Fronzizzle

I've had this intermittent hesitation/running/RPM fluctuation issue ever since I did my engine install. I've tried various things to fix it with no luck. I sat down and did a pretty extensive search today and I found a post on another message board. The poster had a 350 SBC and said:

"I determined that with the PCV valve hooked up, my SBC 350 will not idle right, it just fluctautes up and down. With the line removed and capped off it sounds great, no RPM fluctuations."


One of the responses:
"The PCV valve is designed to be opened by engine vacuum during idle. It also can be opened by blowby pressure in the crankcase, especially when vacuum is low (heavy load).

Fuel injection airflow sensors are more sensitive to PCV operation than carbed applications. It could be that your setup is allowing too much aurflow through the PCV at idle, giving a result that acts like an intermittent vacuum leak.

To get around this, the PCV airflow is "metered". Metering means that the amount of air getting to the fuel injection system is restricted in such a way that it won't upset idle.

My guess is that your PCV valve is either defective, or more likely you are using an earlier PCV valve that has no metering. See about getting one from a late application 350."


And finally, from in response to that from the original poster:
"WOW!!! that was it. I bought a PCV for a 1990 Corvette with a FI 350, now it runs the same as when I had the PCV out of the system. Thanks!!"



I've posted about my issue before and one of the suggestions was to check for a vacuum leak; I have been unsuccessful in finding one but I wonder if this could be the issue if the PCV is "acting" like a leak? I plan on removing it, capping the port & taking the car for a ride but I've never really heard about metering PCV valves and the like.

Does this make sense?

vetteoz

iTrader: (13)

Except on a MAF car like yours, the air into / through the PCV valve is metered ( measured ) by the MAF sensor so it doesn't matter how much air flows through the PCV system ;
the computer has already taken account of it and made adjustments accordingly
The MAP ('90+ ) setup apparently being talked about above is different ,the PCV airflow is not directly metered so that " could " have an effect on the idle

Maverick H1L

iTrader: (8)

Just buy another one if you're worried about it. They're what, $5?

And make sure it seats in the grommet tight along with the hose end fitting tight onto it.

Fronzizzle

I changed my computer system to the '90 MAP sensor, I no longer have the MAF sensor.

I was thinking that, too - just wondering if there was a "correct" one to buy or should I get the same one listed above - for a '90 Corvette with a 350?

Maverick H1L

iTrader: (8)

Doesn't the 90 Vette still run the TPI? Should be the same thing, IIRC.

Fronzizzle

Yeah, good point. I didn't know if it would be "better" than the one my car originally called for as I went from a 305 with the tuned port injection to a 383 with MPFI. Kind of grasping at straws here to find the reason my car falls on its face when I hit the gas...

Maverick H1L

iTrader: (8)

Well, seeing as how you're running a MAP system...

1. If you think you need it, replace the PCV valve.
2. Check both ends of the PCV tube/hose/whatever it is to make sure they seal tight to their fittings.
3. Make sure your valve covers are tight.
4. Check the breather nipple in the opposite valve cover to make sure that seals tight and it's undamaged.
5. Check the intake bellows connection for the breather pipe and make sure that's not damaged (if you're not running a breather filter and still plumbing the breather into the bellows).

Other than that, there really isn't much new to say. Also, a malfunctioning PCV doesn't "act like" a vacuum leak... It IS a vacuum leak. Most PCV systems are plumbed right into the intake in some way on FI vehicles (except a lot of TBI's, I think).

vetteoz

iTrader: (13)

Need to mention little facts like that when asking for help; in this case it completely changes the situation

Fronzizzle

I took off my PCV valve, found that it & the hose had some oil in it. Is that normal? I swapped it out anyway, just wondering. With the new PCV valve, the cold running/hesitation issue is still there so as far as I can tell, it didn't make a difference.

mcrandrz

Cold running hesitation. Sounds like a fuel problem. Not vacuum


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Fronzizzle

Thanks for the response. I just created a new post with some more fuel information.

george88gta

Time to revive a 10 yo post, so I was running lean at idle had alot of noise from a new pcv valve, just replaced that pcv with another new one, no more clicking noise seems to slowly draw air with no Fluctuating in idle like before. In a lot of posts that say it should be noisy but after changing mine I now believe that it shouldn't be it should be drawn a specific amount of air from the valve then when you hit the gas and the vacuum increases it should just open up more. My thought is that one that is noisy is taking small amounts of air then the spring is opening and taking more back and forth back and forth, it should be steady ! what do the rest of you think? The new one seems to have improved the Idle and No More clicking sound but the test will be to check it on the computer. Thoughts?

Vader

Unless there is a valve spring problem, stuck or poorly sealing intake valve, or combustion leak causing pulsations, a PCV should operate a relatively steady state when at idle. A poorly constructed or damaged PCV valve can enter a state of oscillation, and never settle down to steady state.

OrangeBird

iTrader: (1)

Small correction here;

Hook a vacuum gauge up to your manifold and observe the readings. At idle you will see a higher vacuum, and when you "hit the gas" you will see the reading decrease, and not increase as you stated above

george88gta

thanks!!

george88gta

post after post I read on here states that loud and noisey is normal. Thanks for your input. I would agree that it's the state of oscillation.