Idle speed question

[Jeb]

How long does it take a cold started L98 to settle down into an idle speed in the neighborhood of from 500 to 700 RPM?

I just finished adjusting my idle speed, IAC and TPS and now she doesn't want to settle in on an idle speed until the engine warms up well.

That'll teach me to fix something that wasn't broke!

 

[redvett]

What are your IAC counts at idle hot? Must get over about 157 deg. coolant temp and TPS set at .54 V should have about 15 - 20 counts on IAC. If A4 it must be in Drive.

 

[Jeb]

What are your IAC counts at idle hot? Must get over about 157 deg. coolant temp and TPS set at .54 V should have about 15 - 20 counts on IAC. If A4 it must be in Drive.

I have no idea what an IAC count is nor do I know what is meant by A-4.

Here's what I did....

1. Jumped terminals A & B (or is it one and two?) on the diagnostic plug under the dash.

2. Turned key to 'On'..did not start engine...waited a minute or two.

3. Disconnected IAC plug

4. Unjumped the terminals

5. Started engine, warmed up to oper. temp. and adjusted idle with the idle adj. screw, in drive to 400, in neutral to 450. Shut down engine.

6. Reconnected IAC plug

7. Turned key to 'on'...did not start engine

8. Adjusted TPS to 5.4 V at idle

9. Tightened TPS screws and rechecked voltage....5.4V @ idle

10. Disconnected meter from TPS.

11. Finished.

Now, the idle doesn't want to settle down until the engine is fully warmed up whereas before it idled down within a minute or so of starting cold.

I don't like to put it in gear (auto trans) while the idle is high so I have to wait a few minutes after starting (even when it's a hot start) to put it in gear and drive.

I think something's amiss.

I don't have one of those fancy tools that can read codes and tell you what the computer is doing but I can jump the terminals and see if it displays a code........and it doesn't.

I'm thinking of disconnecting the battery for a few minutes to clear the computer's throat and seeing what happens.

 

[wishuwerehere82]

A4's an automatic, which you have.

Idle should be 650 in drive, with the TPS voltage at .54V at that RPM. The EMC will whack out if it senses the idle going below 500 and will increase the IAC's to conpensate.
That's the problem. Just turn up your idle to 650 and reset you TPS voltage again.

 

[redvett]

I have no idea what an IAC count is nor do I know what is meant by A-4.

Here's what I did....

1. Jumped terminals A & B (or is it one and two?) on the diagnostic plug under the dash.

2. Turned key to 'On'..did not start engine...waited a minute or two.

3. Disconnected IAC plug

4. Unjumped the terminals

5. Started engine, warmed up to oper. temp. and adjusted idle with the idle adj. screw, in drive to 400, in neutral to 450. Shut down engine.

6. Reconnected IAC plug

7. Turned key to 'on'...did not start engine

8. Adjusted TPS to 5.4 V at idle

9. Tightened TPS screws and rechecked voltage....5.4V @ idle

10. Disconnected meter from TPS.

11. Finished.

Now, the idle doesn't want to settle down until the engine is fully warmed up whereas before it idled down within a minute or so of starting cold.

I don't like to put it in gear (auto trans) while the idle is high so I have to wait a few minutes after starting (even when it's a hot start) to put it in gear and drive.

I think something's amiss.

I don't have one of those fancy tools that can read codes and tell you what the computer is doing but I can jump the terminals and see if it displays a code........and it doesn't.

I'm thinking of disconnecting the battery for a few minutes to clear the computer's throat and seeing what happens.

IAC counts can only be checked with a scanner must be around 15-20 counts and A4 is 4 speed automatic trans.
Idle should be at least about 550rpm for A4 and i believe 650 for stick. You cannot change the speed its controlled by the calibrations.
My guess is that you are overiding the IAC and the result is high idle. Most likely the IAC is closed because you opened the throttle.

 

[Jeb]

Thanks, Guys. I'll give it a whirl.

Appreciate it.

 

[tigmaned]

jeb

this is new to me, i have seen were it says idle with the iac disconnected should be 400rpms too? so maybe i should adjust mine to 550 rpms since mines a 700r4 tranny.

 

[Vettefan87]

Mine does not right as well, I know my TPS is set higher then the base .54 volts, but I wanted to achieve the closest to 5 volts at WOT. I would recheck your TPS voltages, it is very easy to bump that and the voltsge get messed up.

 

[tigmaned]

you know i think i read in my GM service manual that when you do this you also disconnect the wire to the dist too. any body do it that way or just like its writen here above?

 

[WhalePirot]

Per 1987 Corvete service manual, 6E3-C2-18, the ECM uses battery voltage, coolant temperature, TPS, engine speed, A/C clutch signal and Park/Neutral switch to set the idle speed.

How long does it take a cold started L98 to settle down into an idle speed in the neighborhood of from 500 to 700 RPM?

Depending upon how cold the engine is and how it is driven/run, the idle should settle down soon after the temperature indicates a real temperature. I wonder if the temperatiure sending unit that sends to the ECM is erring, causing the "long" high-idle speed. A sticking EGR, faulty A/C switch signal or a vacuum leak can affect the idle.

I just finished adjusting my idle speed, IAC and TPS.

The idle speed is not adjustable outside of the ECM; i.e., it is programmed. The TPS is adjusted to .54+/-.08vdc. This is done with the ignition 'ON', SCAN tool and TPS in place. The voltage can be measured at the TPS, between terminals A & B. A is ground; B is the varied voltage, going to the ECM; C is the voltage feed to the TPS.

The IAC is checked (ONLY, not adjusted) w/engine idling-warm, @ normal temperature. Note RPM (park or neutral). Shut IGN off for 10 seconds, then restart, immediately noticing RPM. If it does not return to normal idle speed w/in one minute, test the IAC terminals w/ test light, with the engine shutdown, then the IGN 'on', per the manual. You can check the resistance between terminal pairs (opposite harness connector) A/B & C/D @ >20 ohms.

That'll teach me to fix something that wasn't broke!

Yeah, well haven't we all learned that one at some time. Chalk it up to learning about your car and how to work on it.

The EST is required to be disconnected at the distributor (brown wire) when setting the base ignition TIMING.

 

[tigmaned]

http://www.batee.com/corvette/dcrg/readerstips/larsstuff/this_tech_paper_will_discuss_sim5.htm

heres how i did it to my 86.