low idle miss.

[Chas Henderson #28127]

Hi all. My car is a 71 SB with AC, Auto. I have been chasing a low engine miss for a few weeks now. I have changed all the plugs and they were all ashy white. New points and rotor and cap even though when I checked the tune up settings everything was correct. So I have reset the timing and dwell. I pulled the plug wires one by one and really didn't notice any differance. I need to check the boots to see if any arcing is going on, but I don't think thats a problem. I checked the engine vacuum and it was pulling 17-18. When the engine is on high idle, the "miss" goes away or is just masked so thats why I am calling it a low idle miss. It isn't rythmic. The gas gets changed about every two weeks as I drive it alot. I know it could be a vacuum leak but I really don't know where to start and it probably could use the new tune up parts anyway. This car has CEC control system on it. I have read the GM service manual, and it isn't really offering any help and is confusing me. The part about the CEC valve, It works when the car starts up and on shut down. But, if you check the vacuum between the line from the distributor to the CEC valve, once the valve pulls the plunger back down, all vacuum to the distributor stops. I thought you should have vacuum all the time while at idle? The lines are hooked up correctly as I have check it against the AIM and the Service manual. It is also set to the correct setting. (CEC valve) I have checked the carb bolts and all is tight. Everything on the car that is vacuum operated works. Sorry that this is long but this is really putting a cramp in my driving time.

Thanks for any suggestions.

Chasman
28127

[Wayne W.]

Re: low idle miss.

Pull off and plug the hose to the accessory vacuum fitting at the rear of the intake manifold. That is the most likely suspect.

[Jack H.]

Disagree with Wayne....

My '71 SB with A/C and manual trans runs like a champ and the CEC works without a hiccup. Understand the one year only CEC system is VERY misunderstood....

Your copy of the '71 Chassis Service Manual will give you a theory of operation thumbnail on how the CEC functions. Basically, there are two positions for the carb solenoid (energized and non-energized).

In the energized mode, the solenoid increases curb idle (about 75 RPM) and ports vac to the distributor. In the non-energized mode, the solenoid relaxes and you have standard curb idle as set mechanically on the LH side of your Q-Jet OR as the choke's fast idle on the RH side of the carb dictates and vac to the distributor is squelched (cut off).

A set of smart relays (on the LH center of the firewall) control the CEC solenoid in conjunction with other factors (the temp switch in the RH cylinder head, the transmission switch, and whether or not the engine has JUST been started). The Chassis Service Manual's tutorial on the subject (look under Emissions) will explain the hierarchy of who does what/when to fire the CEC solenoid.

I say the CEC system is NOT the source of your rough idle! You should round up and interrogate the 'usual suspects' for your problem: (1) worn plug wires, (2) bad rotor/dist cap, (3) bad spark plug(s), (4) improper idle mixture, Etc. A decent mechanic with an engine analyzer ought to be able to put your ignition waveforms up on the display scope and nail the problem(s) fast....

I wouldn't blame the CEC because all it's doing is modestly changing curb idle RPM and blocking/passing vac to the distributor. Remember, we tune these cars by disconnecting and plugging the vac advance line to the distributor altogether to set timing, dwell and the carb's idle mixture.... There's no reason for an engine to run rough or intermittently just because you have no vac advance at idle!

[Wayne W.]

Re: Now Jack.

Are you disagreing just to be disagreable or what? Where did I say a thing about the CEC? I just suggested that the low idle miss may be related to a vacuum leak on the runner that supplies vacuum systems on the car. That port is on the 5-8 runner and "IS" the most common source of low idle miss that I have experienced in more than 30 years of working on these cars. I just suggested that he quickly diagnose the primary problem area by stopping the leak at the manifold. He could then diagnose the various potential leak sources on that port. You can disagree if you want, but it is a fact.

[Chas Henderson #28127]

Re: Disagree with Wayne....

Jack,
I have read and re-read the book on the CEC valve. While I don't think it is the problem, I don't quit understand why when you tune up the engine at normal operating tempeture: you disconect the line from the distributor and plug it when the CEC valve has already done that for you. If I am following the diagram in the service manual properly, then when the engine is at idle, warmed up, the CEC valve is in the relaxed position cutting off all vacuum to the distributor. So when you tune your car up, why bother?

Chasman

[Chas Henderson #28127]

Re: low idle miss.

Wayne,
I have two ports, one for the tranny and one for everything else. Should I leave the one to the tranny alone to see if the leak is there? then block the other if it isnt to elimenate the other ?
Chasman

[Jack H.]

Re: Disagree with Wayne....

It's done as a routine precaution.... What if the CEC solenoid's vac switch isn't cutting off fully? The needle valve could be worn or clogged. The solenoid's foam plastic vent screen could be plugged with underhood grease/dirt causing the venting process to delay or 'stall'.

You'll find other/similar tune-up precautions along these lines. Consider the CSM tells us to open the gas filler door and remove the tank gas cap during tune-up.

Why? Well, that's a precaution to safeguard against an artificially rich mixture at the carb due to residual vapors in the car's vapor recovery system from affecting tune-up adjustment.

[Joe C.]

I Like Jack's Line Of Reasoning

It is always better to err on the side of caution, and that's what GM (and all other corporations did before the Age Of Disclaimers). That being said, if you don't have a "dead miss", then I think that you have what's known as a lean misfire, especially if it it random, and ALL 8 plugs show white. The insulators should show tan, with some greyish-white dust on the electrodes.
The seventies were known for some of the most boneheaded solutions to the "emissions problem" known to man. If you are not going to subject your car to a P.V. then myself and others would be more than happy to help you re-engineer your ignition/induction system. If you prefer to retain the silly ported vacuum system, then simply go after all sources of vacuum leaks, and look toward the possibility of a carb. rebuild.

Joe