1969_CM2590D
April 19th, 2014, 10:13 PM
I was looking around to get ready to adjust the valve lash and check the PCV valves. I did some searches on those things and a couple of questions came up.
The truck is a 3/4 ton, non-California. It's free of emission controls other than the PCV's in the head, and the breather cap in the valve cover. So, I think this is the last of the line before Thermacs and air pumps on all pickups.
First one is, I see a lot of references to a draft tube or a breather/filter on the rear of the older engines. Nothing is on the back of mine, but there is a breather that is on the rear of the passenger's valve cover, plumbed into the air cleaner. Same function, I assume? The ventilation schematic just calls out the PCV's and the breather, showing the airflow.
Second, on the searches I found two numbers for PCV's. Other than having a threaded end, they look quite different, but are they? These look like they have a provision for a hose or something opposite the threads, while mine looks like a piece of pipe threaded at the bottom with a 9/16" hex head at the top to extract it. The PCV itself seems fine, rattles freely, spring has tension, and seals when you blow air through it- So do I really need to change them? The GMC manual says do it every 12,000 mi. regardless.
Picture below.
Thanks
The truck is a 3/4 ton, non-California. It's free of emission controls other than the PCV's in the head, and the breather cap in the valve cover. So, I think this is the last of the line before Thermacs and air pumps on all pickups.
First one is, I see a lot of references to a draft tube or a breather/filter on the rear of the older engines. Nothing is on the back of mine, but there is a breather that is on the rear of the passenger's valve cover, plumbed into the air cleaner. Same function, I assume? The ventilation schematic just calls out the PCV's and the breather, showing the airflow.
Second, on the searches I found two numbers for PCV's. Other than having a threaded end, they look quite different, but are they? These look like they have a provision for a hose or something opposite the threads, while mine looks like a piece of pipe threaded at the bottom with a 9/16" hex head at the top to extract it. The PCV itself seems fine, rattles freely, spring has tension, and seals when you blow air through it- So do I really need to change them? The GMC manual says do it every 12,000 mi. regardless.
Picture below.
Thanks