So my Dad hauled in this 90 RS with the 3.1L a few months ago. He picked the thing up for about $300 because the PO couldn't get it to run. Needless to say they had replaced well over $300 in parts trying to get it running again.

We swapped parts around for a while and finally got it running again, but not good. It'd miss and sputter and cut out and if you hit the throttle hard it would backfire out of the throttle body. You could see the Check Engine light flickering a little and after it wouldn't flash a 12 code we replaced the computer and it's good to go. After that Julie laid claim to it and we've been trying to get it roadworthy for her.

However, the tach reads about ~2500RPM at an idle. I was wondering if anyone had any suggestions on what was going on? The car has a Hypertech chip in it one of the PO's put in it and while I wasn't excited about that fact, I did put it in the new computer when we replaced it because I don't have the original prom to go in. Could that be causing it or is the tach still driven by a signal from the coil like the older cars?

 

The tach is driven directly from the dizzy, theres a white wire on one of the plugs for it and thats the tach wire. Theres a thread on here somewhere about how to fix a misreading tach. If its backfiring out of the TB, the timing could be way off. Or the plug wires could be switched around.

 

Well, the new ECM fixed the back firing. Computer was dead and didn't know what it was doing. I'll search some this evening after work and see if I can find the tach thread. Thanks man!

 

Found the article. Thanks man! Just gotta re-read it and I think I'm going to use a potentiometer instead of messing around trying to find the resistor that will work best.

 

IIRC the potentiometer was used to find what value of resistor you need to fix the issue. Its then removed and the resistor was put in it place.

 

Hey admins, can we get both tach correction links stickied so there are fewer threads trying to find the info and it's more readily available.

 

i just got a 1990 rs 305 and my tach doesnt work at all. i have talked a few other guys with cars like mine and nobodys tach works...so im thinking we all probably have the same problem. anybody been here before?