Does anyone have any insight on what PCV valve to use for the 235? I'm using a Purolator PV609.
The reason I ask is that after two short drives and at operating temperature the idle dropped and started to sputter and the oil pressure dropped a bit at the stop light. It did the same thing when I pulled into the garage.
I rebuilt my carburetor (Carter YF 2100s) about 8 months ago. Runs great, my plugs are clean and it always starts up and idles great. The truck is driven at least once or twice a week. All short hops to the store or bank etc. No issues except the one I just mentioned. Just trying to check off possible causes and was wondering what other valves are usable and if I'm using the correct one for the 235.
If you use that method of installing a PCV valve, be sure to stock up on oil. Your engine will consume massive amounts of it due to splash frim the crankcase. The PCV valve needs to be at approximately the level of the valve cover on a tall, baffled standpipe that replaces the road draft tube to avoid sucking up oli along with crankcase vapors. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Thanks. Some good info. I can read Deve's site for hours. It seems like there isn't much information on these valves for the old trucks since most used the road draft tube. Thanks Hank!
If you noticed, Deve was smart to do a leak test. If the system leaks, the PCV system is useless. In the early days, AC made a kit that checked vacuum level to see if the PCV was working. See attached picture. Everybody knows that cork gaskets (valve cover, side cover, oil pan) can leak as the cork ages...so an periodic on-going vacuum level test has to be done. If you are not willing to do this then don't install a PCV system.
Jerry's comment about the location of the PCV valve has merit. The picture shows the top of a road draft tube with the cover removed. Notice the holes inside. This design collects oil mist. Oil drips back down the tube and into the crankcase. Also notice the orientation of the PCV valve and right angle connector. Another deterrent for oil making it to the valve (I think everyone knows that oil can fowl PCV valve operation). GM designed the system for the early AD trucks. It works well on my 55. I can take my valve apart, clean and reuse.
Which PCV valve to use? See pictures (Chevrolet Master Parts List and AC PCV Catalog 660). At idle, vacuum is higher and overcomes the PCV valve's calibrated internal spring. Valve flow is at a calibrated minimum (too much flow can lean out the carburetor). At high speed, vacuum is lower and the PCV valve spring opens the valve a lot more. The valve is designed to work with a specific diameter hose. AC calls for CV670 hose which is 5/8". The diameter of the hoses, pipes and connectors can negatively affect the operation of the PCV valve so to be effective, use the sizes that the manufacturer recommends.
The amount of "blow-by" is not linear plus it gradually increases as the engine wears. Over time more vacuum flow is needed to remove blow-by. The AC PCV valve design is basically a crude on/off device to meet California and Federal emissions requirements at the time. There are better PCV systems if you are willing to install an oil trap and tune the PCV system to the condition of your engine and have a thick wallet.
In the end, why should we old truck drivers even consider a PVC system? The aroma of slightly burnt oil smell wafting up into the cab is not pleasant to some noses.
Last edited by buoymaker; 04/14/20226:00 PM.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
I've been running mine (thanks to Dave for the idea) that way for many a years no issue with pulling oil into the intake. Ran a steel line on mine. Removing moisture from crankcase is not something road draft systems do all that well, need some road speed to make it work at all.
Last edited by BC59; 04/15/20221:47 AM.
BC 1960 Chevy C10 driver 261 T5 4.10 dana 44 power loc 1949 GMC 250 project in waiting 1960 C60 pasture art Retired GM dealer tech. 1980 - 2022
This is another way that a PCV system has been installed (picture). The road draft tube is removed and the hole in the block is sealed. The slots in the valve cover have been sealed. Intake vacuum on the valve cover side connection and inlet air from the elbow on top (via hose connection to air cleaner). The part that bugs me is that if there is no deflector, oil from the rocker arm assembly is being sucked into the intake.
Moisture is a different matter. It's usually caused by short driving distances or cranking the engine, letting it idle a while, then turn it off. I've seen it happen to a 50 Chevy 216 car. My cousin's wife only drove her car 2 miles to park at a gas station then car pooled to work. The dipstick would rust. The oil had to be changed frequently. It takes time and miles to get a cast iron engine hot enough to evaporate the moisture. Modern aluminum engines heat up quickly.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
With that setup, there would also need to be a baffle installed in the center of the valve cover to force the inlet air to the crankcase, then back up to the valve cover again. Siomply flowing air across the top of the valve cover would result in very inefficient purging of the crankcase. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
I added an optional stock GMC PCV valve using stock fittings and PCV type valve cover (no vents) to my '52 228. I cut the draft tube off the oil filler tube and plugged the hole. Then I used a 3" piece of rubber hose zip-tied to a Mr. Gasket oil filler/breather to sit in the top of the oil filler tube as a source of incoming air. Worked well until I realized my average gas mileage had increased from around 15 MPG to 16.5. (Carb is a Carter YF). I pulled some spark plugs and sure enough, they were looking lean. A PCV system is just a controlled vacuum leak after all.
In an attempt to fix the lean problem before it got too serious I temporarily installed an AFR gauge and tried various jets in the YF. Never could get to where I wanted to be so ....
I removed the stock PVC valve from the intake manifold and installed an small oil separator from an air compressor between the fitting in the top of the valve cover and a tube that I welded to the side of the air cleaner. Fresh air source remains the Mr. Gasket breather. About once every 5 tanks of gas I drain about 5 cc of oil from the oil separator. I'm back to the original 0.099" jet in the Carter YF. Gas mileage is where it was before the experiment started and plugs look good.
Last edited by Bill Hanlon; 04/16/202212:04 PM.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
Bill, I don't see a PCV valve in your pictures so I guess you have none at this point? You said, " I removed the stock PVC valve from the intake manifold" but you didn't say what you did with the valve and/or plugging the hole in the intake manifold (from the removed valve). So judging by your pictures, a small amount of vacuum is created by the carburetor inlet air moving past the connector that you welded into the air cleaner.
Attached pictures are of a 1948 GMC with the system connected. Ignore the rubber hose that was disconnected from the gas filter. I think the original flow is as follows: The oil fill is part of the draft tube which is vented (I think). So flow is from the draft tube, crankcase, lifter galley, head, valve cover, tubing, PCV valve to intake manifold vacuum.
I think having an oil trap inline with the PCV valve, regardless of where it is connected has merit. Oil and sludge can cause the valve to stick. Being the internal spring is trying to push the valve wide open (engine not running), this is where the valve sticks after the engine cools down. Wide open valve means lean fuel mixture.
To me the Chevrolet location of the PCV valve above the closed-up draft tube would be better to avoid oil related problems.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
Bill, I don't see a PCV valve in your pictures so I guess you have none at this point? CORRECT
but you didn't say what you did with the valve and/or plugging the hole in the intake manifold (from the removed valve). Swapped the valve to a friend for other GMC engine parts and plugged the hole with a brass pipe plug.
So judging by your pictures, a small amount of vacuum is created by the carburetor inlet air moving past the connector that you welded into the air cleaner. Correct.
Attached pictures are of a 1948 GMC with the system connected. Ignore the rubber hose that was disconnected from the gas filter. I think the original flow is as follows: The oil fill is part of the draft tube which is vented (I think). So flow is from the draft tube, crankcase, lifter galley, head, valve cover, tubing, PCV valve to intake manifold vacuum. That is how mine WAS, but I didn't have the correct vented oil fill tube. That is why I used (and continue to use) my "Mr. Gasket breather in the top of the tube I have.
I think having an oil trap inline with the PCV valve, regardless of where it is connected has merit. Oil and sludge can cause the valve to stick. Being the internal spring is trying to push the valve wide open (engine not running), this is where the valve sticks after the engine cools down. Wide open valve means lean fuel mixture.
To me the Chevrolet location of the PCV valve above the closed-up draft tube would be better to avoid oil related problems.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
Thanks Bill. Your arrangement is similar to what I did for a well worn 235 engine with original draft tube. I was a kid in high school and couldn't afford an engine rebuild. It cut down on the burning oil smell inside the cab. Unfortunately it didn't help the driver's behind me at the stoplight...big grin.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
Mine doesn't smoke at all and the PCV addition seems to have reduced oil drips to practically zero. (Yes I checked to make sure there was still oil in the engine)
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
