Recently I timed the engine and balanced the throttle bodies and noticed today when I went to fire up the engine today that the idle was very rough. It went from around 1275 at first start down to 650 after a few minutes. Although not fully warmed up, the idle was consistent at 650 but the engine was very rough. After taking it for a drive I noticed that the engine was still a bit rough but consistent as far as RPMs go.

With that said I am also curious if I've balanced the TBs correctly. I followed the shop manual procedures which are as follows (will paraphrase some for brevity):

1. Get engine to operating temp.
2. Turn engine off, disconnect IAC and plug passages.
3. Restart engine and put in drive (parking brake applied and wheels chocked).
4. Connect (homemade) manometer to rear TBI port (center port)
5. Adjust idle stop screw to obtain 6 inches of vacuum
6. Cap rear TBI and connect manometer to front TBI (passenger side) and adjust screw to again get 6 inches of vacuum on front.
7. Opted to reconnected TBI to both front and rear a few times to fine-tune the 6 inches.
8. Once balance achieved, adjusted throttle stop screw on rear TBI to 475 RPM (per shop manual)
9. Turned engine off, reconnected IACs, adjusted TPS with multi meter to .525v.
10. Disconnected battery for 10 minutes then took for a drive to reset IAC.

I double checked timing afterwards and confirmed 6 degrees BTDC as well. When I checked, the timing mark had zero bounce to it.

Only things I can think of at the moment is:

• Balance screw has adjusted (applied blue thread locker when reinstalled)
• Harmonic balancer shifted
  • Not confident this is the case as timing mark doesn't appear to move while running.
  • Previously I installed new valve stem seals and mark was at TDC. When rotating engine I observed valve rockers and saw that they were closed when mark on balancer matched pointer on valve cover.
• Worn timing gear / timing chain
  • I have not yet confirmed this yet but have a feeling it needs to be changed.

What are some other things to check? Once balanced and RPMs adjusted down to 475, is the vacuum still supposed to be 6 inches or is it supposed to drop?

Engine has around 73k miles.

 

The procedure looks fine to me. Was idle OK before you balanced the TB's? Was it idling OK when you set the base idle?

If it was OK before, one thing I found easy to mess up is to make sure the idle passage ways are indeed blocked.

 

I will say I have no true way of knowing if the passage is fully blocked with the method that I am using. I may get some groans but it has seemed to work for me. I take a piece of duct tape and make a V with the sticky side out and use a flathead screwdriver to work the tape into the passage and gently massage it over the hole. I leave enough tape sticking out that I can easily remove it afterwards. I then take a piece of insulation foam (similar to a pool noodle) and shove it down in.

As for the idle before I balanced, it was still rough. There is a decent "shake" to the engine that's felt in the car. Now I know that there will be some "shake"/vibration during normal operation, it just seems excessive to me. I'll see if I can get a recording of it shaking while running after startup.

I re-balanced the TB's earlier this morning following the same procedure and checked timing again to make sure. I noticed the timing mark would occasionally hit 8-10 before TDC but it wasn't super consistent.

 

You can verify the tape is sealing the whole by sticking your finger over the opening. You'll hear the difference. But it sounds like that's not your issue anyway.

When you were setting the base idle after balancing, how was the idle? Was it completely smooth without the IACs controlling the idle?

Just an aside, I went through this balancing my TB's a couple of times just like you did but found out my EGR was stuck open. Kind of sounding to me that the balance isn't your issue.

 

Without IACs it seemed to be rough still. When setting the balance on the TBs and the vacuum was at 6inHg for each, the idle reflected the same dips if you will.

How can I check that the EGR is stuck open? I don't have a block off plate and when applying vacuum to the EGR it does open up. Do I just disconnect the vacuum to it?

 

Since you are going to need to do some troubleshooting, I'd suggest getting a hold of the shop manual. In it you find procedures to check it. To test the EGR, while at idle, you reach underneath it push up on the bellows. The idle will get worse if the EGR is not stuck and already open. Your's sounds like it it's working though if you see it moving with vacuum applied.

Since the idle is bad without the IAC working and the timing issues, the ignition system might need some looking into.

 

Funny you mention the SM, I was looking at that before I realized you replied and saw the same test. However, I also saw another post elsewhere where someone provided info on the EGR valves and another way to bench test. According to the other post (not on this site) the EGR needed for a 1982 Corvette is a positive back pressure one. The test is as simple as applying vacuum to the EGR. If the vacuum bleeds down then it is positive back pressure, if it holds a vacuum then it is a negative back pressure. In doing that same test to my EGR valve it bleeds down to which it indicates that I have the wrong EGR installed.

 

The difference was between CA emissions vs everyone else (don't remember which one was which).

 

Do you mean between CA and Federal? From what I gathered the Federal is the one that I would want for my specific engine as it requires a vacuum from the TB/Solenoid to be applied to allow exhaust into the intake where as the CA EGR appears to allow exhaust so long as there is enough pressure generated by the exhaust. Of course I could be wrong with my understanding of what is supposed to be going on.

 

Yes Federal. At idle, you do not want vacuum at the EGR, otherwise it opens up the EGR and you'll have rough idle. I learned this one the hard way. For federal emissions, the ECM will ground the solenoid, sending vacuum to the EGR at cruising speeds.

This should be a simple test for you. While your engine is idling pull the vacuum line off the EGR. Does the idle improve because there was vacuum on the line to the EGR? There's your problem!