my 88 coupe 3.4L 4 speed, idles alittle high when its cold, about 1900 RPM according to the laptop and ALDL connector, but the dash gauge reads it as 2400 RPM, hot idle the car shows 1300-1400 RPM while the ECM is reading 975-1050, i also had the idle at 4400 RPM according to the ECM and the dash gauge was reading 5200 RPM, do you guys think the tach filter could be causing this problem, or would it more than likely be a ignition related problem? i have cheap plug wires, and i think they might be arcing,

also the temp gauge on the dash moves when the key is triggered, but it doesnt move ANY after that, the ECM sensor is reading properly, i got the car upto 170* but the gauge was not showing anything, it stayed at the same place it was once the key was triggered.....

think the gauge on the dash is bad, or the sensor on the engine?

matthew

edit: forgot to mention that the car does have a 120 v6 gauge pod and RPM gauge. from a 86GT

[This message has been edited by m0sh_man (edited 09-29-2004).]

The idle RPM # coming from the ECM is the accurate one. Something's fishy with the tach reading. The first thing I would check would be the tach filter, and the wiring that connects it with the tach in the dash.

Ed

More than likely the trouble is with the tach. If it was in the filter you would have false readings to the ECM too. Mine is off about the same amount as yours, just have not taken the time to fix it.

The scan tool data provides the most accurate RPM information, even though the update rate is quite slow in the Fieros. The factory tach in the Fiero is calibrated with a capacitor, which breaks down over time which causes the tach accuracy to deminish. I replaced the cal capacitor in my tach with a new one and it fixed my problem.

do you know which cap to get? can i get it at a local radio shack?

matthew

looks like your tach is reading about 500rpm high.. could be the needle needs to be moved - could be that the capacitor is bad

as for the temp guage - the sender going to the ECM is not the same sender as for the guage - the sender for the guage is bellow the ignition coil on the left side of the rear head.. easy to replace..

For what it's worth, this is a pretty common problem with V6 Fiero tachs. A number of folks on the racing list (myself included) have experienced this problem to varying degrees.

Oliver Scholz explains how to fix it: http://www.fieros.de/en/articles/tach.html

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Doug Chase
Chase Race
Custom roll cage and exhaust fabrication

quote Originally posted by m0sh_man: do you know which cap to get? can i get it at a local radio shack? matthew

I don't think Radio Shack carries the right unit. For a 6 cyl tach I believe the correct capacitor is 0.0068 uf with a 100v rating. Problem is most of the capacitors I have found to replace this unit have a 10% tolerance which means their actual values were all over the place. What I do is just buy a bunch of them, and swap them out until I find the one that gets it the closest.

For the temp gauge. I don't remember which of the two wires BUT if you disconnect the sensor connector for the temp gauge and short one wire to ground the needle will peg. That says the wireing and gauge are good --- replace the sensor

Frank

If the needle is off by 500 rpm then you could easily see that by turning the key to the on position with the engine off. If it resets to zero then it's not the needle.