Sudden high idle - 76 L-48 automatic

[Patrick H.]

I've been helping a friend with his 76 L-48 automatic car the last few weeks. When we started it was running rich and rough, probably on 6 1/2 cylinders.

What we've done -
New plugs and wires (R-45 and ACDelco wires). Previous plugs were covered in carbon, and 2 plug wires were bad.
New ACDelco cap, new rotor.
New carb float (original brass one was sinking), new carb gaskets, new fuel filter.
Changed oil.

With some adjusting yesterday we got it to idle at 600rpm in gear as per spec, 900 rpm in Park. It was running very well, the exhaust no longer made your eyes tear, and we took it for a 4 mile drive after we'd run it for 15 minutes or more in his driveway. At the halfway point of the drive we let it idle for a while and zero problems were noted.

Upon arrival back at his garage, the idle is now 1700 rpm, and no amount of idle stop adjustment will change it. The choke is not stuck on high idle, I do not see any loose or fallen-off vacuum hoses, and everything appears as it did before we left. The last 1000 feet to his house is a 15mph private drive, and the car had no drivability issues with it at all. The high idle came on just as he shifted into Park in his garage.

In addition, it now seems to be running hot, and just started having "run on" once he shuts it off. I did not have my IR temp gauge with me so it's an entirely subjective hot, but having worked on the car yesterday for a couple of hours previous the temp difference was notable. And, it never "ran on" before.

I checked as much as I thought I could, but I'm stumped on this one.

If anyone has thoughts on what might have suddenly changed or caused this, I'm all ears.

The down side is that it's an hour away from me, and he's not that mechanical. So, I can't get to it again for at least a week.

[Peter M.]

Re: Sudden high idle - 76 L-48 automatic

Patrick,
Is the throttle lever returning all the way to the stop?

[Patrick H.]

Re: Sudden high idle - 76 L-48 automatic

Yes. I even looked down, and the throttle plates "appear" closed when viewed from above.

I did not change the idle mixture screws through any of this, so I'm not suspecting they are part of the problem. Just thought I'd add that piece of info.

[Mark E.]

Re: Sudden high idle - 76 L-48 automatic

A couple of thoughts:

- Does this car have an idle stop solenoid? If yes, was the idle speed adjusted properly with the solenoid, and not the throttle stop screw?

- Does this car have some sort of transmission controlled spark vacuum advance (different names in different years- TCS, CES,...)? This system can fail by causing a vacuum leak only when in Park or selected gears. A clue is the high idle only occurs in Park, that's why I thought of this system. Some systems only switch on when a threshold coolant temperature is reached (in other words, it's inactive when the engine is cold). Maybe that's why it idled fine for awhile after start up?

Also, these Quadrajets used plastic, not brass floats. Although this probably is not causing the problem, to avoid fuel level issues in the bowl, be sure to use an original style plastic float.

[Patrick H.]

Re: Sudden high idle - 76 L-48 automatic

1. No idle stop solenoid.
2. No TCS, or at least none installed. The high idle also persisted when put into gear ("chirped" the tires when placed in drive despite brakes being held) once it started. It does have EGR/ EGE/ TFS as original.

It had a brass float; the new one is nitrophyl. It took a bit of tweaking, but we got the float level correct previously.

One of my thoughts is to plug off every possible vacuum source to the carb, and see if it still happens. Just an idea. But, that still doesn't explain why it went from OK to not OK so quickly.

[Chris H.]

Re: Sudden high idle - 76 L-48 automatic

I would check the timing........

[Mark E.]

Re: Sudden high idle - 76 L-48 automatic

One of my thoughts is to plug off every possible vacuum source to the carb, and see if it still happens. Just an idea. But, that still doesn't explain why it went from OK to not OK so quickly.

That and checking the timing as suggested may be a good plan. A lose distributor or a vacuum leak are consistent with what's happening.

[Peter M.]

Re: Sudden high idle - 76 L-48 automatic

Patrick,
Did you try pulling the PCV valve from the valve cover? I wonder if you have a fuel pump leaking into the crankcase?

[Duke W.]

Re: Sudden high idle - 76 L-48 automatic

My thought is that a vacuum leak suddenly developed, and with all the vacuum hoses/devices/connections on that engine, there's a lot to check.

Duke

[Patrick H.]

Re: Sudden high idle - 76 L-48 automatic

My thought is that a vacuum leak suddenly developed, and with all the vacuum hoses/devices/connections on that engine, there's a lot to check.

Duke

That’s what it acts like, but I thought others might assist me in thinking of options.

[Jeffrey S.]

Re: Sudden high idle - 76 L-48 automatic

Patrick,
Did you drive the car after the high idle started and, if so, did it shift from 1st to 2nd properly? Check vacuum modulator rubber hose to vacuum modulator on transmission. If this broke thre is a serious vacuum leak. Ask me how I know.
Jeff

[Patrick H.]

Re: Sudden high idle - 76 L-48 automatic

Patrick,
Did you drive the car after the high idle started and, if so, did it shift from 1st to 2nd properly? Check vacuum modulator rubber hose to vacuum modulator on transmission. If this broke thre is a serious vacuum leak. Ask me how I know.
Jeff

No, we didn't move it at all.
I'll have to check that one, as I thought I checked the vast majority of those under the hood.
Thanks.