View Full Version : 350TBI - PCV Valve... Is it even needed on a non-smog vehicle?
so my cars been idleing funny since the day i bought it - rough and low idle. i decided to look at the pcv valve, with the engine running i unpluged the pcv valve from its hose(completely open) and to my amazment the idle evened out and felt like it was runnning stronger. i understand that the pcv pulls out unburned vapors fron the crank case and that it is important, so is there any way it could pull the vapors while still not blocking the vaccum pressure.
also the pcv valve seems to be in working order / but just incase you guys dont recommend running a open pvc i could try out a new one from my local kragen
02-19-2011, 10:11 PM
Running it open is a bad idea. My guess is that the other end of the hose which goes to the standoff for the air cleaner is plugged up. A plugged pcv valve is most likely what you are experiencing. PCV valves are cheap, but like I said it could be plugged at the other end. You can buy a little filter to plug in to your valve cover that protects you from dirt, and bypass the problem (also cheap)
what would happen if i gutted the internals to my existing pcv valve? the pcv valve opens under throttle so what would happen if it was open under idle also(other than a increased idle) - performance loss?
something like this/maybe bigger holes .. or i could just go out and buy the new style gm pvc valve(pictured above)
02-20-2011, 03:33 AM
The PCV valve opens under moderate vacuum. It closes at WOT until RPM rises enough to raise vacuum.
At high vacuum, the valve partially closes to provide some restriction to keep from sucking oil from the case.
Drill the valve and you'll be burning oil, and your WOT performance will suffer (you'll run lean).
Replace the valve and the hose. You can get both at the 'Zone
While you've got the valve out, it's probably a good idea to replace the grommet. The valve on my '95 was always working it's way out. I replaced the grommet, and the old one was so hard and brittle that it came out in 8 pieces. After I got everything out, I "reassembled" the old grommet to make sure that I hadn't left anything inside the top of the baffle in the valve cover.
thanks guys once again - ill slap in a new pcv and see how it runs - i think i figured out the true underlying problem... when i talked to the guy who sold me the vehicle it wasnt really starting, but he reassured me that he had recently installed a 100 AMP alternator... when driving at night with the headlughts on and the radio i drop to about 10 volts or lower at idle (turning my radio to turn off when i pull up to stop lights)
where these engines supposed to be running 150 AMP alternators?
02-20-2011, 11:50 PM
where these engines supposed to be running 150 AMP alternators?
Nope. 105 was standard on the '95, and older models went even lower. I seem to recall 104 being the "big one" for large cars when I worked auto parts in the 80s.
If you're dropping to 10v at idle, there's something going on... either a bad alternator or something is drawing WAY too much current (which will eventually kill the alternator). You should be idling between 13.5 and 15 volts with the headlights and stock radio on.
These aren't like a 1981 Honda motorcycle that had to be above 2k before they'd charge, and you should NEVER drop below 12.5 other than while cranking.
02-21-2011, 12:37 AM
if with the valve unplugged, and running completely open, its running normally, i would start to look for something causing the engine to run rich. a vac leak like an open port will cause the engine to go lean... if its running normal, its obviously correcting itself using the vac leak. you're talking like you just bought this truck... if so, sounds like it might be time to disregard anything the previous owner said, and give it a good tuneup.
02-21-2011, 04:57 PM
sounds like it might be time to disregard anything the previous owner said
Any time the previous owner says "It's only __________", simply fill in the blank with "the beginning of my problems"
If "It's only a $100 sensor" then why is he selling a $3000 truck for $1800?
ahhh this things driving me crazy -- maybe i should run my alternator into a parts store and get it tested. basicly i just bought the car about 2 months ago... and yes i said car lol. its a chevelle 65 running a 350tbi, the battery is brand new/ changed it and the wires running to it... along with a mini starter when i put in my long tube headers a few weeks back. so what its basiclly coming down to is the alternator right now or something more serious. all the vacumm lines seem to be ok, but i will take another look at it tonight... except one - the deceleration valve has no hose coming off it. could that be causeing a vacuum leak? the voltage at idle is bad enough that even when i just have my turn signal on its jumping back and forth 2 volts.
is there a way to test the alternator using a voltometer while its still attched to the vehicle? without getting a mixed reading of stored energy(battery) and the alternators output.
02-27-2011, 04:29 PM
Quick and dirty... plug a voltmeter into the cig lighter and have someone monitor it while driving.
Try various load levels... headlights on/off, etc...
If voltage is higher than engine-off battery voltage, it's coming from the alternator.
If it is lower, the alternator is not putting out.
A little less quick and dirty is to install an amp-meter on the alternator's output (series connection, lug on the alternator goes to one terminal on the gauge, the other gauge terminal goes to the wire feeding the vehicle).
That will tell you exactly what the output of the alternator is.
Likewise, you could make a similar connection on the battery (NOT the big starter cable), and that will tell whether you are charging or discharging the battery... but if the voltmeter is above 13v, you are charging, and if it is below 12.5, you are definitely discharging.
Is the secondary connection to the alternator in place? On the 350TBI engines, there is a single wire plugged into the side of the alternator. This plug is run from the "BATT" light on the dash, the other side of the light goes to a switched hot (hot in start and run).
If that is missing, the alternator may not regulate properly.
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