85 idle issues

[patrickj]

Hi all my 85 does not idle correctly it surges then dies I replaced the IAC with an genuine ac delco made no difference I have a question about a change I made recently.the oil pressure switch failed so I bypassed it with a toggle switch did not think it was necessary as I installed mechanical gauges.the switch runs the fuel pump.the engine is new with all new vaccum lines I checked for vaccum leaks at the intake mating surfaces etc.no vaccum leaks anywhere any ideas? Thanks

 

[Schrade]

Vacuum leak detection is hard to do right: I know of only one guy who did it right and found a leak (in IAC gasket):

Originally Posted by Schrade View Post
No one got the proper technique for vacuum leak detection...

FIRST (fully warmed up motor); block the gas pedal, or block the throttle linkage. YOU HAVE TO DO THIS TO GET A STEADY IDLE , around 1,250 rpms, or a little higher. If you don't, you can fish starting fluid, a propane hose, or WD40, or Krylon, or hair spray, or pee, anything, until you need a sweater in Hades, but if the motor is lopin' idle already from a vacuum leak, you ain't gonna' find any leak, unless it's so big that Little Jack Horner can stick his thumb in it and pull out a pineapple NOPE.

AFTER you get a steady idle, THEN, you can spray starting fluid everywhere - that's the best detector, because it's vapor pressure is very high - a LITTLE at a time, all around the motor, until you get the rpm SURGE. Follow the vacuum lines to the EVAP cannister too. You might even have to follow the EVAP lines from the EVAP cannister to the gas tank too, but that vacuum leak will show up as a DTC 32 EGR fault, and won't normally show as loping idle, unless the EVAP cannister has been by-passed (been there, saw that uh-HUH yup).

If you get a surge in a tight spot, and can't tell exactly where it is, light a book of matches, blow out the match heads while they're still burnin', and feed the smoke to the tight spot to see it get drawn in.

Fear not the starting fluid; you cannot put the can down fast enough and light the matches fast enough to catch the starting fluid, because the vapor pressure is so high.

Bookmark this post YUP.

Wrench Wizard OUT!

 

[muskiemike]

Hello,

I have a '84, similar problems as yours, I replaced the Throttle Position Sensor (TPS), and immediately solved my problem:

Go To: http://www.corvetteactioncenter.com...rottle-position-sensor-tps-2.html#post1126392