4.2 Manifold Vacum

[Jeepskate]

I'm looking for info or links to info. I've got an '84 CJ7 with the 258 six.

I'm going to pickup a vacum gauge to do some diagnostics. I'm getting a lot of oil through my breather tube into my air filter housing. So what I'm getting at is I need to decide if I'm getting a lot of blow-by or maybe my cam is so worn that my manifold vacum is low. Seems like these two possibilities would have similar consequences. My jeep lacks the power that I think it should have, but then I don't have a good reference point since it was in worse shape when I bought it. I checked the compression and that seemed fine ~150 in all cylinders. I also put in a PCV valve from a big V-8(I was thinking more flow), that helped, but I had to tune the carb a little.

My instincts are leaning towards the cam. Does anyone have any thoughts on putting a new cam in an old engine? Also does anyone know much about interpretting vacum :?:

 

[MudderChuck]

A fairly normal reading (at the manifold) would be 14 + inches of mercury at idle. A little wiggle in the needle is OK, if it vibrates much and/or has wild swings, it´s usually valve trouble. Rev it up to around 2000 needle should be pretty steady (but it will drop a lot, which is normal). Your vacume is highest at idle and drops to almost nothing at full open throttle, at the manifold, pretty much just the opposite if measured above the throttle valve.
The crankcase ventilation, comes in two parts, one works with the throttle closed (PVC) and one with the throttle open (scavenger). I´ve seen some pretty large ventilation hoses get plugged (3/4") and at the connections to the valve cover. I blow through everything to check for stopages or restrictions..
A quick test for excessive blow-by, is to put your hand over the oil filler hole (warm, not hot motor) and blip the throttle, a little pressure is OK, alot (like blowing your hand off of the opening), usually means rings. You can sometimes see a motor blow smoke out of the oil filler opening, when there is excessive blow-by or a seriously plugged crankcase ventilation system.
Another test you can make is with the throttle closed all the way (back out the idle screw, keeping track of the turns) disconnect the coil wire to the distributor and crank it over with a vacume gauge hooked to the intake manifold. Should be around 5 inches, lower than 2 usually means, a serious vacume leak or bad valves. With 150 compression, not really too bad, it should probably be around 4 or maybe 3 minimum. You can raise this number by pinching the hose or plugging the PVC valve. The plunger on the valve is often stamped with a number 4 or 5 this relates to inches of mercury (vacumn).

 

[Jeepskate]

MudderChuck,

Were does the "Scavenger" Ventilation come from? Is it from a port above the throttle body? If so that's new to me and I'll have to check it out. FYI, I've got the Weber 32/36 carb if you know anything about them. The only PCV hoses I've got are the vent from the rear of the valve cover to the air filter housing and the PCV valve from the front of the valve cover to the intake manifold. I suppose the vent to the air filter would provide some vacuum at WOT, if that's the case I'm probably just missing a deflectore to keep the oil from blowing up the hose.

Aside from altering the fuel/air, would there be any negative consequences to installing a PCV valve with a lower number, ie. lower number=more PCV at lower vacuum?

Good info, thanks

Oh yeah, my plan is to hunt down and kill those pesky vacuum leaks first.

 

[MudderChuck]

The scavenger system is an old name, for the hookup to the air filter, Which does most of the ventilation duties, doesn´t take much to get below 5 inches of vacumn and the hose running to the air filter is usually some bigger than the one to the PVC valve. A lower number (lighter) PVC valve would/could make you run a little lean at idle. Don´t underestimate the amount of pull in the air cleaner. Many of the inlets at the air filter, OEM had a piece of filter/foam at the end, this was to help, oil vapor turn back to a liguid and coating the bottom of the filter housing, instead of the filter element.
Seems when the PVC side of the system (hooked at the intake) becomes clogged or restricted, the motor builds a little pressure, in the crankcase and when the throttle is opened, there is a surge, of crankcase gasses and oil droplets. Must have been a good day for me, when I did the hour of crankcase ventilation theory in school. I seem to remeber most of it.