I see that one of my PCV Valves goes into the air breather and the other one goes into the intake manifold. My question is if replace them with ones that just are open to atmosphere, will affect how my engine runs? Of course I would block off the path into the intake manifold.

I'm in the process of hunting down a rough starting issue and overall just not running right. I just got a P0172 (Fuel System Rich). I had a new exhaust put on the truck last year with new cat and O2 sensors. It's a rebuilt engine with about 15k miles on it. Fuel pump maintaining 50 psi while running.

The plugs btw don't look like it's been running rich. If anything, lean

 

Flow goes from the airbox, thru the motor, regulated via the pcv valve (or, a fixed-orifice pipe) into the intake. The idea is to draw a slight vacuum on the crank case, to both reduce emissions, and aid in piston ring sealing. You actually WANT the PCV system to work the way it is supposed to. It doesn't cost any power, and it keeps our air cleaner.

 

The flow path you described is for the passenger side. My driver side PCV goes straight into the throttle body underneath the throttle valve. If a leak develops on that driver side, I effectively have a vacuum leak. What I'd like to do is tie the driver side PCV valve into the passenger side that goes into the air breather, or just replace both with breathers.

 

I went to the parts store and got some vacuum line rubber caps. I unplugged the driver side PCV hose and capped it at the intake manifold. I also unplugged what looks like a purge line that is next to the MAP sensor and capped it too. When I started the tuck I noticed it was idling better and was averaging 18.5mm VAC instead of the 17mm VAC.

Since I had more caps, I went ahead and unplugged and capped the passenger side vacuum connection that goes into the dash and I think also goes to the 4x4 system. The truck still maintained about 18.5 mm VAC, but this is where it gets weird. Each time I moved the A/C switch (from Max A/C to regular A/C or any other position) I got a sharp drop in vacuum. I was watching vacuum on my handheld and with each position change it would quickly drop 1 to 2mm VAC and then climb back up to 18.5. With all the vacuum lines disconnected at the throttle body and intake manifold, how is the A/C switch affecting vacuum?

 

The Positive Crankcase Ventilation valve is one of the few emissions systems that clean the air AND help the engine. It seals the atmosphere away from the internal engine systems. At low speed, it pulls straight from the valve into the intake to burn it. At higher speeds, it closes and the crankcase gasses go through the vent tube to be diluted before going into the intake. When you bypass it, you go back to the days of road tubes where the engine gasses were vented to atmosphere. Those also allowed road dirt, moisture and just general crud to get into the engine. When I was training in mechanics, I worked on engines both ways. Even at similar miles, the older road tube engines were a LOT nastier inside. These were maintained engines, not ones that people just put gas in and drove.

It was designed by guys with lots of letters after their names. The parts cost money. The car companies wouldn't put it on there unless it was needed. Put it back like it was designed.

 

The pcv also keeps moisture from building up in the crankcase.
AC compressor puts a load on the engine, pcm sees the rpm drop slightly and it allows more air in to the motor to compensate. Changing the vac.

 

That makes sense. After doing some research today I think I found my boogieman. When I had the rebuilt engine put in the truck, my mechanic took everything off my old engine and put it on the new one. Apparently our intake manifold bolts can't be reused on these engines because they are torque to yield. He definitely reused the old ones. Supposedly this can create a vacuum leak. That got me thinking even more, what if the owner before me never replaced the plenum plate...

The old engine started and ran great, that's why I never have thought plenum plate. It didn't do the hissing out of the IACV port thing and rpms surging and settling on startup that I've got now. It always had power. I had to replace the old engine because it was burning a quart of oil every tank of gas. I struck it up to rings, not the plenum plate.

How I look at it is a new engine needs a new intake manifold, or at least a fully functioning intake manifold. I'll do the magnet test through the throttle body tomorrow and if it's steel, I'll plan on replacing the plenum plate. Maybe just maybe I'll get that Hughes Engines intake manifold for the 5.2. That thing looks sweet.

We'll find out tomorrow. It always shows its butt the worst on a completely cold start up. With all the vacuum ports now blocked off, if it still acts up I'll get ready to do the intake manifold bolts/ plenum plate

 

Well, she threw her normal temper tantrum this morning. Cold start, all vacuum lines blocked off at intake manifold and throttle body. I started the truck with 1/4 throttle. RPMs fell from 2000 to about 400 with the truck shaking. The truck went on to do its by now infamous "three calibration surges" and then settled out. I'm including a photo of MAP Vacuum during the surges and a photo of when it settles out. The orange lines are when vacuum was dropping and the loud hissing was coming from the IACV port.

BTW times are not accurate in the photos. From time 0 to 1:30 I was having to pump the throttle to keep the truck running. From 1:30 to 2:25 my foot was off the throttle. I then did the screen shot and somehow backed out of the diagnostic program. Once I reloaded the program, I took the second screen shot that shows steady MAP Vacuum

 

Think I would start with giving the throttle body a very thorough cleaning. Remove it from the engine, take all the sensors off, use some carb cleaner, and high pressure air to clean it out, making real sure the IAC well is spotless. (disconnect battery while cleaning.) Check that the little vacuum line for the map sensor is in good shape, and not clogged up.... Put it all back together, reconnect the battery, turn ignition to "on" (not start), count to 10 (don't touch anything), THEN start the engine. If everything is as it should be, the PCM should figure out idle fairly quickly.