PCV valve help! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Jesse 67 c-10 · 12-23-2002, 02:16 PM

Someone on here must have experience with this, My truck isn't running right, the idle is eomewhat erratic, as in it always seems to be changing speed! Sometimes its fine at around 750, 800 others it sits at 1500 for a while and if it idles for a minute it'll slowly drop right down and almost stall, this is when its allready warmed up, choke is working properly. also the way it runs is erratic too, sometimes runs ok, other times I get hesitation on initail pressing of the gas, and low power. I'm really thinking its the pcv valve, it's good, but I think its opening and closing at the wrong times for my combination, like maybe right when I press on the gas or at intermittent times during idle, its a valve for an 84 chev 350, initialy picked it for the right hose size and bend! what are people with similar combos using? shown below, anyone else had problems similar to this? Can't seem to find any one who knows! or even some basic theory on how these things work in the first place, or any inventive ideas?
Thanks!
Jesse

my new 72 · 12-23-2002, 02:59 PM

are you running a fuel pressure regulator for that carb. If your fuel psi is too high it will cause what you are describeing to happen. I doubt it is a vaccum problem though. Check the float height and adjust as necessary also, but make sure to put a regulator and gauge on that thing if you don't have one already.

72K10 · 12-23-2002, 03:05 PM

Jesse:

Might not be your pcv valve, but could be a different vacuum leak. That's the easiest to check for first. Just inspect vacuum hoses, or squirt WD40 around base / backside of carb, listening for a change in rpms.
If not that, then it might be your idle mixture. My 1406 had similar problems, but after I let it warm up and then set the idel mixture, I solved the problem. One at a time, adjust your two idle screws in until rpms falter, then back off a half turn. Hope this helps.

Project1970 · 12-23-2002, 04:09 PM

PCV valves don't 'open' or 'close' in that sense - all they are is a one-way check valve to make sure the crankcase can't suck air from the intake. The only two problems you could have with it would be either a clogged one, or one that was leaking vacuum.

lukecp · 12-23-2002, 06:22 PM

A mechanic taught me a very easy way to find vaccum leaks. First, adjust the idle on your carb wayyyy lean, and drop the speed down to around 300 RPM. Then spray carb cleaner all over any vaccum hoses or carb gaskets. When the engine is that lean, the carb cleaner will make the RPM go up to about 400, which makes it much easier to detect a vaccum leak. Then reset your idle mixture and drive off

.

Jesse 67 c-10 · 12-24-2002, 01:53 AM

Thanks for the advice, hmm, I'll look into those things, one prob though, my engine won't run below 500 rpm, but I see what you mean yellow 72. So a pcv is just a one way check valve huh? hmm, maybe the fuel pressure being to high would account for the only so so gas milage, despite an overdrive, I have a stock fuel pump, i'd kind of like that to be the prob, then my gas milage would improve too! I'll definatly check for vacume leaks and reset my idle, might try blocking off pcv temporarily, but who knows, I'll try.
Thanks!
Jesse

*Black67Devil* · 12-24-2002, 02:40 AM

gaskets are the leading cause of irratic idling
replace the carb gasket with a good holley one.
then get back to us.

mikep · 12-24-2002, 03:19 AM

I know they kind of seem that way but PCV valves are not one way check valves. They are designed to close under periods of very hi vacuum such as idle as well as times as very low vacuum such as acceleration. They function as a check valve in as much as they close in the event of a backfire but the airflow potential in them is proportional to manifold vacuum. cut one open. You'll see what I mean.

Project1970 · 12-24-2002, 05:06 AM

Really? Learn something new every day.

I'd still find it odd that a PCV would be responsible for something other than a good old fashioned vacuum leak, though...

Long Knight · 12-24-2002, 02:17 PM

I had similar problems when my points started deteriorating. Truck would pretend to run good but before you knew it, the thing would start to gag and caugh then fart and die!

I hate it when my truck is sick!

*67/72junkie* · 12-24-2002, 03:17 PM

Probably some kind of vacumn leak like everyone says.Be sure to check intake for vacumn leaks also especially if fresh gaskets .

12-23-2002, 02:16 PM	#1
Jesse 67 c-10 Registered User Join Date: Feb 2000 Location: Edmonton, Alberta, Canada Posts: 599	PCV valve help! Someone on here must have experience with this, My truck isn't running right, the idle is eomewhat erratic, as in it always seems to be changing speed! Sometimes its fine at around 750, 800 others it sits at 1500 for a while and if it idles for a minute it'll slowly drop right down and almost stall, this is when its allready warmed up, choke is working properly. also the way it runs is erratic too, sometimes runs ok, other times I get hesitation on initail pressing of the gas, and low power. I'm really thinking its the pcv valve, it's good, but I think its opening and closing at the wrong times for my combination, like maybe right when I press on the gas or at intermittent times during idle, its a valve for an 84 chev 350, initialy picked it for the right hose size and bend! what are people with similar combos using? shown below, anyone else had problems similar to this? Can't seem to find any one who knows! or even some basic theory on how these things work in the first place, or any inventive ideas? Thanks! Jesse __________________ Edmonton, Alberta, 67 c-10, Long fleet, front disks, 5 lug rear end, 327 with Vortechs, edlbrock manifold, comp cams XE 256, 600cfm carb. Backed by a getrag 5 speed and 1 piece driveshaft. 1993 Dihatsu Hijet Jumbo cab 4x4, currently converting to battery electric power.

12-23-2002, 02:59 PM	#2
my new 72 faster, faster, faster Join Date: Dec 2001 Location: Houston ,Texas ,USA Posts: 1,354	are you running a fuel pressure regulator for that carb. If your fuel psi is too high it will cause what you are describeing to happen. I doubt it is a vaccum problem though. Check the float height and adjust as necessary also, but make sure to put a regulator and gauge on that thing if you don't have one already. __________________ I'm going to Law School, wanna debate? It's good practice.

12-23-2002, 03:05 PM	#3
72K10 Registered User Join Date: Dec 1999 Location: New York Posts: 288	Jesse: Might not be your pcv valve, but could be a different vacuum leak. That's the easiest to check for first. Just inspect vacuum hoses, or squirt WD40 around base / backside of carb, listening for a change in rpms. If not that, then it might be your idle mixture. My 1406 had similar problems, but after I let it warm up and then set the idel mixture, I solved the problem. One at a time, adjust your two idle screws in until rpms falter, then back off a half turn. Hope this helps. __________________ RLTW 70C10 72K10 72K10 Cheyenne 72C10 Cheyenne Super

12-23-2002, 04:09 PM	#4
Project1970 14.1 @ 96MPH Join Date: Jul 2001 Location: Mesa, AZ Posts: 2,811	PCV valves don't 'open' or 'close' in that sense - all they are is a one-way check valve to make sure the crankcase can't suck air from the intake. The only two problems you could have with it would be either a clogged one, or one that was leaking vacuum. __________________ Project1970 - LS1 Swap Complete!

12-23-2002, 06:22 PM	#5
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	A mechanic taught me a very easy way to find vaccum leaks. First, adjust the idle on your carb wayyyy lean, and drop the speed down to around 300 RPM. Then spray carb cleaner all over any vaccum hoses or carb gaskets. When the engine is that lean, the carb cleaner will make the RPM go up to about 400, which makes it much easier to detect a vaccum leak. Then reset your idle mixture and drive off . __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

12-24-2002, 01:53 AM	#6
Jesse 67 c-10 Registered User Join Date: Feb 2000 Location: Edmonton, Alberta, Canada Posts: 599	Thanks for the advice, hmm, I'll look into those things, one prob though, my engine won't run below 500 rpm, but I see what you mean yellow 72. So a pcv is just a one way check valve huh? hmm, maybe the fuel pressure being to high would account for the only so so gas milage, despite an overdrive, I have a stock fuel pump, i'd kind of like that to be the prob, then my gas milage would improve too! I'll definatly check for vacume leaks and reset my idle, might try blocking off pcv temporarily, but who knows, I'll try. Thanks! Jesse __________________ Edmonton, Alberta, 67 c-10, Long fleet, front disks, 5 lug rear end, 327 with Vortechs, edlbrock manifold, comp cams XE 256, 600cfm carb. Backed by a getrag 5 speed and 1 piece driveshaft. 1993 Dihatsu Hijet Jumbo cab 4x4, currently converting to battery electric power.

12-24-2002, 02:40 AM	#7
Black67Devil Account Suspended Join Date: Dec 2002 Location: Purgatory ( no, not Colorodo) Posts: 81	gaskets!! gaskets are the leading cause of irratic idling replace the carb gasket with a good holley one. then get back to us.

12-24-2002, 03:19 AM	#8
mikep Used to have a truck Join Date: May 2002 Location: port orchard WA Posts: 1,552	I know they kind of seem that way but PCV valves are not one way check valves. They are designed to close under periods of very hi vacuum such as idle as well as times as very low vacuum such as acceleration. They function as a check valve in as much as they close in the event of a backfire but the airflow potential in them is proportional to manifold vacuum. cut one open. You'll see what I mean. __________________ No truck :-(

12-24-2002, 05:06 AM	#9
Project1970 14.1 @ 96MPH Join Date: Jul 2001 Location: Mesa, AZ Posts: 2,811	Really? Learn something new every day. I'd still find it odd that a PCV would be responsible for something other than a good old fashioned vacuum leak, though... __________________ Project1970 - LS1 Swap Complete!

12-24-2002, 02:17 PM	#10
Long Knight Spank 'em if you got 'em! Join Date: Nov 2002 Location: Kansas City Posts: 628	I had similar problems when my points started deteriorating. Truck would pretend to run good but before you knew it, the thing would start to gag and caugh then fart and die! I hate it when my truck is sick! __________________ Long Knight '71 C-20, 350ci, TH350, Edl. 1406 Carb w/elec.choke, Ignitor ignition, Viper Tires, Orange Dipstick

12-24-2002, 03:17 PM	#11
67/72junkie Account Suspended Join Date: Oct 2002 Location: NC Posts: 1,129	Probably some kind of vacumn leak like everyone says.Be sure to check intake for vacumn leaks also especially if fresh gaskets .



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