just wondering about this. the tach in my 90 reads about 400rpms over what it is. would the reading from the tach on the dash affect the computer or is it  a separate system? is there any way to adjust it so it reads properly? thanks for any help. dave

If memory serves, the C4 tach is electric.  Gordon Killebrew would be the man to answer this one.

How do you know it is reading "fast"?
The dash read-out gets it's info from the ECM, so it should be the same as what the ECM shows in data.
It is possible the distributer has the wrong ignition module, too, but that would also effect the ECM reading for the engine rpm.

Then again, it could just be a problem with the gauge itself, or the tach filter...again... Weren't you having a filter issue last summer, or was that someone else?

Anyway, the tach reading higher than actual engine rpm shouldn't effect the way the engine runs, unless the ECM is getting the higher reading too...

when i hook up the scanner, the scanner shows 800 but the tach on the dash says 1200.as far as i know it has the right module in it as i installed it. dont remember about bringing this up this summer but had other problems. will try a new tach filter.

my iac number is too high and all the other numbers are correct. i have installed a new iac and numbers dropped a hair. was wondering if it was seeing the wrong reading on the tach and messing with the a setting on the computer. i will be installing new injectors soon (which i planned anyway). thanks for the help. dave

Dave, I'll look and see if I have any troubleshooting info on the tach when I get to work...if so, I'll send it via e-mail this eve.

IAC too high could be due to a dirty, or otherwise contaminated throttle body. Have you cleaned it, or done anything to it recently? If the idle speed is too low, the IAC is gonna want to raise it to where it should be.

thanks joel. i thoroughly cleaned the throttle body out a couple months ago and about two weeks ago had it apart again to make sure it was still clean. all new gaskets were installed.

i replaced the iac as the idle will not go below 800. i set the min. idle the way snapon troubleshooter said to and still wont go lower. the scanner says the desired idle should be 7-750 while i had the car running.

haynes manual listed as part of things to check was leaking injectors and i was going to change them anyway as someone who saw them told me they were changed already. with some of the things i found that were done mechanically to the car i wouldn't be surprized if they put the wrong injectors in or didn't install correctly. i have a set of new injectors so i will have to get off my butt and replace them.

with the tach issue, i was just checking into that because it's the only thing that shows wrong right now. guess i was trying to think like these computers do in the cars. these computers can drive you nuttier than a home computer.

thanks again for any help.

Have you also adjusted the TPS to spec? If it is higher than it should be the idle will also be high. There is a specific method for adjusting the TPS, and it should be followed. There is really no need to ever adjust the idle speed screw on the side of the throttle body...in theory, anyway. If you have the factory service manual, the procedure is in it.
A skewed coolant temp sensor could also cause a high idle...