I've been having an issue with my idle for a while now. The issue began a couple months after a replaced my MAF per this thread: https://www.corvetteforum.com/forums...given-gas.html .

Essentially once warmed up, and only once warmed up, and only while idling my rpms will die down from 700 to 550 and keep bouncing back and forth. It will kind of seem like it's going to die once the idle goes down but it will never die, it just keeps bouncing back and forth.

Now, this will never happen while I'm driving, no surging or hesitation while I'm driving, in fact while it is idling like this I can slam down the gas and peel out without issue, it doesn't actually seem to affect driving at all, only the idle.

This isn't super consistent either, sometimes when I drive it this will only happen once, other times it will happen every time I stop.

Plugs/Wires/Cap/Rotor are all new, TPS/IAC and throttle body have been replaced and cleaned within the last year. As I showed in the other thread, I replaced the MAF with a reman MAF, I'm wondering if it is possible the MAF is causing this issue?

I haven't had a chance to check my timing yet, will be doing that soon.

Could this be a fuel pressure/regulator issue? I replaced the fuel filter over last winter.

Vacuum leak issue?

Not sure entirely which direction to go from here.

Other relevant information: Previous owner deleted EGR and Air system, I've double checked everything and it appears everything is blocked off properly.

 

Also, I can't spell, Inconsistent Idle.

 

Codes? Any repairs or servicing prior to this condition returning? Vacuum leak(s) is where I would start. Scanner?

 

Lots of little odds and ends since then. Not exactly the same issue as the MAF issue I had before, a new issue I'd say.

No codes, and I have that USB hookup to scan but I haven't had a chance to get it working on my laptop.

Vacuum leaks - will see if I can find anything.

Also, forgot to include the year in my post, this is on a 1985.

 

Have you serviced your IAC recently? That means you take the TB off completely, take the cover off, take the IAC out, take the IAC housing off, clean all passages, gently spray brake cleaner on the IAC pintle and wipe and FINALLY, NEW GASKETS for reassembly?

Check timing and start scanning. If you have done the above service on the IAC system, I would try see what the IAC counts are when everything else is off but the motor. Let us know what you see.

 

Ok guys, sorry for the late follow up, attempting to balance work, College, and hobbies can get tricky.

Looked over all the vac lines, it appears I have a few that are not hooked up.

I found this guy sitting on top of the wiper motor, it goes down the side of the wiper motor, I can't seem to see any more of where it goes after that. Looking at the VAC diagram is not showing me much as far as what it is.




I also found this guy, this one goes to the charcoal canister, it connects with another vac line in a Y. It looks like this might go to the throttle body?


Tracing this one it connects to the top of the charcoal canister.

 

Also, I keep trying to downsize those images as they are currently really big, doesn't appear to be taking my edits. Sorry about that. Also, I know my wiper motor cover is unplugged, it is currently bad and I will be swapping it out when I change the valve cover gaskets when I have additional free time.

 

top pic looks like a vent hose. I'm having about the same problem on my 85. had a 33 code replaced maf tps and computer still have a intermittent 33 today it idles up and down.

 

Vent hose for what? And wouldn't it be shown in the vacuum diagram?

 

Ok, I figured out what the smaller hose is, it appears that it is the line from the charcoal canister that goes to the TB, under the TB was a nipple that was capped off. Based on the diagram I'm assuming both the EGR and Charcoal canister went into this nipple with a little T adapter of some sorts and the previous owner just capped it off.

It's rainy up here in Northern Ohio, hopefully the rain stops on Saturday and I'll take her for a spin to see if there is any difference.

 

looks like transmission vent hose

 

hi there. i have an 85 and just totally overhauled it right down to the bead gaskets over last winter. i might be able to help.

the nipple under the throttle body supplied vacuum power to the egr sole noid and to the vacum cannister solinoid.

that said, the main vacuum line that does the sucking of the fumes in the cannister is actually tied i to the steel tube that runs along the driver side valve cover, picks up the pc. valve, and goes into the intake niple

there is a small piece of hose that bridges between the intake vacuum port and the egr/cannister tube. my bridge hose was very old and stiff and there was no seal. huge vacuum leak. if i sprayed brake clean around the area i could hear the vauum power around that old hose schucking it up intothe intake and it would speed up the rpm.

there are numerous places that a vac leaking could be sucking in unmetered air. ive never had luck shooting propane in around the. e gine but a bottle of carb cleaner does the trick nice.

you will hear the car rev up slightly flamible fumes are sucked in and burned in the cylinder.

pull the flr vac line off and see if its got fuel smell on it.

and aslo dou have a fuel pres gauge

 

Sorry for bumping an old thread, but I wanted to make sure I put the solution in here as to not frustrate future Corvette owners when they search.

I had a separate issue a while ago with my TB shaft binding under vacuum, I put an auxiliary return spring on it and left it alone. I ended up finding out that the shaft was severely leaking vacuum.

I bought a new TB and she now idles like a champ.

 

excellent, would you mind explaining how you went about identifying the throttlebody hinge as being the issue? I think I may have that on my 85.

 

I was able to wiggle the shaft around when the issue was occurring and it stopped it, it explains why if I gave it a couple of quick short revs it would stop going up and down.

My TB shaft also had A LOT of play.