I asked this question yesterday in the general discussion forum, so I thought I would check here today. Here goes:

OK, I need some ideas on what may be wrong. My 91 started running very rough at 1000 rpm or less. When I get on it or am going over 1200 rpm or so, every things ok. At stop signs it runs so rough I have to put it in neutral or it almost dies. I ran thru a pretty good water puddle the day before it started happening, but I know it has dried since then and it still does it. Any ideas?

 

Clean the IAC and TB. Check TPS voltage (.57 V is stock, i think you can run it up to .67 to get a little better throttle response), check plugs and wires as it may need a tune-up


You not gettin any codes right?


[Modified by vader86, 5:56 PM 9/10/2002]

 

No, I'm not getting any codes. Now, you have to understand, I am a real vette noobie so... what is the IAC, TB (throttle body?) and TPS voltage :confused:

Thanks!

 

You must get a Helm shop manual.

The Idle Air Control valve allows air into the motor at idle, its part of the Throttle Body, remove the TB and disassemble, and clean everything.

I dont have my manual here so im sorry that i cant give more specific directions.

 

This is how I did my '89, I think your '91 should be pretty much the same but I'm not positive. I'm sure someone else will chime in to confirm.

Throttle Body maintenance 101:

(YOU WILL NEED A DIGITAL MULTIMETER IF YOU TAKE THE TPS OFF! DON’T ATTEMPT TO REMOVE THE TPS IF YOU DON’T HAVE ONE, IT IS MANDATORY!)

Using your screwdriver remove the air duct in front of the throttle body, now open the throttle by hand (engine off of course) and feel behind the throttle blades. Did your fingers go all black and gungy (yucky isn't it). You need to get that throttle body off and scrub it cleaner than clean can be. Preparation: Go and buy some throttle body cleaner (best in spray cans), and buy yourself a new toothbrush, your old one just got a new life.

Get an old bowl or something similar, rags and perhaps some of those lovely rubber surgical gloves. Getting right on in there remove the 3 pin connector from the throttle position sensor, and remove all the pipe work going to the throttle body, there is a little water spillage, not much. Also remove the IAC connector (a 4 wire square connector) it's a bit fiddly to get to. Now the tricky part, the throttle cables need to be released, (manual and auto cars are different), the cables are fixed with wonderful things called Jesus clips, you slide a screwdriver under it and ping it disappears, you mutter 'Jesus where did that go' - what a name for an item. Best to cover the area with a cloth to capture any pinging that goes on.

With the cables released the throttle body should be free to remove. Undo the four long bolts holding it to the plenum, careful when prizing it apart, do not damage the gasket. Now you thought you were clear didn't you, nope, GM put a little booby trap underneath. There is a small vacuum tube with a rubber elbow that needs to be pulled off. You're free!

Now comes the really dirty part. Put the throttle body in the bowl and spray with throttle body cleaner and scrub like crazy, especially around the throttle blades. Its great, really messy but the final spray just cleans it all off and there you have a gleaming throttle body.

Now using a 31 or 32mm spanner remove the IAC valve (that's the large black looking thing on the side), mind the gasket. Clean the spring and pintle and if you can get inside the orifice and clean the seat area. Make sure that the small hole going from the throttle mouth to the IAC is clear. Finally just spray the spring and pintle with WD40 and replace the IAC valve.

Final check - and perhaps another blast with the cleaner to remove any surface muck still there. You may want to soak a rag with carb cleaner and clean as far in the plenum as you can get. Allow to dry and replace the throttle body on the plenum, taker great care with the gasket ensure it is positioned correctly. Ensure all the connectors cables and pipes are back on (don't forget the little tube underneath).

This may be a good point to check and adjust your throttle position sensor. Get two paper clips, straighten out and slide into the end of the TPS sensor connector, A (earth) and B (tps output), keep the wires separate and turn the ignition on, measure the voltage at the wires and adjust by loosening the torx bolts, if necessary, to 0.54 volts - this is the normal idle setting.

You actually should buy a new gasket kit, they sell them at the dealer, mine was around $20.

Self satisfaction. Job done now you should start your Corvette and stand back to admire the even tickover and you will marvel at the smooth response the car now seems to have. If nothing else you have pampered and tinkered for about an hour, got filthy, therefore justifying your time and got a tangible result afterwards, all that remains to do is test drive the car to the nearest (or in fact not so nearest pub). Congratulate yourself with a beer and apply wide beaming grin. :cheers: :seeya





[Modified by VetBoy89, 3:31 PM 9/10/2002]

 

Thanks for the info. My C4 knowledge is going up exponentially. I went to start it this morning and the thing would not even start!!! So... a tow truck is on the way.

 

How about the dreaded optispark?

 

From what I have read the OptiSprark did not appear until 92. Plus as it turns out it was a bad fuel pump.