TheDucci

My cars idle has never been that good seance I picket it up from the PO. The idle has always been a little high and fluctuates 200 - 400 rpms at idle as well. I have been chasing down vacuum leaks. I have disconnected the headlights vacuum tube as I now there is a leak there. The other day I pinched off the tube running to the brake booster and the idle increased quit a bit, if I then let off the engine almost wants to stall until I blip the linkage and then it comes back. My brakes feel fine, however.
The rubber tube to the booster is new and I have looked at the PVC elbow and it dose not look like it has any cracks. Do the PVC valves go bad?

Thanks

LemansBlue68

Take a close look at the PB booster check valve. Mine had a small crack in it that I couldn't see causing an irregular idle. One day I was messing around, checking things over again and the check valve nipple finally broke loose. Easy repair. A new check valve is about $10 and is just a press fit into the grommet in the steel booster enclosure. The grommet is an easy repair too in case it has gotten hard and cracked with age.

It's also possible for the booster diaphragm to go bad and leak vacuum. Then it's time for a booster rebuild or replacement. I understand this can happen if your master cylinder leaks fluid into it so check for that.

Oh, and PCV valves most definitely can go bad and leak vacuum! If you even have a doubt, replace it. They're cheap too!

TheDucci

Ok thanks. I'll just replace the PVC valve and see what happens. It seams the closer i get to getting this motor in tune the more pernounced the other issues get.

Easy Mike

Have you checked the torque on your intake manifold bolts?

TheDucci

What is the TQ on a SBC with an aluminum intake manifold? Its been awhile sence I have done any rebuild work on a chevy motor.

terry82

after you turn off the motor .[wait 5 minutes]see if there is vacuume in the booster .where the hose goes into the booster pull it out and listen if you hear air escaping.

75vetteman

the Positive Crankcase Ventilation valve grommet on mine was totally dry rotted. i pulled off the valve cover to safely replace it. it was causing a vacuum leak and spitting oil drops around that area. simple check for the pcv valve is to pull it out and shake it. the check ball should be rattling around in there.