Corvette Tune/Trade
Thread Starter
8th Gear
Joined: Sep 2008
Posts: 8
Likes: 0
Corvette Tune/Trade
I have a question or rather a couple. 1) what causes my 85 vette to fluctuate when idling at a stop, from 59 to 72 rpm's? Why not a steady rpm reading? I just put plugs, wires, cap and rotor in. Question #2- I have put too much $ into this car and would like to trade up. Where would I go to do this and get a fair shake? Thanks, Bob
Burning Brakes
Joined: Apr 2004
Posts: 762
Likes: 2
From: Huntington ma
Fluctuating idle can be caused by any number of problems. Some of the possibilities can be discarded if you are not getting a check engine light. Assuming you aren't:
Verify the base idle setting and TPS adjustment, verify no vacuum leaks (including that the EGR valve is Shutting OK). Verify Fuel pressure. If Everything is right so far then check/clean the IAC valve, perhaps even replace it "just to be sure".
Still fluctuating? Next is to check the throttle body shaft bushings - how sloppy are they. Then the O2 sensor - is it so carboned up as to be useless. Is the AIR system working right - not adding air at idle. Same question for the EGR system, is the control solenoid OK.
Next up, the distributor. How worn are the shaft bushings, is play there bothering the pickup coil causing a misfire. Is the Knock sensor getting involved (any rattles or knocks in the motor?) and causing the timing to do weird things (ESC circuit).
Then there is the MAF sensor - is it borderline dead and not reading accurately at idle? What about the physical mechanical health of the motor - compression, valve job, etc. Is the Vacuum reading nice and steady at idle?
Could be Anything...
Verify the base idle setting and TPS adjustment, verify no vacuum leaks (including that the EGR valve is Shutting OK). Verify Fuel pressure. If Everything is right so far then check/clean the IAC valve, perhaps even replace it "just to be sure".
Still fluctuating? Next is to check the throttle body shaft bushings - how sloppy are they. Then the O2 sensor - is it so carboned up as to be useless. Is the AIR system working right - not adding air at idle. Same question for the EGR system, is the control solenoid OK.
Next up, the distributor. How worn are the shaft bushings, is play there bothering the pickup coil causing a misfire. Is the Knock sensor getting involved (any rattles or knocks in the motor?) and causing the timing to do weird things (ESC circuit).
Then there is the MAF sensor - is it borderline dead and not reading accurately at idle? What about the physical mechanical health of the motor - compression, valve job, etc. Is the Vacuum reading nice and steady at idle?
Could be Anything...
