrades I have Performed Are Larger Compucam ,TB Airfoil, Accel FP Regulator, Accel Super Coil, and SLP Emission Legal Headers; Flowmaster Muffler. Seeing as to how the Engine Management Is Speed Density, I Know I Need To Look into Eprom Burning. With that Said, I'm just curious If anyone knows If the Tach Is routed through the ECM. Ever Since My Engine Replacement, My tach is extremely Inaccurate and Flys way off the scale. At Idle it is off by about 800 RPM. (Showing Idle Near 1600 when in fact Idling at or about 800) I have the GM Bible For this car, which shows an inline Tach Filter somewhere in the Schematic. I also thought I had read there is an Unspecified Filter In or under the MemCal, yet I am unsure of the purpose of it. The inline filter (which i am currently unable to locate) according to the 1991 Chvrolet Camaro GM Service manual, Is used to Filter out unusual voltage irregularities or spikes. I was actually thinking that perhaps an Aftermarket coil such as the Accel piece I mentioned above could possibly be an awful good source of irregularity. Just a possibility i have considered. All said, The tach worked perfectly before the engine change. Anyone experienced a problem of this nature? Or possibly anyone made some sort of mistake in the past that I may have mage, eg; have I overlooked a simple ground, or connection? Any input on this would be greatly appreciated, as I am at my wits end on this one...Thanks In Advance!
~Jack~

 

I had the same probelem you have, when i swapped out my 305 and dropped in the 350 with hei and carb my tach was like 500 rpm to high at idle and like 1000rpm off while cruising. Although i dont know why this happened i just disconnected the stock tach and installed an autometer one. My tach worked perfectly before the swap also, im pretty sure your tach has nothing to do with your comp mine didnt but mine is an 88. Hope this helps

 

You might have to change your tach because it sounds similar to what i have had to do right now because we are swapping the 2.8L MPFI for my 305HO (carb) and the tach had to be replaced because it was only for a V6 but i don't know if there is a difference for you in the tach if you had a 305 or a 350?

 

86firebird86,
Thanks for your reply. I did however swap a factory relacement 350 in place of a tired 350, so unfortunately, the Tach should have still worked. Good looking out though; i appreciate you trying to help!
~Jack~

 

Sorry i couldn't of helped you solve the problem, hope you get it all worked out.