vanduzmi Posted June 26, 2011 Report Share Posted June 26, 2011 Hi everyone, I have been following this forum for years and have finally run into a problem that I could not find the answer to. I have been trying to track down a vacuum leak for about a year now without much luck. Today I tried something a little different, I hooked up a small compressor (~3 psi) to the intake manifold and then listened. I found that air was leaking from the three nuts holding the rocker cover down. I was surprised by this, I didn't think there would be any vacuum in the rocker cover. I was just wondering if this is normal. Thanks, Mike Quote Link to post Share on other sites
poolboy Posted June 26, 2011 Report Share Posted June 26, 2011 The Valve Cover is where the crankcase pressure is relieved by a vent hose from the valve cover to the air intake manifold. So you found the nut's to be leaking ? An O-ring or sealing washer around the studs would cure that. Quote Link to post Share on other sites
Davidw Posted June 26, 2011 Report Share Posted June 26, 2011 The Valve Cover is where the crankcase pressure is relieved by a vent hose from the valve cover to the air intake manifold. So you found the nut's to be leaking ? An O-ring or sealing washer around the studs would cure that. Provided that the thin steel around the hole is completely flat. Most of the nuts have been overtightened at some point so it probably won't be. Oil gets out here as well. I spent an entire morning once tapping it out with a hammer & dolly. Now I have an alloy rocker cover & no leaks. Quote Link to post Share on other sites
Peter Cobbold Posted June 26, 2011 Report Share Posted June 26, 2011 There should not be any vacuum in the rocker box. It connects down to the sump and is filled with blow-by coming past the pistons, so it can be slightly above above atmospheric pressure. As poolboy says those blowby gases are vented into the air intake plenum box. If your rocker box has been connected to the air intake throttle bodies then it would have vacuum most of the time the engine is running, but its wrong. Quote Link to post Share on other sites
john.r.davies Posted June 26, 2011 Report Share Posted June 26, 2011 What is the 'vacuum problem'? As Peter says, leaking rocker cover nuts are irrelevant to any fuelling problem. And I doubt if pressurising the cover with 3psi represents the true pressure within, anyway, unless you have excess blow-by and poor venting. Give us more details and maybe the wisdom of crowds will solve it for you! JOhn Quote Link to post Share on other sites
vanduzmi Posted June 26, 2011 Author Report Share Posted June 26, 2011 John, It wasn't the rocker cover that was pressurized, it was the intake manifold. I unhooked the vacuum line that goes from the manifold to the brake booster and hooked the compresses to it. I as hoping this would point me to a leak on the intake manifold or the carbs. But I was surprised when air started hissing out of the top of the rocker cover. I'm in the process of installing a fuel injection system (http://www.pattonmachine.com/). Everything seems to be installed correctly, but I'm still getting rough idle at start up, and hesitation on acceleration. It seems the most likely cause of this is a vacuum leak. I have found and fixed a few minor leaks, and the car is running better, but still not great. Mike Quote Link to post Share on other sites
john.r.davies Posted June 26, 2011 Report Share Posted June 26, 2011 John, It wasn't the rocker cover that was pressurized, it was the intake manifold. Ah! My misreading! I unhooked the vacuum line that goes from the manifold to the brake booster and hooked the compresses to it. But also Ah! There should be a non-return valve in the line between the manifold and the rocker cover. No idea if that will prevent 3psi the wrong way getting past it, but if it can that should have stopped the rocker cover (and the block, and the sump) pressurising. John Quote Link to post Share on other sites
poolboy Posted June 26, 2011 Report Share Posted June 26, 2011 John, It wasn't the rocker cover that was pressurized, it was the intake manifold. I unhooked the vacuum line that goes from the manifold to the brake booster and hooked the compresses to it. I as hoping this would point me to a leak on the intake manifold or the carbs. But I was surprised when air started hissing out of the top of the rocker cover. I'm in the process of installing a fuel injection system (http://www.pattonmachine.com/). Everything seems to be installed correctly, but I'm still getting rough idle at start up, and hesitation on acceleration. It seems the most likely cause of this is a vacuum leak. I have found and fixed a few minor leaks, and the car is running better, but still not great. Mike A club member went thru this recently with Patton's Throttle Body Injection. Unlike the fuel injection that came on the fast cars, this one is controlled by a computer chip that has to be programmed. It took several weeks and several different programs until he got one that made the engine run decently. Think of it, a computer chip, a forty year old car with what that implies; if that chip or computer goes down, you'll be dead in the water. You can't carry enough wrenches to get you going again. Quote Link to post Share on other sites
Peter Cobbold Posted June 26, 2011 Report Share Posted June 26, 2011 John, It wasn't the rocker cover that was pressurized, it was the intake manifold. I unhooked the vacuum line that goes from the manifold to the brake booster and hooked the compresses to it. I as hoping this would point me to a leak on the intake manifold or the carbs. But I was surprised when air started hissing out of the top of the rocker cover. I'm in the process of installing a fuel injection system (http://www.pattonmachine.com/). Everything seems to be installed correctly, but I'm still getting rough idle at start up, and hesitation on acceleration. It seems the most likely cause of this is a vacuum leak. I have found and fixed a few minor leaks, and the car is running better, but still not great. Mike Why is that test pressure not vented by the rocker cover vent to plenum? Pressure in inlet manifold could get to rocker cover past piston rings, inlet valve guides (or worse, eg holed pistons) But it should re-appear at the rocker cover vent tube not at the rocker cover gasket. I vent the rocker cover to atmosphere through a catch-tank, but even at 6-10psi supercharger boost I dont get a lot of blow-by or other gas leaks. So your 3psi test pressure has found a large leak pathway. Compression test next? Quote Link to post Share on other sites
Peter Cobbold Posted June 26, 2011 Report Share Posted June 26, 2011 Have you got the tick-over air bleed valve on the front throttle-body connected to the rocker cover vent? Quote Link to post Share on other sites
vanduzmi Posted June 26, 2011 Author Report Share Posted June 26, 2011 Upon further investigation it was the hose running from the carburetors to the rocker cover and carbon canister. After I blocked those I no longer have any pressure in the rocker cover. I was also able to find a few leaks on the carbs and where the carbs connect to the intake manifold. Thanks for the help everyone, Mike Quote Link to post Share on other sites
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