Can anyone recommend a PCV valve part number for a stock 350 Chevy that doesn't pull vacuum at idle. Two I tried pull so much vacuum at idle, the engine stalls when I put the trans in gear, like a big vacuum leak. I suspected one may be defective so I replaced it with a new one, but it's basically doing the same thing. Engine runs fine when I plug off the PCV. As I understand it, a PCV valve should be closed at idle, or almost closed, when vacuum is highest and open at lower vacuum during wide open throttle or cruising. Have I got it wrong?
Unless I'm mistaken it's closed at wide open throttle but should be working at ide or cruising. Maybe you need to bump up the idle slightly. Remember the old vacuum wipers. Flat chat they'd barely operate until you got out of the throttle.
Do you have your pcv system hooked up so that it is a closed system like it's supposed to be or are you running a breather cap?
Google is your friend.... The PCV valve regulates how much vacuum can pull through the crankcase. It's hooked into the intake - when the vacuum is high at idle, the PCV valve stops most of that vacuum. As vacuum drops with an increase in RPM, The PCV valve opens, to allow a higher volume of g***es and pressure to be pulled into the intake.
Your kinda right the valve should limit the amount of vacuum the crankcase sees so it’s constant despite the amount of vacuum in the intake. But it’s never closed completely. First I’d check that your system; valve and filler cap, breather, pipe to air cleaner or whatever is all correct. Have you just added this? If yes then you need to tune your engine to take account of it. If no and something has changed then it’s possible that something else is wrong such as a vacuum leak from other than PCV.
270ci....I ***ume you have the pcv stuck into the valve cover where it belongs and not laying off to the side connected to vacuum?
Your engine will pull the most vacuum at idle. If you think about the engine being a pump, it generates vacuum when pulling against a restriction i.e. your closed throttle blades. What carb/intake are you running? Is the pcv valve the only new addition to your car? Did it run fine before? Is this a new setup? Sounds to me like you could have it hooked to a port on your carburetor that is maybe for vacuum advance (?) Most pcv systems plumb into the intake not the carburetor. Or as mentioned before, if the pcv valve isn't installed, laying on the intake for example, it won't work correctly either.
The PCV valve was factory installed on SBCs from 1965 to the end of the production. There are a bunch of different 'flows' for this over the years, not just a single part. I am not aware of any data or a chart for PCV flows so you could pick another that has a different flow, but an idle air leak is going to raise RPM when the mixture is correct, just like opening the throttle blades. Second, you say it dies when you drop it into gear. I'd guess it's a powerglide, not a TH350. Many early stock PGs had a fairly tight converter stock that would do this if the engine wasn't tuned properly, and even then needed an idle of 900 RPM to counter the idle in gear load. So my guess is that this is a tuning issue, not the PCV valve. If it runs fine with it plugged, the carb is running lean with it installed and open. What are the other specs of this 350 with a PCV and what trans is behind?
Thanks for all the replies. First thing is my PCV is plumbed correctly as a closed system, starting with a "clean" air line from the air cleaner base going to driver side valve cover and vacuum line run from the p***enger side rear port on the base of a Holley 4160 (which is labeled "for PCV" on a Holley diagram) to the PCV valve in p***enger valve cover. It's been this way for some time and seemed to be working fine, although I have very few road miles on it. It was during some long periods of idling that it would occasionally stumble and sometimes die, and if I put it in gear it would stall. I first suspected a brake booster diaphragm leak, but it checked out OK. When I noticed a faint whistling coming from inside my p***enger valve cover, I pulled the (old) PCV valve out and it was ****ing to beat the band....seemed to be stuck wide open. Replaced it with a new one that is supposed to be the correct part number for the engine. It also seems to pull quite a bit of vacuum at idle but not as much as the defective one. When checking it, I'm holding it at the same at***ude as normally installed (not turning it over) as it appears to have a weighted valve to control flow. It just seems like the new one still pulls way too much vacuum at idle and I can easily draw a breath through it. I'll try re-tuning my carb around it, but it doesn't seem right to have to adjust for what is basically a vacuum leak at idle.
What are the other specs of this 350 with a PCV and what trans is behind?[/QUOTE] It's basically a stock 87 350 out of a state police Caprice (9C1 option) with an Edelbrock performer intake, Holley 4160 carb, headers, and a 700R4 trans that it came with from GM. Has only 50K miles on it, is clean as new inside, averages 170 psi compression across all holes and pulls a steady 20 in vacuum at idle.
I cut a few pcv valves apart to see the differences inside. All these appear the same on the outside, common GM pcv valves with a plastic elbow... NAPA# 2-9229 (the body of the valve is stamped 2088) GM 4/6/8 cyl '85-'95 Spring free height is .775" 95 grams compressed .300" NAPA# 2-9333 (the body of the valve is stamped 2221) Ford 5.0 & 460 '87-2002 Spring free height is .800" 110 grams compressed .300" NAPA# 2-9210 (the body of the valve is stamped 2072, pictured in previous post) GM 262/350/454 '80's thru the '90's Spring free height is .775" 117 grams compressed .300" There are different pintel shapes and tapers. All the above have the same dia holes in the internal "washer" that the pintle slides into. 2221 and 2072 use the same pintle with 4 cutouts in the large "nailhead" end and a short taper. 2088 has a pintle that looks like a sawed off nail, no cutouts in the nailhead and a very gradual large dia taper where it slides into the "washer". I also have two other valves in hand, they were special order from my local store. I didn't get them until after i had made my valve, so i have not bothered to cut them apart. They are... NAPA# 2-9230 (stamped V187) '80-'87 Chevette NAPA# 2-9246 (stamped 2108) '74-'79 350/400/454 All the above valves share the same 3/4" body diameter where they plug into the rubber grommet, and have the same plastic elbow pushed onto the top. You could easily cut them apart by grinding the parting line and re-***emble with shrink tube. Grant
PCV valves basically work with a plunger and a pair of springs. The main spring adjusts with the degree or strength of vacuum from the intake. They are more free-flowing at wide open and almost closed at idle. The second spring (at the tip on the right in the diagrams) prevents it from being closed entirely at idle but indeed should be constricted. (Some PCV valves might not have the second spring arrangement.) Because of the springs, they're usually not affected by gravity or the orientation of the valve. The valve can be at the valve cover, at the other end of the hose adjacent to the intake, or mid-hose anywhere in between. The plunger should be easy to rattle when you shake it back and forth. That means the plunger is not sticking or gunked up. PCV valves are maintenance items that need replacing periodically, but in a pinch, you can sometimes give them a big squirt of carb cleaner and they are good to go for a while longer. They are pretty simple and there's not much in them that's prone to fail. I would guess that big blocks require more flow potential than 4-cylinder bangers so it makes sense that various engines need a matching PCV valve. But from the description of the issues, I suspect a vacuum leak. I have pulled my hair out over a leak in a PCV hose that was whipping my ****. It wasn't until I totally removed the hose did I find a spilt under the hose clamp at the barbed fitting. Tightening the clamp was counter-productive to eliminating the leak in that case.
Thank you Weedburner and Shift Wizard for your additional info. I'll keep investigating and possibly try a few different PCV valves till I figure it out.
270ci, thanks for the further info, and the other guys have given more great info. As for adjusting the carb, a good setup can and will respond to idle mixture adjustment after just an oil change after 5k miles. I tend to agree that another vacuum leak may be causing the issue, but you seem competent and have checked the other major possible hose. Wishing you luck on the battle!
Coincidently, in the November issue of Hemming's Muscle Machines magazine, technical writer Ray Bohacz "ASK RAY" has a six column answer to a reader's question on the basics, use and needs for EGR valves. A lot of information/explanation there if interested.
I realize this is a very old post, I know what you want, poor man solution: connect the PCV valve to the ported vacuum port, completely closed at idle. rich mans solution: there is an aluminum adjustable pcv valve that works great, darn near fully closed at idle and it fully adjustable, I think its name is made By Wagner if you need and can’t find hit me up and I’ll find my receipt and send you the contact info
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