Start Up.... Motor Cold...Flat Spot Off Idol...Rats..!!!

[Viet Nam Vett]

86 Vette L98 With SLP Runners and A Carroll Super Charger..

This just started happening a month ago. After you start up from a cold motor ..She goes into fast idol and slowly comes back to normal idol. This part is normal. If you Blip the Go Padel..she cuts out then catches and revs up fine. It's almost like having a Carb with a bad Accelerator pump After she's warm ..No problem. It's just that I never noticed this before until a month ago. No Codes have been set. The motor idols fine and runs nice and strong after shes warm.

I checked the Service manual..but nothing listed about off idol problems. There is a section with trouble and drivability but that's got 2 Billion things to check.

Any ideas..???

 

[G Winter]

Throttle postion sensor. Feul pressure.

Glenn

 

[Viet Nam Vett]

Throttle postion sensor. Feul pressure.

Glenn

Fuel Pressure Is Good... I'll run a check on the TPS Tomorrow....

 

[nyernga]

Throttle postion sensor. Feul pressure.

Glenn

That was my guess along with the IAC valve.

 

[Viet Nam Vett]

That was my guess along with the IAC valve.

I have the TPS and IAC Test Connectors .. I ran a check and both are right on spec according to the GM Shop Manual...At 130* the problem goes away...You would think it would be either listed above.. but no banana. It's feels like a choke problem ..like a carb system and the choke not being on enough..You would think that the IAC would be the problem as it controls air flow. But according to the discription in the manual ..it's there to control idol.

 

[nyernga]

I have the TPS and IAC Test Connectors .. I ran a check and both are right on spec according to the GM Shop Manual...At 130* the problem goes away...You would think it would be either listed above.. but no banana. It's feels like a choke problem ..like a carb system and the choke not being on enough..You would think that the IAC would be the problem as it controls air flow. But according to the discription in the manual ..it's there to control idol.

O2 sensor? My problem last month was an idle issue and not a running issue but after warming up it idles like crap. Replaced the O2 sensor and all is well.........

 

[JonM]

IAC may need to be removed and cleaned

 

[Viet Nam Vett]

IAC may need to be removed and cleaned

Ok... That sounds like a plan....I have the throttle body water inlet bypassed and run directly to the heater. So maybe she's gacked up ??

 

[Viet Nam Vett]

O2 sensor? My problem last month was an idle issue and not a running issue but after warming up it idles like crap. Replaced the O2 sensor and all is well.........

My Idol is fine after cold start.. She goes to about 1500 Rev's and works down to a nice idol and is very drivable. Hot idol and normal driving are fine...

But..heh...an O2 Sensor is another great thing to throw money at...
:L

 

[JonM]

Ok... That sounds like a plan....I have the throttle body water inlet bypassed and run directly to the heater. So maybe she's gacked up ??

The IAC is a stepper motor, that receives pulses from the ECM which in turn make the IAC; move in and out according to the ECM signals. The IAC allows air to enter thru a hole in the bottom of the throttle body while the throttle body is closed, say at idle.

Look in your manual, it should tell you how to clean it up and set it...be careful its delicate.

 

[Viet Nam Vett]

The IAC is a stepper motor, that receives pulses from the ECM which in turn make the IAC; move in and out according to the ECM signals. The IAC allows air to enter thru a hole in the bottom of the throttle body while the throttle body is closed, say at idle.

Look in your manual, it should tell you how to clean it up and set it...be careful its delicate.

Yep..allready have read the story on the IAC in my manual. This IAC was replaced a few years back and should be in fine shape. The manual only talks about idol control...it would seem to me that the TPS should be the device responsable for telling the injectors that the throttle has been opened and to add more fuel when the throttle is blipped.

As the name says.. IAC "Idol Air Control" .. So then what device would be the one to act as the Accelerator pump in a carb in relation to Electronic TPI.

The manual only speakes to Idol in relation to the IAC Motor..

My Idol is fine after cold start.. She goes to about 1500 Rev's and works down to a nice idol and is very drivable. Hot idol and normal driving are fine...

Thanks for your reply..

 

[Viet Nam Vett]

And The Correct Answer Is......????

I decided to Call "Gordon Killebrew" .. The Renown C-4 Corvette Expert who use to work at the Corvette Action Center at The Corvette Plant In BG. Gordon is the man when it comes to Fuel and Electrical Problems with The C-4...

Rather then throwing money at the Vette replacing a sensor here and a sensor their.. I feel this was the best move..

Gordon recommended a Min Idle Adjustment covered in the manual. This reset the IAC and TPS and Synchronized them with the ECM. So now.. there all on the same page..

Problem solved .. Motor running excellent..!!

Thanks Gordon..