Racing up and down at idle

[Money pit Beretta]

You can clean out the PCV and it will do a little better. Welding it shut is a very bad idea, that could blow the rear main seal.

[Rettax3]

The crank-case still vents out through the port in the front valve-cover. I wouldn't weld shut the PCV valve either, perhaps replace it with a vent-filter...

[Money pit Beretta]

That is true, must be thinking/dreaming of the old SBC. Been doing that alot these days.

[Rettax3]

That is true, must be thinking/dreaming of the old SBC. Been doing that alot these days.

Well honestly, I've been questioning the PCV valve's function a lot lately. It should be replaced with every tune-up. Yeah sure, okay, but why? The crank-case vents out just fine whether the PCV is there or working or not. Is it just a $3 flim-flam? Or does it actually help vent the case a little differently because it is piped into the intake after the throttle-plate? On the other hand, it isn't supposed to open under high vacuum conditions anyway, so when it does open up, it is acting just like the constantly-open port up front, isn't it? Okay, I know this is getting a little off topic, but if someone here has real knowledge that controverts this view, and not just a sheep response of 'well, they put it in so it must do something', then I am all ears (or actually eyes, as this is all type ). It is not uncommon for performance-oriented engines to ditch the breather tube and even the PCV altogether in favor of a filtered vent. The blow-by gasses recirculated into the intake are dirty, hot, and generally inert, taking just a tiny bit away from the fresh, usable intake airstream on a stock engine. And, if the PCV valve actually ever prevents these gasses' escape from the crank-case, then it would (as you said earlier) cause a build-up of pressure inside the crank-case, which causes inefficiencies and is potentially damaging to the engine. None of it makes any sense to me from an engineering perspective. Personally, I just don't buy it anymore. If I am wrong, someone with more knowledge here please educate me.

[3X00-Modified]

You have the idea all wrong...

The air is pulled through the PCV valve from the crankcase into the intake under VAC situations, in doing that it needs a supply of clean air in the crankcase... That air is coming from the vent tube on the back valve cover. That goes to the Intake tube so the air your pulling into the crankcase is clean air that has gone through the air cleaner.

When there is no intake VAC then the PCV closes...

If you have oil dripping out of the rear valve cover port, or dripping into your intake from that tube you have excessive blow by and the crankcase is getting air needed for the PCV system from that as well as having excess pressure that's being discharged through that tube and into the pre TB side of the intake.

yes its a very dumb educational video but it explains the purpose and use of it...

http://www.youtube.com/watch?v=c1Jb_sfpZsI

I still have one on my car as well as a working EGR and neither cause any issues for me.

[Rettax3]

Okay, thanks Jon. Yes, I did have it all wrong so far as when the PCV opens -vacuum vs 'high' manifold pressure. It also sounds like they use the PCV system to keep the crank-case air 'fresh' -possibly as a safety measure to keep those more volatile gasses from building up? I can't remember ever hearing of an engine-fire related to crank-case emissions though, so I still think it is more of an environmental concern issue, but I was unaware of that function of the PCV system. Thanks for setting me straight.

It still shouldn't cause seals to blow out though, because of the other vent hose. I do remember going rounds with you back on bstuff regarding EGRs, and whether to vent the crank-case through a filter to the atmosphere or to use engine vacuum to draw out the crank-case pressures, as I recall you were going to set up a catch-can on Mr Pink to keep the heavier deposits from re-entering the intake and sludging it up... Did that ever happen?

I suspect the reasons you've never seen issues with either the EGR or the crank-case ventilation into the intake is your engine's low mileage, meticulous maintenance, and good condition. If it were a daily-driver for 150,000 miles without ever having the engine upgraded, rebuilt, exchanged, or otherwise pulled apart, you would likely see the same carbon build-up in your 3X00 that most other engines get under the same circumstances. Don't get me wrong -I am not saying EGRs and PCVs have no function at all, I am just saying that for the strategic plans I have for my engines, I believe they are counter-productive in the long-run. I expect my engines to get 200k+ miles, without having to be torn-down regularly to get there. As many cars as I have and as little as I actually drive, that should be enough to last my lifetime...

Okay, so back on track?

[No Quarter]

Damn. Tonight it drove fine until it didn't. Stalled at a light, when I restarted it raced up and down the rest of the way home. At home I stopped it, restarted and it idled fine. What is going on???

[3X00-Modified]

No I never did a catch can... No space really and no time to make one up.

And yes it's environmental as well as preventative on the bottom end, It's way easier to clean out some light amount of oil in an intake than replace a rod bearing because it had moisture condense on it and starved it for oil.

If you seal off both ports and don't give the crankcase a way to breathe then yes you can blow out a seal; It would take quite a bit of RPM but it is possible. Think about how much air your moving around when the pistons are going up and down... Why do you think so much HP is gained when you install a crankcase vacuum system, nothing to move in a vacuum.

[3X00-Modified]

Damn. Tonight it drove fine until it didn't. Stalled at a light, when I restarted it raced up and down the rest of the way home. At home I stopped it, restarted and it idled fine. What is going on???

Best check would be to get a scan tool on it and see if you can find any odd readings from some sensors, otherwise your chasing it blind and just need to throw parts at it.

[woody90gtz]

I agree, it's time to get a datalog.

[No Quarter]

Is a scanning tool something I can buy, and where?

Thanks

[3X00-Modified]

You would need to get an ALDL cable and then some software... PM Woody he has the software and the definitions to log you just need to source a cable if you have a laptop you can work with.

[ifixalot]

Check the wiring harnesses to the computer for corrosion. This was the case with my daughter's "90" 3.1, intermittent, high idle problem.
It seems logical to me that it could also cause racing, low idle as the pin contacts connect and disconnect at will.
The IAC has two wires that open it and two wires that close it. The computer energizes one or the other to get the idle correct. If one set is intermittent, the computer may have a heck of a time getting the idle right.
I cleaned the pins with a small wire brush and found a drill bit and used the non fluted end to push through the connectors.
I used WD-40 as a cleaner and to protect after.
Basically, try removing and spraying WD-40 on the pins and connector and put it back on and see if you have any change. If so, clean as I described for a more permanent fix.

[No Quarter]

I checked the wiring harness and WD40'd it, no change.

Soon I'm going on a long trip and I have a Chevy dealer next door to my job, so I bit the bullet and asked them to test it. They did, said the battery had been cut recently, and that they had to adjust the idle because of that. And any battery change in the future would mean the same procedure. Can that be right?

Since then it has run fine, wonder if that will continue. It cost me $300, so is there a tool you can do it yourself?

[ifixalot]

Everything I've heard and experienced, idle relearn gets done automatically as you drive the car.
I don't believe there is anything they can do to reset it.
I've changed out batteries in many of my cars and never had a problem after changing the battery except resetting the clock and radio station presets.
What did they mean the battery had been cut?