I have read all the archives on this subject and I believe the Opti is on the way out. Or could it be the IAC Valve? What do you all think.
Symptoms: Surges (300 to 500 rpms jumpes from idle) when at operating temperature only, Fans and AC are off when it surges, Car seems perfectly drivable otherwise (good power no hesitation), no obvious vacumn leaks.
Also can someone explain how this problem only occures at operating temp?
Thanks for the help.

 

Just dealt with that on a chevy van and it was a vacuum hose was not connected. Don't know if this is your problem but it cured the vans problem with surging.

 

Sounds like you need to have a throttle body/IAC cleaning.

I would do that first before anything else. :thumbs:

 

I will try the IAC and throttle body cleaning. Took the car out for a long run yesterday and the latest is the ARS service light came on. Havn't done anything for the idle problem yet but could the idle issue and the ASR service light be related? This morning I am going to pull the codes and will report my findings.

 

My 94 did this last week. Checked vacuum lines and I do not have any codes. I did the search also. Hope it is not the opti. The first time it did this I was at a stoplight, darn near hit the guy up front. :confused:

 

I would check the IAC, but if the IAC is failing, or gummed up, it will fluctuate and nearly crash... however, if it's the opti, the idle will start at about 650~700rpms and move up from there.

If it stalls, then it is definately the IAC.

 

This was the first symptom when my opti started going bad. Next was a random miss at higher RPM. I never saw any codes.

When I finally pulled the opti (after playing with the IAC, replacing the injectors(rebuilt set), checking all the vacuum lines, replacing the fuel filter, checking fuel pressure, spent 2 hours at the Vette shop with a TECH II cutting individual plugs and injectors off and on and on...) I found the hi-voltage contacts had grown green fuzz!

 

Well, I found I have the OBD 2which means I need a scan tool to see if it has set and codes. Since the check engine light is not on I would assume there are no codes to be read. Is this true or should I go invest in a scan tool?

 

If you have a 94, you have OBD I with an OBD II connector.

You don't need a scan tool. You can do it directly from the car.

Check this link: http://www.c4vettes.com/codes.htm


It's unlikely that you have an current codes if the SES is not on. You may have some History codes.

Once you are in maintenance mode and all the codes have flashed by, you can review the codes by pressing the Fuel Info/Eng Met and the Gauges/Trip ODO to page back and forth between all the modules.




[Modified by John Row, 3:31 PM 4/19/2004]

 

Thanks John. That was an excellent link.
I did as the link described and here is what I got
1 C12
1 ---
4 ---
9 H62
9 ---
A ---
1.0

Can anyone tell me what this means?
Thanks for the help

 

C12 - Normal Communications on bus. You must have this to go any farther.

H62: Engine Oil Temperature Sensor Circuit Voltage Low. This is a History code. Not currently set. May indicate a faulty sensor. Just keep it in the back of your mind.

Here's another link: http://www.corvettedoctor.com/1994-96_Codes.htm


[Modified by John Row, 7:50 PM 4/19/2004]

 

That was a helpful link. Thanks. I guess without any codes I start with cleaning the TB and from the posts on this subject dive right into the opti after the cleaning proves not to be the problem.
Thanks for all the help.

 

My 'rough idle' problems on a 93 coupe

2000 rough idle-brutal surging, almost hit cars. This was the throttle body. Cleaned it up, new OEM gaskets, did not replace IAC. Idled smoothly when done

2001 rough idle, annoying minor surging. Opti replaced with OEM with 100k on it. Idled smoothly again.

2004 rough idle and annoying hesitation. EGR valve completely dead. Replaced with OEM. Smooth idle and no hesitation, drives like new car.

Somewhere in there I also R&R'd the intake to stop the oil leaks and replace gaskets. In hindsight. I probably should have done it all at once, since the parts all together were less than $500 :yesnod:

 

Now I am at a loss, I cleaned the TB and the idle is smooth. :party:
I just wish someone could explain how a dirty TB would cause a surging idle only when the car is at operating temperature.
What I found was a lot of sludge around the IAC valve seat, and a deterorating vacumn hose on the right side of the TB. I don't thing it was leaking but who knows. I am just happy to be at the point where something I did with all your help may have actually solved the problem.
Thanks for the help. See you all in the fast lane.

 

OK, my rudimentary understanding of how idle works is something like this...

Until the engine reaches a predetermined temperature (I think it has to do with how the O2 sensors are reporting) the computer holds the idle speed with the injectors enriching the mix (kind of like how a choke in a carb works). Once the O2s are working the computer uses the IAC to add/reduce air to lean the mixture along with the injectors to hold the idle speed.

The problem is that the 'puter can't tell if the IAC is clogged. It just keeps telling it to open until the fuel mix is in the correct ratio. If it's clogged it might be hitting a point where instead of a continuously varying air stream a sudden blast hits, thins the mix suddenly and the computer compensates.

Also if the throttle plates are gunked, they may not be closing far enough adding more air to the mix as well.

If I'd been thinking :smash: I would have recommended unplugging the IAC to see if it would have smoothed the idle. The 'puter appearently knows when the IAC is totally gone. This is another diagnostic to see if the opti is going bad. If you pull the IAC and the idle still surges it's another brick in the path to a new opti.

It's a nice feeling to actually fix something. :cheers: