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Ford F150 300 in line Six Engine Problem

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MGTF1250Dave

MGTF1250Dave

Jedi Knight
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Aloha All,

My son has had a recurring problem with his older truck. This is a 70's vintage truck with a carburated engine, hydraulic lifters and smog controls removed. A previous engine that had been recently rebuild developed what appeared to be high crank case pressure as large quantities of oil were blown out of the valve cover breather. That engine was replace a few weeks ago and within a week or two the same situation developed. Worn rings seemed the apparent problem, but compression is fine. I think the removal of the smog controls and plumbing rerouting may be the problem. There is a hose running from the valve cover to the intake manifold, could this be sucking oil by creating a strong vacuum in the valve cover? It just seemed odd to me that two engines would have the same problem in such a short time.

Safety Fast,
Dave
 
Yes, you see the issue. The breather from the valve cover should go into the air filter (or dump to atmosphere if you must), but not into the manifold which is under low pressure.
 
There should be a PCV valve in that line, to prevent just that problem.
 
The line you describe probably would have a PCV valve, as stated earlier. However, the intake manifold should generally contain a vacuum, not pressure, so it should draw fumes from the crankcase. One routing that's common is to have two lines from/to the crankcase. One, to the manifold or somewhere else behind the throttle butterfly draws fumes out of the crankcase. The other, connected to the air cleaner, draws air in.

Does oil get blown out all the time? Can you feel pressure at idle if you remove the oil cap? If so, disconnect the hose to the manifold and check it again. Is the oil cap vented?
 
Aloha and thanks for the help. My son just let me know that after another compression test he had a low reading on one cylinder. Seems to be badly worn or broken rings in that cylinder. The valve cover is vented and the hose to the intake manifold has a PCV in it. The oil is blowing out of the valve cover vent. One or more cylinder with worn rings is causing high crankcase pressure. I think he has found the problem. Thanks again Paul, Eric and John.

Safety Fast,
Dave
 
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