Redhook98

I have a weird problem with idle creep. After running and driving for a bit, my idle out of drive will creep up to 1000-1050 or so(850 in drive). I can not find any vacuum leaks the traditional way with carb cleaner. My choke appears to be fully off by this time and the high idle cam is not making contact (I backed it all the way out to make sure).

I clamped off my PCV hose and the idle dropped to about where it should be in park. 850rpm. Is the PCV system supposed to create a controlled vacuum leak? Or does this point to another issue? I pull out the valve and cap it with my finger and rpm goes down. It is connected to the correct location in the front center of the carb. Carb looks brand new and is the correct numbered matching carb. I had it apart hoping to find dirt and crap, but it looked brand new. ?? Idle air mixture is set good. I have about 17-18 lbs of vacuum at idle before it goes up.

Ideas?

Also, what are the symptoms of a power valve going bad or dirty? This is unrelated to the idle issue, I believe. I cut out at mid throttle under a load, but not at easy driving or flooring it.

Car is a 1973 Vette. Factory everything. No mods other then the chambered pipes.

DUB

QuadraJet carbs do not have a 'power valve'. Holley's do. If that is what you are referring to what you write 'power valve'

When I tune and set up an engine..I remove and cap off the vacuum hose(s) that are attached to the fitting in the intake and take all vacuum operated components out of the equation. The only ones that I will leave attached (when I test drive it) are the hose to the power brake booster and the one for the automatic transmission if it is equipped with that.

Without checking the timing and making sure it is correct ..either by using a vacuum gauge or a timing light ( and that is ASSUMING that the notch on your harmonic balancer is CORRECTLY positioned and has not spun)...and checking the idle mixture screws to make sure that they are actually able to meter the fuel during idle.....and that the FAST IDLE is working as needed...and the choke works....it is hard to advise you exactly where this issue is.

DUB

gkull

I'm not there and I can only guess on a few things. It is normal for a hot engine to idle faster that a cold motor. You set you idle speed when the motor is hot.

The power valve if the correct inches of vacuum for your setup should have no flow unless the vacuum drops like at WOT. Power valves usually fail the diaphram so it never opens. So you have lean poping or hesitation under wot

toobroketoretire

Anything that operates under a vacuum could be the cause so if a vacuum leak is causing your "idle creep" you have to ask yourself which component would likely be affected by the engine's heat and start leaking vacuum. The EGR valve's diaphragm would be a possibility because it gets so hot during it's operation. If you have a Mighty Vac try sucking on the diaphragm's 3/16" nipple and see if it holds vacuum.

OMF

The PCV valve is a controlled vacuum leak. Leaks a little at idle...alittle more at heavy throttle. Sounds like something is buggered inside the valve, for the couple of bucks, I'd just replace it.

It's hard to tell from the picture, but make sure you have dual throttle return springs...could be a weak spring also.

resdoggie

We may never know. Thread is 5 yrs old.