Idles high when hot

[jtminton]

My truck idles high when the engine has been running awhile. If I exit off the highway, especially. If I give it a little gas, it goes down almost back to normal, but the next time I stop, it's the same thing.

What causes this? Is it a problem, other than annoying me?

BTW, it's a 72 350 with a quadrajet.

Thanks

[67ChevyRedneck]

Had the exact same problem with an 86 camaro, it had a 305 and q-jet. Never did figure out what caused it, (was "borrowing" the car while I was saving up for my z-71 so I never fixed it.) I suspected it was either a malfunctioning choke or a vacuum leak somewhere. I have also read that q-jets are great until they get old. They start to deteriorate and develop gasket leaks and other little quirks that cause them to start "acting up." Hope you find the problem.

[jtminton]

Originally Posted by 67ChevyRedneck

I have also read that q-jets are great until they get old. They start to deteriorate and develop gasket leaks and other little quirks that cause them to start "acting up." Hope you find the problem.

The puddle of gas on my intake manifold is evidence of that!

[gman]

Check the throttle return spring. Pop the hood and manipulate the throttle by hand. Then pull a bit on the linkage to see if it will move and thus drop the idle. The spring may just be worn out and not pulling the throttle completely back.

[jtminton]

I'll check that when I get home. Thanks!

[franko72]

Vac. leak on hoses, Vac leak on carb gasket, Vac leak on intake manifold gasket. Do carb. gumout on all carb linkage and throttle shafts

[still w.i.p '68]

while its running spray b12 around the carb to see if any vac leaks'
my thought is you have bad carb base gasket. good luck

[4x4Poet]

After the suggestions above, check the vacuum booster for a leak. Does the rig stall sometimes when you step on the brakes? Are the brakes hard to apply. The booster check valve is also in play as is the hose connection (could be cracked).

Brake booster troubleshooting

*A restricted vacuum hose will cause boost to drop off when the brakes are applied in rapid succession. This happens because the blockage slows the return of vacuum in the booster.

To check engine vacuum, connect a vacuum gauge to the supply hose that runs from the intake manifold to the booster. A low reading (below 16 inches) may indicate a hose leak or obstruction, a blockage in the exhaust system (plugged catalytic converter, crushed pipe, bad muffler, etc.), or a problem in the engine itself (manifold vacuum leak, bad valve, head gasket, etc.).

The condition of the diaphragm inside the booster is also important. If cracked, ruptured or leaking, it won't hold vacuum and can't provide much power assist. Leaks in the master cylinder can allow brake fluid to be siphoned into the booster, accelerating the demise of the diaphragm. So if there's brake fluid inside the vacuum hose, it's a good indication the master cylinder is leaking and needs to be rebuilt or replaced. Wetness around the back of the master cylinder would be another clue to this kind of problem.

To check the vacuum booster, pump the brake pedal with the engine off until you've bled off all the vacuum from the unit. Then hold the pedal down and start the engine. You should feel the pedal depress slightly as engine vacuum enters the booster and pulls on the diaphragm. No change? Then check the vacuum hose connection and engine vacuum. If okay, the problem is in the booster and the booster needs to be replaced.

Vacuum boosters also have an external one-way check valve at the hose inlet that closes when the engine is either shut off or stalls. This traps vacuum inside the booster so it can still provide one or two power assisted stops until the engine is restarted. The valve also helps maintain vacuum when intake vacuum is low (when the engine is under load or is running at wide open throttle). You can check the valve by removing it and trying to blow through it from both sides. It should pass air from the rear but not from the front.*

[cowboy33713]

Howdy;
recalled that some QJets had a "hot idle compensator", which was ( usually) a small bi-metal heat sensitive strip in a small recess in the rear of the carb.
Don't recall exactly how they worked, (it's been 20+years since I needed to know this), but a Rochester carb book should describe it and it's function, as well as how to tell if it needs replacement. If other avenues don't pan out, check if your QJet has one.
cowboy33713