89VetteGirl

My 1989 TPI Corvette seems to me to have a very low idle speed, although I do not know what the idle speed should be. It was idling around 600rpm when I bought it. I replaced the spark plugs, wires, distributor cap and rotor, and it now idles around 700rpm. I sometimes have to give it a tap of gas to keep it going or it will drop to 500rpm and stall out, but once I give it that tap of gas it is fine and runs strong, just stays around 700rpm. I also smell gas when I first start it up and it blows a bit of smoke for about 30 seconds out of the tailpipes. I think it may be running rich? What are the most likely causes, and how do I fix it?

engle1147

Get some Throttle Body cleaner and clean the TB. A good place to start is by check your TPS for ~ .54 volts max is .62 or .63. You can slightly adjust the sensor's valuse just by loosening up the 2 mounting bolts and bumping the sensor clockwise or counter clockise. Small changes in the voltage of the TPS will have an effect on the idle RPM.

The start-up smoke your describing sound like "old/ worn out" valve seals....the somke shouldn't hurt anything. Valve seals can take quite a bit of effort to change.

Good Luck!

19vette89

after start-up, my 89 L98 settles down to around 6 on the rpm readout. i believe that is the proper low idle speed for that car.

SunCr

Specs are an Actual Idle 50 +/- rpms from Targeted Idle in Drive; 100 +/- rpms from Targeted Idle in Park/Neutral. There is nothing to fool with until you know what those numbers are - hookup a scanner to find out; otherwise, you're wasting your time.

vader86

First thing I would do is a TPS check and IAC/TB cleaning, a procedure is on my website, click my name.

tequilaboy

Warm desired idle speeds for 89:

Auto trans: 575 rpm in drive
+ 50 rpm offset for park/neutral = 625 rpm.

Manual trans: 700 rpm

Here's what the table looks like (auto trans shown):

;----------------------------------------------
; IAC TARGET SPEED vs COOLANT TEMP
;
; Dissassemby of ARAP, LINES = 17
;
; TBL = .08 * RPM
;----------------------------------------------
ORG $052E ; RPM Deg c COOL
;----------------------------------
LC52E FCB 84 ; 1050 -40
LC52F FCB 84 ; 1050 -28
LC530 FCB 84 ; 1050 -16
LC531 FCB 84 ; 1050 -4
LC532 FCB 84 ; 1050 8
LC533 FCB 76 ; 950 20
LC534 FCB 72 ; 900 32
LC535 FCB 60 ; 750 44
LC536 FCB 48 ; 600 56
LC537 FCB 48 ; 600 68
LC538 FCB 46 ; 575 80
LC539 FCB 46 ; 575 92
LC53A FCB 46 ; 575 104
LC53B FCB 46 ; 575 116
LC53C FCB 46 ; 575 128
LC53D FCB 46 ; 575 140
LC53E FCB 46 ; 575 152
;----------------------------------------------

Kubs

Speaking of idle.. I just installed a ZZ4 cam and had a chip burned. Now the idle is around 900 RPM. Is this ok for the cam, or should it be at 700 'cause its a manual trans?

juanway

[QUOTE=engle1147;1566881054]Get some Throttle Body cleaner and clean the TB. A good place to start is by check your TPS for ~ .54 volts max is .62 or .63. You can slightly adjust the sensor's valuse just by loosening up the 2 mounting bolts and bumping the sensor clockwise or counter clockise. Small changes in the voltage of the TPS will have an effect on the idle RPM.

The start-up smoke your describing sound like "old/ worn out" valve seals....the somke shouldn't hurt anything. Valve seals can take quite a bit of effort to change.

Good Luck!

Do You know if .54volts gives you a lower rpm or higher?
is the lower voltage = to lower rpm ?

tequilaboy

TPS voltage does not add or subtract rpm directly.

If it is way off in the positive direction, it can take you out of idle PID control (like when opening the throttle) and will add IAC counts to follow the throttle which will tend to raise rpm.

If you're seeing this effect, the setting is too high.

engle1147

[QUOTE=juanway;1566882407]The higher the TPS sensor voltage is set = lower RPM....if I remember right. Example if the TPS is set @ .54 and the idle is @ 600 then the TPS is changed to .60 the RPM will drop lower than 600.

Hope that helps.

89VetteGirl

Thanks guys! It sounds like my idle speed is ok. Since I changed out the spark plugs and wires for new OEM replacement spark plugs and wires and replaced the alternator this past week, it has been a lot steadier, but it still wants to stall out on start-up. I will clean the throttle body and check the voltage on the TPS next.

BADDUCK

700-750 rpm, the ZZ4 isn't that much of a step up from stock.