point me right direction on this?

[tigmaned]

my 86 L98 has been running really good expect at idle and foot off gas pedal. so far i have done a complete tune-up, cleaned the T/B, adjusted tps to .54V, set IAC idle to 400rpms. new fuel pump and fuel filter about 3 months ago.
at a stop light i notice it the most, idle goes back and forth 5 to 6 and back, and i notice when driving on city streets and coasting it feels like some one is pulling the car back and letting off like a jerking feel. yeah pretty weird huh!! i am going to check the fuel pressure again and see if thats still right like it was, and do a vac check too. i just wanted to ask you all this incase i am missing some thing and something else i should check??
Thanks!!

 

[vette-dude]

Check the EGR valve, may have broken diaphram. You should get a code 32 I think. Also maybe the O2 sensor has gone south. Just some ideas.

Randy

 

[Agent 86]

Try raising your min idle.(and reset TPS)

If it is a vaccum leak, this can cause your IAC to constantly try and correct your target idle, thus the RPM goes up and down like a yoyo.

 

[Jeb]

Is your A/C cycling? If it's low on R-12, it will cycle and the computer will constantly try to adjust. Also, I believe you'll find that 400 RPMs at idle is a tad on the low side.

 

[redvett]

my 86 L98 has been running really good expect at idle and foot off gas pedal. so far i have done a complete tune-up, cleaned the T/B, adjusted tps to .54V, set IAC idle to 400rpms. new fuel pump and fuel filter about 3 months ago.
at a stop light i notice it the most, idle goes back and forth 5 to 6 and back, and i notice when driving on city streets and coasting it feels like some one is pulling the car back and letting off like a jerking feel. yeah pretty weird huh!! i am going to check the fuel pressure again and see if thats still right like it was, and do a vac check too. i just wanted to ask you all this incase i am missing some thing and something else i should check??
Thanks!!

If i remember correctly those cars were supposed to idle at 500rpm in drive and 600 with M/T IAC counts should be at 18 counts.

 

[tigmaned]

i used this.
Procedure
There are two electrical components on the TB that you will be working with: The TPS and the Idle Air Control Valve (IAC). Make sure that the connectors for these two components are easily accessible and that you can easily disconnect the IAC.
You will also be playing with the diagnostic connector under the dash. Remove the cover (if it's still in place). Bend your paper clip into a "U" shape. You will be playing with the two top right hand terminals ("A" and "B") in the connector.

• First step is to set the minimum idle speed. If nobody has messed with this on your car before, the set screw will be covered by a pressed-in plug. It's located on the driver's side of the TB. Remove this plug if it's there.
• With the IAC connected and the ignition "OFF," stick the paper clip into the diagnostic connector from "A" to "B." This grounds the diagnostic lead.
• Turn the ignition to the "ON" position without starting the engine. Wait 30 seconds.
• Now, with the ignition still in the "ON" position, disconnect the IAC connector at the IAC.
• Remove the paper clip from the diagnostic connector.
• Start the engine and allow it to reach normal operating temperature. The idle speed will probably be really low, and you may have to coax the engine a bit with the gas pedal to keep it running for a while.
• If your car is an automatic, set the parking brake and put the transmission in "DRIVE." If your car is a manual, leave it in neutral.
• Adjust the idle speed screw to obtain 400 rpm in drive or 450 in neutral.
• Shut off the engine and re-connect the IAC.

That's it for idle speed. Now on to the TPS.
There are 3 wires stacked vertically on the TPS. You will need to be able to measure the voltage between the two top wires. You can either buy a special harness connector that breaks these wires out (from Mid America), or gently pierce the insulation of the wires with the pointy prongs on your volt meter. You can also stick a paper clip into each of the two top locations of the connector and clamp onto the paper clips to measure the voltage. Whatever is easiest for you.

• Turn the ignition to the "ON" position without starting the engine.
• Loosen the TPS Torx adjustment screws.
• Set your volt meter to a low scale DC volt setting that will accurately read less than 1 volt.
• Measure the voltage between the two top TPS wires.
• Adjust the TPS by rotating its position until you get a reading of .54 volts.
• Tighten the Torx screws and recheck the voltage. Re-adjust if necessary to make sure voltage is right at .54.
• Turn the ignition "OFF."

 

[tigmaned]

well it almost died when i finally got it home?? well i checked the TPS and some how it moved?? and was at .49 volts so i reset it to .54 volts, adjusted the IAC one more time. check fuel pressure and its right ar 39psi. have to take her for a quick spin so the computer will relearn it all now and see what happens!

 

[tcolyer]

well it almost died when i finally got it home?? well i checked the TPS and some how it moved?? and was at .49 volts so i reset it to .54 volts, adjusted the IAC one more time. check fuel pressure and its right ar 39psi. have to take her for a quick spin so the computer will relearn it all now and see what happens!

You must set the IAC & IDLE RPM correctly first before adjusting the TPS to .54 volts this may be you problem.

Hope this helps.

 

[Mad-Mic]

if everything is checking out right change out your O2 sensor. long tube headers use a heated. regular manifolds use a regular 1 wire. this will also make the idle hunt or the proper rpm due to the car thinking it's running lean then it recalculates the proper AF and realize that it is rich and runs back lean thinking it's rich because it thinks it's lean when it's really rich...lol so as you can see the idle will always fluxuate when an O2 sensor is on it's last leg or already broke.

 

[tigmaned]

i need to re-set the timing again, can't seem to keep it at 6 while i tighten the bolt back down, after i thighten it go back and check it and its advance more? crap!! but since i adjusted the IAC, and re-set the TPS back to .54 shes idleing alot better right at 600rpm in drive. but if i let it idle and Maryland long long red stop lights it starts to seem to loose it some? my fuel pressure is a steady 39psi all the time, which i am reading it right that correct. but i did notice when i shut it off and check the pressure gage she drops to 29 psi pretty quick and stays there with the key off.
Mad-Mic. i have a new O3 sensor from when i was trying to figure out why it failed Maryland Emissions testing, but sinve then has passed and good for 2 more years!!!!!!

 

[tyrel]

You should never have to adjust the idle screw, that's why it's sealed. The ECM sets the idle speed.

 

[redvett]

i need to re-set the timing again, can't seem to keep it at 6 while i tighten the bolt back down, after i thighten it go back and check it and its advance more? crap!! but since i adjusted the IAC, and re-set the TPS back to .54 shes idleing alot better right at 600rpm in drive. but if i let it idle and Maryland long long red stop lights it starts to seem to loose it some? my fuel pressure is a steady 39psi all the time, which i am reading it right that correct. but i did notice when i shut it off and check the pressure gage she drops to 29 psi pretty quick and stays there with the key off.
Mad-Mic. i have a new O3 sensor from when i was trying to figure out why it failed Maryland Emissions testing, but sinve then has passed and good for 2 more years!!!!!!

You shouldnt loosen the dist. anymore than enough to turn by hand. Any too loose and dist. will push it self up and change timing. Seems like you either have bad injectors (leaking down) or fuel regulator is bad. Seems like fuel pressure is low i think its supposed to be 45 psi at idle.

 

[tigmaned]

fuel perssure for a L98 vette is between 34pis and 39psi. i think LT-1's require more then that??
i'll get the timing next try. i've done it before but it started to rain so i said try and see how it is?
and i have seen it writen by alot of guys to adjust idle, you just have to follow the steps to disconect the IAC.

 

[tigmaned]

just had to say thanks for all the help!!! I know some know more then others here but this site as a whole saves corvette guys alot of headachs and cash!! WELL done!!
i got the timing set right a 6, IAC set and TPS re-set at .54 volts. which i must have moved when i removed the throttle body to clean? when warm seating at a stop long stop light she idles at 600rpm.
i am sure it past do for fuel injectors but my fuel pressure stayed steady at 39psi which i find is right for a L98. but i do see a pretty good drop off of fuel perssure when the key is turned off, (from 39psi to 29psi then stays there a while) so i am not sure thats just leak down or some line i don't have right with all the fuel and emissions stuff?