Oil Filler Neck Sucking Air

[behan]

Hi Guys!
Love your forum.

This might be a silly question,
I've been having some issues with my SES light throwing a code #35 Idle air control,

It's a 1994 S-10 Blazer LT
4.3l Vortec (code W)
K&N Intake, Magnaflow exhaust,
aprox, 3000km on rebuilt engine.

So far I have replaced

-IAC valve
-TPS
-All vaccum lines
-IAT sensor

Most of the problem occurs when the truck is stopped at a light,
the idle gets a little rough, and the SES light comes on, then goes out when i start moving again, It seems pretty minor but it sometimes makes me want to push it off a cliff, i can't stand lights where there should be no lights,

[behan]

Sorry guys I submitted before I was finished,

Also just noticed, when I changed the oil, I started the engine with the oil filler cap off, when i was putting it back on "engine still running" I noticed a whole lot of suction, as if it was trying to suck the filler cap into the engine, this may be normal but after the many oil changes i've done over the years I have never noticed this before.

[old_master]

Pull the PCV valve out and inspect it. It has a check valve in it that should rattle when you shake the valve. A slight amount of vacuum in the crankcase is normal, but the way you describe it sounds excessive. If the valve came apart, it would explain the excess vacuum and rough idle too.

[behan]

Thanks,
I changed the PCV today, the old one seemed fine it still rattled, but for a couple of bucks i thought i would try a new one,

No Fix

The idle increases to around 1200rpm at idle with the filler cap removed, and around 1000rpm in park at idle with cap in place, it seems high to me, but i have a digital dash, and rpm is measured at 250rpm increments,

[old_master]

Removing the oil filler cap should not affect idle speed. Have you checked fuel pressure and the systems ability to hold pressure? It's possible there is an internal fuel leak allowing fuel into the oil.

[vtmecheng]

There should be two connections from your intake to the valve covers. One has the PCV valve and connects to downstream of your throttle plates. The second should be located on the other valve cover and connect to upstream of the throttle plates. Make sure this second hose is connected and free for air passage. When you say there is suction at the oil fill neck, how much are you talking?

[behan]

I have one connection on the passenger side "PCV" it leads to the front of the intake plenum, just behind the IAC,

The connection on the other valve cover, leads in from air intake box (filter housing)

Both seem to be clear, if i pinch off the line from the air filter housing to valve cover with oil cap on, it will stall the engine, I noticed there is also some resitance "vacuum" when I remove the oil dipstick, with the engine on,

I won't be able to get a hold of a vacuum gauge till monday,

I know if i place the palm of my hand over the filler neck, the vacuum will cause a red circle on my hand, comparable to a crappy shop vac,

I have higer than normal oil pressure as well, 30-40psi at idle and around 60psi under load,

[old_master]

You don't need a vacuum gauge for this test. Disconnect the filtered air intake for the PCV where it connects just behind the IAT sensor. With the engine at idle, there should be a slight vacuum on that line. Seal the end of the line, with your thumb, and pull the PCV valve out of the driver side rocker arm cover. There should be no vacuum at the hole in the rocker arm cover while the engine is at idle. If there is vacuum present, the intake manifold gasket is leaking internally and must be replaced.

Your oil pressure at 30-40 psi at idle and 60psi under load is great! No problem there.

Post your results.

[behan]

My engine seems to be set up the opposite way,

Filtered air supply comes through the air cleaner box on the drivers side, and PCV on passenger side,

I disconnected the fresh air supply, thumbed it shut. and still had vacuum on the other side in both the PCV valve, and the hole in the rocker cover,

I guess this means I have an intake manifold gasket leak,
Being that I'm not losing any coolant, could it be the upper intake gasket,
just under the Plenum, where the CPI is? " this would be much easier for me to replace"

Thanks!

[old_master]

It's possible that the upper intake gasket is leaking, but more than likely it's the lower one on the underside. Lower intake gasket leaks are common on the 4.3L. When you remove the upper, look very carefully at the sealing surfaces around the ports. Usually you can see the witness marks from the old gasket. You'll be looking mainly on the lower side of each port for a place that the gasket sucked in. When the upper is off, look for fuel "washing" from a leaking fuel regulator or lines leaking, now's the time to do them if they're leaking. Post back your results.

[behan]

If the vacuum in the crankcase was caused by a leak, would I not be able to hear it when all system's and caps were installed?

I'm now wondering if something internal could be plugged/blocked,

The engine runs better than ever!
but has this vacuum issue and SES light, also noticed yesterday my oil level is right on the mark, and spotless clean, as always, but when it's running there is NO oil on the dipstick, shut it down and right away it re-appears, start it up and voila! No oil on dipstick, I run 5W30 no leaks/don't burn

I have lost all trust in my mechanic, fortunately my wife and I just bought a house with a 30'x30' heated garage, but don't move in till June 22nd,
I am wondering if these problems can wait till then?

[old_master]

Any time there is an internal vacuum leak, the only time it will leak is when the engine is running and you will not be able to hear it over normal engine noise. If the leak is in the upper intake gasket, there is no oil in the plenum that could be sucked in and burned. If the leak is in the lower intake gasket, the leak is very close to the underside of the intake manifold where only oil vapor would be present. In either case, oil consumption would not be affected.

The fact that the oil level drops when running may not be an issue. As soon as the engine is running, the oil pump starts circulating oil from the oil pan through the oiling system and the oil level will go down some. You mentioned the oil level rises as soon as the engine is shut down, this would indicate the oil returns are clear. IF the oil fill cap and the PCV valve are in the same rocker arm cover, there would be a possibility that the oil returns are restricted on that side which would also explain high vacuum at the oil fill cap. That's pretty much a long shot...but possible. Simple enough to check... remove the rocker arm cover and look.

As far as waiting to repair it... if in fact there is a vacuum leak, the engine is running lean, (The PCM can only richen the mixture a certain amount). A lean mixture burns hotter and you run the risk of burning a valve or piston. You make the call. Congrats on the house, and especially the garage, you'll love it!

[behan]

Sorry for the delay,
It took 2 months to get my new garage up and running,

You were right on the money, 1 of the lower intake bolts had backed right out, I found it wedged between the fuel pressure regulator and the intake,
the other bolts were only finger tight. I removed the old gaskets and torqued everything down to 35 ft. lbs. and Voila! it no longer tries to suck in flies and other stuff, also idle is smooth, oil pressure is still at 60psi on start up the it hangs around at 40psi at idle once warm,

thanks again, you guys rock!!
Will post more as My Blazer and I evolve together,

[DINO55]

Good Call Chuck,(old master) as Usual............

[old_master]

Thanks Dino, I've been around the block a few times

I learned a few lessons from my Dad, rest his soul, one of them being: "The hard part is understanding how it works, then you can understand why it doesn't work, fixin' it is the easy part." Holds true with most things in life too.