Question: 1990 L98 vette engine idle & tach problem

[JOE BERA]

I have a 1990 L98, 6 speed. When cold, engine runs good. as soon as it warms up, it hunts for idle (drops to approx 250 rpm & knocks a little & then increases to approx 1000 rpm & knock goes away. Each phase lasts about 2-3 seconds. The odd thing is, when cold, the tach seems to work ok, (idles at about 550 rpm (by the sound). The tach reads way too high when it warms up (the same time as the engine acts up)., shows 2000rpm at idle & if engine revved to approx 3000rpm tack pinsat top end.
Any ideas?

 

[gw962]

Any trouble codes?

My best uneducated guess on the idle would be a weak O2 sensor.

Sorry, but I have no clue on the tach. Supposedly, we have no tach filter on the 90, but I've never looked for one on mine. That may be something for you to check.

Good luck

 

[G Winter]

Sorry, but I have no clue on the tach. Supposedly, we have no tach filter on the 90, but I've never looked for one on mine. That may be something for you to check.

Good luck

There is a tach filter attached to the top left bell housing bolt. It has white wires going to and from it. My tach is off some and bypassing the filter did nothing.
I did some searches and found that the tach in our vets is the same as the F body of the same yrs. A resistor bank on the tach is used to calibrate the tach when it is manufactured and part of this goes bad. Repair is quite a process and a little bit of a hit and miss situation. :ugh

Glenn

 

[vetteoz]

Dodgy tacho in '90 models is well known problem.Some have fixed as stated by replacing tacho filter.Most have had to have work done on the tacho itself to fix.

http://forums.corvetteforum.com/showthread.php?t=1876888

http://www.thirdgen.org/techboard/electronics/357893-90-92-tachometer-fix.html