So, I am having a strange issue with my vette that I can’t seem to figure out. I’ve had a rough idle for a while now, and the engine mostly just shakes and misfires (does surge sometimes to), so I put my vacuum gauge on the intake vacuum port, and it reads its pulling between 15-17 inches of Mercury. I’ve tested different throttle speeds and it (aside from opening the throttle quickly) kept the same vacuum and the engine is responsive and snappy. I replaced a lot of the vacuum system already (Headlights and AC vacuum lines and some of the intake stuff), but I haven't touched the purge canister, or any cruise control items. I borrowed a vacuum pump and tested the line going to the cruise control solenoid and it slowly lost vacuum. I tested lot of other sections (including AC and Headlights), and they all held vacuum.

I’ve changed a lot idle related items such as IAC's and new plugs, wires, cap and rotor and I’ve rebuilt the throttle bodies with new gaskets and fuel regulators and injectors, and they spray evenly. I have tested the plenum gasket and the base gaskets for each throttle body using carb cleaner and found that one side was leaking slightly (I replaced both before taking this current measurement). I haven’t checked the timing but again, I was told its electric and I shouldn’t worry about it.

One thing that I might add is I’m getting a sound coming from the front of my engine. It sounds like if you tap to wine bottles together softly but to the speed of the valve train. I’m not sure if my valve train might be screwed or if I have a more internal issue, Or if I still have more vacuum leaks to chase down.

 

Just trying to decipher what your saying here. Your saying you know you have a slight intake manifold vacuum leak? If so, fix it.
your saying that your vacuum is bouncing between 15-17 inches? Not steady? And you have an engine tick?
Bouncing vacuum at idle is an indication of a bad valve. A engine tick is a indication of a bad cam lobe, or lifter which opens and closes a valve.
40 year old car. Lots of things are possible. Any manifold vacuum leak no matter if you consider it minor or not needs to be repaired. Valve train tick needs to be sorted out.

 

Sorry, i think i got things mixed up. So i replaced the throttle body base gaskets before i took 15-17 inches measurement. Which did bring up the vacuum to 17 inches cold and its 15 when warm and i tested and i have not intake manifold (Plenum) leak. (sorry i forgot to clarify). It doesn't fluctuate when idling unless there is an rpm change from going into open loop. The glass bottle noise to me is whats concerning me. I took it to a couple of mechanic shops, there saying saying a whole range of different things from "its the whole valve train and it needs a rebuild" to "its a worn belt or a pully thats chirping". which is why its part of my general question on to why it could be running rough. There is no ticking or tapping from the lifters.

I also forgot to mention, that glass bottle noise is only coming when the car is under load. when reving at a standstill, its perfectly quiet and makes no noise at all RPM's.

 

Your timing gear might be out to lunch. GM used plastic coated timing gears during the C3 years and they wear out and cause excessive slop, or worse, jumps a tooth or two.

 

When I read this I thought of the movie "The Warriors".

​​​​Remove the valve covers and start the engine and observe the rocker arms.
https://youtu.be/6emWN2tEPMk

 

Are you getting a Check engine lite?If so check the codes its throwing

 

I am not getting any codes at all.

 

Any luck since your last post?

 

So I’m not entirely sure, I think I have a vacuum leak near the plenum. Could a gasket near or on the distributor cause one? I don’t have a vacuum advance but I heard there’s a gasket at the base of the distributor itself which may cause problems?

 

Stop! You mentioned that you have a clacking noise from your engine. It could be related or not. But I would be more concerned about that clacking sound before the rough idling.

 

So, I figured it out. After a long period of trial and error, I found that my intake gaskets where leaking, especially on cylinder 5 and 7. it was causing me to lean out under load. I decided to replace every gasket in the upper engine bay and found that my China wall gaskets where also destroyed which I guess is typical. Found also that my distributer was a few degrees out of factory. With all of this and new fuel lines, etc. I should be good to go. Thank you, guys, for the help

 

What about the clanking noise from the front of the engine?

 

Just a guess, maybe what he was hearing/describing was 'pinging' from detonation! He mentions only happens under a load...like I said, just a guess👍

 

The real issue is people who don't know what end of the screwdriver to hold ask for answers and give **** poor description of what's going on.

​​​​​​I would think most professional mechanics look at the lifter, cam, or rocker arms rather a vacuum leak.