Still trying to troubleshoot why our heads/cam/headers combo has been such a failure.
Vaccuum reading at idle is about 12 -13 inches. TPIS says the ZZ9 cam should yield 17 to 20 inches at idle.

Could the EGR valve be causing the vaccuum loss? I have checked all the hoses and routing for vaccuum leaks.

What about the PCV valve? Why is that thing routed so strange? the holes in the manifold are a couple inches apart, and just a short arc of tubing between them. I am used to seeing a PCV valve go from and intake port, over to a valve cover. Is th second hole in the intake routed to the crankcase in the lifter valley?

Any ideas?

 

I'd be very surprised to see 20"Hg out of a ZZ9. Stock LT1 cam will only yield 20-21"Hg. 12"Hg does seem exceedingly low. Could be in part due to poor tuning... throttle blades need to be too far open to keep it idling.

EDIT: another thought, are the valves adjusted correctly?

The ports in the intake manifold are routed to allow the PCV valve to pull from the bottom of the intake manifold, directly over the lifter valley, and to discharge into the plenum. There is a sheet metal baffle bolted to the bottom of the intake to keep oil out of the valve. The u-bend tube is only a 93/94 thing... later models use a port on the front of the intake, under the TB for the connection.

 

Thanks for the info, Fred. It seemed like that was the only logical way that PCV could work.

The valves were adjusted by the fellow who did the wrenching on the H/C install for us. He says he used the old method of "adjusting the adjacent valve, when the other is opened". Gave a half turn past zero lash with that method.

I am thinking that we may pull the valve covers and use the TDC #1, then 180 Deg, #6 TDC method, with 1/4 turn past zero.

We installed Harland Sharp 1.5 RR as part of the mods. These are std SBC items. I have had prior exp with AFR heads, and question whether or not we might need longer pushrods too.........

I have a OBD1 cable coming, and am going to run some scans soon. But I doubt we are gonna pick up all the gains we are missing with these mods from just PCM tuning/Troubleshooting.
However, I would be very happy if that could be the root cause.

I checked static compression, and have 180 psi. Fuel pump pressure is fairly steady at 50 psi, during road testing. Plugs look nice and clean. this should be an easy 12 second car, not a high 13's , with these mods

 

Are you sure there isn't an intake manifold leak?

Do you know how to check for this?

 

Once you get LT1 edit up and running, play with the closed throttle spark advance (idle advance). Adding a few degrees at idle will usually stabilize the idle and increase vacuum. Not sure if that's the only problem you have or if there are performance issues, but assuming there's not a vacuum leak that is a way to increase your idle stability.

 

I havent ruled out an intake manifold leak yet. Is that a common occurence on LT1's?
I dont hear a leak. I dont see oil burning which might indicate a leak on the underside of the manifold.

Would one spray some sort of combustible fluid around edges of manifold with engine runnig, and look for rpm increase? Or is there a better way?

would the engine tend to run rich with a vaccuum leak?