Mavrick70004

Hoping you guys can help me out. I have a 1984 Corvette with Crossfire, about 75k miles on it. My main problem is the car has an extremely high idle. It had other idle issues besides this one (would not stay running when cold) but I think I tracked most of that down to open/missing vacuum lines and a CTS that had bare wires touching.
The car does not look like it was every abused but it was surely neglected.

This is what happens:

- Start car, engine idles around 800-1000 RPM and is very rough at anything below that - already too high
- After a couple of minutes the car warms up but is not yet at
operating temperature. 120-150 Degrees.
- The engine slowly starts to idle faster.
- Then faster, and tops out at 2000RPM. Then hunts between 1800-
2000RPM.
- The engine hunts up and down 100-300 RPM no matter what the temp
is. This is obviously a problem but not the main concern right now.
- The IAC ports seem to be sucking in as much air as they can at all
times. If I stick a rag in the IAC ports then the idle drops to around
700-900 Rpm and stays there. When I disconnect the distributor
advance to check the timing the engine drops down to a normal 400-
600rpm and idle is smooth. (Again still hunting a couple hundred
Rpms).
- The timing is correct at 6 degrees.
- I took one of the IACs out and it was a little dirty but nothing extreme, I tried moving the plunger up and down but it wouldn't really move.
- Cap,Rotor,Plugs,Wires,EGR,CTS,misc vacuum lines, and possibly o2 sensors(bright shiny silver metal, looks new) have been replaced already.

I have been over and over trying to find any more vacuum leaks but I don't think there are any. Next step is to just replace the intake gasket and see if that helps the rpm hunting. But what is going on with the IAC's?? Is there a way to test them? Could it be a bad ECM? Any other ideas?

JoBy

Forcing or turning the pintle will break the IAC. Shaft should not rotate, if it does then it is broken. Mounting the IAC with the pintle too far out will break them.

There is a pin that keeps the shaft from rotating and inside there is a nut rotating to move the pintle in or out.

Check for vacuum leaks.

Buccaneer

It does sound like you have a classic vacuum leak. The motor should idle at 1,200 - 1,300rpm when cold and then settle down to idle at 500ish or where it is set to. Do you have a CEL light? If so, troubleshoot the code(s). The rough running could also be the CTS sensor in the front of the motor, does it smell rich when idling? Are both IACs cycling in and out trying to keep it running? If they move they are at least OK.

Have you checked the TPS sensor which tells the ECM where the blade angle is for idle? Engine OFF, key ON, check for .525 volts. Adjust for as close to that as possible. If you play with the TB balance in any way, you MUST reset the TPS sensor back to .525.

For the vacuum issue, check the TB base gaskets and the top lid gasket. If the lid is loose and tighten it, you stand a chance of cracking it and having even for issues. Tighten it and give it a shot, If loose, just replace it IMO. Good luck and post up what you find.

Also, do you have a GM Service Manual? If not, get one, sometimes found on eBay. You'll need it for a CFI motor and is a great manual. One more thing, once you get that figured out, check the fuel pressure in-between the TB, set it at 13psi on a stock motor and it will run great.

Mavrick70004

No CEL light or codes. The CTS and connector were just replaced with Acdelco parts yesterday, the CTS shouldn't be a problem. How do I know if the IACs are moving? Should I be able to see the pintle sticking out through the IAC port on top of the TB? The fact that the engine runs at a normal-decent speed when I block off the IAC ports was leading me toward an IAC problem.

If there is a vacuum leak then it must be the intake gasket. I have been over every line so many times, the only one I cannot follow is a black plastic one that comes off the rear of the intake, goes into what looks like some sort of check valve and then splits off. One split goes to the cruise control and the other spilt disappears down behind the engine. ( you can see it in the first picture i posted, it's between the distributor and the MAP sensor

I will check the TPS voltage reading tomorrow, I don't have a manual yet alldatadiy has been giving me what i need so far.

Gibbles

The iac's should auto adjust when the engine is warm and the car hits about 25mph iirc.

You should also hear the little motors making a buzzing noise when you first turn on the ignition.

If not done yet, disconnect the battery for a few minutes, turn on the ignition engine off, and listen for it.

Take it for a drive and see if the idle comes down.

And like others said check for vacuum leaks, even try plugging off the evap port on the driver side of the intake.

Gibbles

Another thing that comes to mind, the temperature sensor that plugs into the front of the intake, the ECM uses that to tell how warm the engine is.

If it's bad the iac may never adjust.

Common for those early ones to fail from what i understand...

​​​​​​​When i bought my 84, the first thing i did was to update any sensors i could...

Tom400CFI

I believe that you can jump pins "A" and "B" on your ALDL connector it that will extend (close) the IAC valves on both TB's. Since the rag brings down the idle, I'd run this test to see if the IAC's are even working. If they are, then jumping those terminals should accomplish the same thing as the rags, but even more so.

I'd check that the TB's are relatively synched and that no one has messed w/the throttle stop.

Surging idle is symptomatic of a lean A/F ratio so after checking the above, I'd try to figure out if or why it's lean. Have you checked fuel pressure? Does it have the right injectors in it? Have you checked the vacuum line from the rear TB to the MAP sensor for cracks/slits/leaks?

Here is a basic procedure for setting "minimum air" (idle speed) but the IAC's have to both be working before you can do this procedure.

jumper pins A&B on the ALDL (Assembly Line Data Link) connector under the dash. Pins A&B are on the upper-right-hand side. These are the same two pins to jumper to read codes from the ECM. Now turn the key on (the Check Engine light should be lit) and leave the key on for at least 30 seconds. The computer will extend the IAC plunger
all the way out to allow adjustment of the idle speed.

After the 30 second wait, unplug the IAC (square 4-pin connector on the throttle body) WHILE THE KEY IS STILL ON. This prevents the ECM from adjusting the idle speed while you make your adjustments.

Block the drive wheels, set the emergency brake, and start the engine.
Set the idle speed by adjusting the idle speed screw. The engine should be at operating temperature for this.

I believe the spec is 400 RPM in gear.

Mavrick70004

Kinda screwed up. Any idea how to get the IAC back down? I took it out of the TB, re-connected it and forgot that the key was on still. The IAC turned on and pushed the pintle out a tad. At least I know the IAC is operational :/

Mavrick70004

Figured it out, ended up over tightening the IAC though and the threads started to crack. Great, new IAC time

ChumpVette

I'm not familiar with the 84 crossfire system, but on later cars with TB injection I have been able to use my Tech 1 and control the IAC. Helps to reset and verify that the idle can be controlled by the IAC. If you get down to zero counts and you are still idling to high, you start hunting for other vacuum leaks,

JoBy

Read Toms post again, Tom is right. The procedure is to close IAC, then disconnect wire. Then start engine and adjust throttle blades to low idle. When done the IAC has a good base to regulate up idle from.

JoBy

As I wrote in post #2

divotdug

+1, the high idle on my '84 was resolved when I replaced the front coolant sensor.

Mavrick70004

It already has a new coolant sensor from AC Delco with the new style connector.

Got the IAC and installed it. Both of the IACs move when I ground the diag terminal so it looks like they are working. I said screw it, ill just go drive around to reset the IACs. Could not even drive it. Even the smallest amount of throttle would make it stumble, it took me feathering the throttle in and out just to get it down the street and back into the driveway. Barely stays idling now. TPS is set to .54V. looked over the TBs and nothing has been tampered with, still has the cap and the weld.

The car is putting codes out though (apparently my OBD scanner is not compatible with this vehicle or something.) I am getting codes 12,33,44.
The first step in diagnosing Code 33 is : "If engine idle is unstable, correct before using this chart". OK well great that does me a lot of good. The vac hose going to the MAP has already been replaced.

So from what I understand an unstable Idle could cause a Code 33 and a Code 33 can cause a Code 44.

Im not too sure where to go from here. I am thinking of checking the MAP voltages and the Fuel pressure.. Vacuum leak from the manifold is still on the table but to make it act like this it would have to be a pretty good size leak?

Mavrick70004

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Buccaneer

Tom has some good points and would follow them to move forward. Also, I would buy the 84 corvette GM shop manual off of eBay, you can usually find them at a good price, sometimes not. You really need a manual for a car that is 33 years old so that you have it readily available anytime if needed. It would appear that All Data DIY is NOT giving all the information you need to troubleshoot this issue. The 84 manual IMO is a better manual than the 82 in a lot of respects and goes more into detail on troubleshooting.

This is right out of the 84 manual and there is lots more on the issues you are having. It just takes patience and a logical approach to fix some of these issues with CFI...jus sayin'. Good luck with your 84.