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Could Fixing PCV Also Fix Fuel Pressure?

My street rod has a 74 Chev truck 350 - stock except for Mallory ignition and Holley 4-barrel. When I got the car, the PCV valve and hose was intact but the breather hole in the opposite valve cover had been plugged - no clue why. I fixed this by installing a nipple in the valve cover and running a hose to another nipple that opens inside the filter element of the carb air cleaner (aftermarket round "sandwich" type). I installed a new element, PCV valve and hose and re-gapped the spark plugs to spec (.035) - no other changes to fuel or ignition systems.

The engine does run better but here's the question. The fuel line has a mechanical pressure gauge just before it goes into the carb. Previously, the gauge fluctuated so much that it was unreadable - would seem to indicate a faulty pump (stock Chev mechanical type), fuel filter, and/or gauge. But now it reads a steady 5 1/2 lbs. I don't understand how fixing the PCV would fix the pressure - could it? If so, how?

Reply to
chudwuh
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That's interesting.....

I can't see how the addition of a second PCV intake could affect the pressure.

The pump is a mechanical diaphragm type open to the crankcase so if there was extreme vacuum on the crankcase maybe it could make it flutter.

Ok, are you saying there was no intake for the PCV system, just the suction valve?

If so, well the physical connection is there so maybe the vacuum at idle caused the pump diaphragm to flutter. Strange though.... I think I would put a spare pump in my tool box....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >
Reply to
Mike Romain

You MAY have hit the nail on the head with that answer..... About 25 years ago, I knew a guy who had an almost identical vehicle. He had the PCV valve hooked up, but where there should have been a breather in the valve cover, he had removed it and stuck a spark plug in the hole to keep the dirt out. Anyway, he had me look at it because there was a noticeable howl or whine coming from deep in the engine. While the engine was running, I pulled the spark plug out and heard a big "whoosh". The howl had stopped after that also. Turns out there was a vacuum in the crankcase from the closed breather and the howl was the front and rear main seals trying to be pulled in from the extreme vacuum. It actually took longer to describe the fix to the customer than the fix itself.

Reply to
Kruse

Yeah, there was no intake - only suction (still can't figure out why somebody plugged it!) It now intakes filtered air through the carb air cleaner - similar to OEM setup. It had some leaks from the valve cover gaskets and main seals which I figured were caused by the plugged PCV - never heard a howl/whine but the low restriction air cleaner and exhaust are pretty loud. Since fixing the PCV, I haven't run it enough to determine whether the leaks have stopped - hopefully the seals aren't trashed!

Anyway, it never occurred to me that the fuel pump could be affected - guess that vacuum has to get relieved somewhere.

Reply to
chudwuh

The fuel pressure problem when the pcv was not working properly was likely a result of too much vacuum in the crankcase. The diaphram of the stock fuel pump is exposed to this vacuum in addition to the return spring pressure which cause it to return pressure against the pump intake check valve causing the momentary drop in pressure as the pump pushrod reciprocated and, thus, your indicated fluctuation. With the crankcase now properly vented, the pump diaphram can maintain more constant pressure.

Lugnut

Reply to
lugnut

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