Minimal idle and timing
Thread Starter
Instructor
Joined: May 2006
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From: Gautier MS
Minimal idle and timing
On my stock 90 vette I have set my timing at 6* with and tried to set my minimal idle and TPS. I followed a few threads I found, and I seem to be doing it right. When I disconnect the IAC and start it up the engine idles very low at about 350-400rpm. It is idling just enough it doesn't shut off. TPS is at .54v
When I go to start the engine after connecting the IAC it idles at about 850-900rpm. I have read that the computer contols the idle speed. Any help on this one....Thanks.
When I go to start the engine after connecting the IAC it idles at about 850-900rpm. I have read that the computer contols the idle speed. Any help on this one....Thanks.
Advanced
Joined: Feb 2005
Posts: 77
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From: N. Tonawanda NY
Send me your email address and I will send you a pdf of the "Mininmum air set to adjust the idle" that I got from Gorden Kilabrew at one of his seminars. I just did it and it works well. Yes with the the IAC disconnected your suppose to adj idle to 450 rpm. The normal TPS idle voltage on a 90 and 91 is .72 volts
I have a 52mm throtle body with an adj TPS and just set it at .72. My car never idled correctly after putting the new throtle body on last year. It would idle high and very up and down.
I have a 52mm throtle body with an adj TPS and just set it at .72. My car never idled correctly after putting the new throtle body on last year. It would idle high and very up and down.
Melting Slicks
Joined: Apr 2002
Posts: 2,118
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From: St. Charles, MO Route 66 Corvette Club
MB Vette: Did you disconnect the brown wire timing connector just above #7 cylinder? The tech tip on this does not indicate the need to do this step...but the ECM must not have the ability to control the timing while you adjust the base idle. If you need more info on this, please advise. This info is from the GM Helms manual!
Thread Starter
Instructor
Joined: May 2006
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From: Gautier MS
Thanks, I did disconnect the EST wire when setting timing and I readjusted the TPS closer to .72 volts and adjusted the minimal idle screw. It still idled the same so I backed the timing to about 4* or 5* BTDC and it idled a little lower and seems to run good now. The idle is around 800 RPM now and I drove it without any knocks. Is it safe to have the timing at this spot?
Melting Slicks
Joined: Apr 2002
Posts: 2,118
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From: St. Charles, MO Route 66 Corvette Club
The idle speed should be controlled by the ECM...assuming all sensors are functioning correctly and you do not have vac leaks. There is a program built into you ECM that controls the engine idle RPM based upon the engine coolant temperature. Here are the ECM programmed values for my 89:
Idle RPM vs. Coolant Temperature
Deg F RPM
306 575.00
284 575.00
262 575.00
241 575.00
219 575.00
198 575.00
176 575.00
154 600.00
133 600.00
111 750.00
90 900.00
68 950.00
46 1050.00
24 1050.00
03 1050.00
-18 1050.00
-40 1050.00
Running the car with retarted timing should not do any damage, but you will lose power from the engine and have reduced gas mileage. I agree with NT Vetteman that you should review some other base idle procedures to be sure you used the correct steps for this activity. I too, have a procedure on this for my 89. One thing you did not state was how the car was idling prior to your base idle adjustment. Let me know if you are interested in my 89 base idle procedure.
Idle RPM vs. Coolant Temperature
Deg F RPM
306 575.00
284 575.00
262 575.00
241 575.00
219 575.00
198 575.00
176 575.00
154 600.00
133 600.00
111 750.00
90 900.00
68 950.00
46 1050.00
24 1050.00
03 1050.00
-18 1050.00
-40 1050.00
Running the car with retarted timing should not do any damage, but you will lose power from the engine and have reduced gas mileage. I agree with NT Vetteman that you should review some other base idle procedures to be sure you used the correct steps for this activity. I too, have a procedure on this for my 89. One thing you did not state was how the car was idling prior to your base idle adjustment. Let me know if you are interested in my 89 base idle procedure.
Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
Originally Posted by MB vette
Thanks, I did disconnect the EST wire when setting timing and I readjusted the TPS closer to .72 volts and adjusted the minimal idle screw.
Is it safe to have the timing at this spot?
Is it safe to have the timing at this spot?
As you said you thought, it is true, that the idle speed is programmed into the prom. Minor TPS and minimum air adjustment won't make any difference in your idle speed. The retarded timing is "safe" but you are sacrificing performance, fuel mileage, and risking running hotter than necessary. Your faster than spec idle is most likely caused by a vacuum leak or "false air" from one source or another. It is also possible that a defective (check it, don't just throw it away) ECT sensor. If the ECM thinks the engine is cool (when it isn't) it will command a faster, cold, idle than the actual conditions call for. Quit trying to "adjust out", via "band aid" methods, a symptom that has a cause and a cure.
RACE ON!!!
