Strange intermittent low idle
Thread Starter
Intermediate
Joined: Jul 2006
Posts: 34
Likes: 1
From: Willow Spring NC
Strange intermittent low idle
This just started happening. 350 with a correct-for-'76 Quadrajet on it. I shot video of what it does, probably easier just to view:
As I mentioned in the video, I did cap the PCV line at the carb, no effect. To recap:
As I mentioned in the video, I did cap the PCV line at the carb, no effect. To recap:
- Idle drops to 400 RPM or less
- Seems to occur only on/after deceleration
- Recoverable, left alone for a few seconds or with a blip of the throttle
- Driven with all carb and intake vacuum ports capped, no effect
- Idle vacuum: 21.5", rock steady
- Deceleration vacuum: 25"
- Starting fluid sprayed around intake, intake bolts, carb base and shafts, no effect
- When occurring, A/F ratio drops down to 10.0-11.0
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
I had a very similar problem with my 1970 L46 after I did an intake job on it. I think you're lean. You're already looking at the possibility of a vacuum leak of course, so my reply might not be of much help to you. I would check the torque on your intake manifold bolts, despite what the starting fluid spray might indicate, just to be sure. I would do the same on the carb bolts to make sure they are snug - if they are too tight, especially on the front, you can have a nearly imperceptible leak at the base gasket.
Did you recently rebuild the carb? It could also be calibrated too lean, or the screws mounting the base to the fuel bowl could be loose. The choke housing can actually be a source of vacuum leaks on a 76 quadrajet - there's a little seal in there.
I would also check the timing, just to rule it out as a potential issue.
I'm nearly certain your issue is a lean condition...the challenge is finding the source of that condition.
Did you recently rebuild the carb? It could also be calibrated too lean, or the screws mounting the base to the fuel bowl could be loose. The choke housing can actually be a source of vacuum leaks on a 76 quadrajet - there's a little seal in there.
I would also check the timing, just to rule it out as a potential issue.
I'm nearly certain your issue is a lean condition...the challenge is finding the source of that condition.
Thread Starter
Intermediate
Joined: Jul 2006
Posts: 34
Likes: 1
From: Willow Spring NC
I was thinking the same, just can't see much to indicate it. Air/Fuel ratio goes rich when it occurs, but I'm not sure if it's a function of the low idle or not. I have another carb here, think I'll swap it out and see if I can isolate the issue. If I can narrow it down to the carb that will be a big step in locating the source of the problem.
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
Check the snugness of the big main vacuum fitting on the rear of the intake manifold (the one that operates the headlights). I also have a 76, and last time I rebuilt my carb I was having problems with poor idle, and I found that fitting to be so loose I could turn it with a pinky finger.
Thread Starter
Intermediate
Joined: Jul 2006
Posts: 34
Likes: 1
From: Willow Spring NC
Update
I swapped the carburetor for a Holley I have and the driveability is improved. The pedal still goes slowly but smoothly to the floor on deceleration to a stop, so I think I was conflating a bad master cylinder and/or needing to bleed the brakes with the engine issue. The idle doesn't drop down low now, so I'll rebuild the carb and see if I can eliminate the problem, which does look like a vacuum leak. I'll also check for leaky calipers, then replace the MC if all else looks good.
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
I swapped the carburetor for a Holley I have and the driveability is improved. The pedal still goes slowly but smoothly to the floor on deceleration to a stop, so I think I was conflating a bad master cylinder and/or needing to bleed the brakes with the engine issue. The idle doesn't drop down low now, so I'll rebuild the carb and see if I can eliminate the problem, which does look like a vacuum leak. I'll also check for leaky calipers, then replace the MC if all else looks good.
Re: the brakes - definitely have air in your system. The question is how it got there. Often on the these cars, if they are driven, the source of the air is run-out on the rear rotors. With original lip seals and run-out over about .007 or so, the calipers will suck in air over time. The fix is to eliminate the run-out, and a bandaid is o-ring seals in the calipers.
