Why in the world would my 1970 Series-1 240Z L-24 have vacuum UNDER the cam cover?

The L24 engine is freshly rebuilt, during which a professional shop redid the original head, planing about 0.1" off it. No cracks noted, and the engine has fewer than 2,000 miles on it since. No oil in the radiator or water in the oil. Manifold vacuum is over 90" and steady, which is better than stock. Compression on all cylinders is even and excellent. I do get about a shotglass of clear water (condensation) out the rear upon startup.

I removed the original "breadbox" aircleaner to mount twin individual units. Because I did this, there was noplace to connect the hose which originally ran from the breather vent on top of the cam cover to the breadbox. That original setup would have exposed the camshaft chamber to intake vacuum, to pull any vapors from under the cam cover and into the carbs to be burned off. Even the EGR diagrams in my manuals show arrows flowing from the cam cover to the carbs. Since there was no place to connect the vent hose, I removed it as I've seen many other owners do. The vent tube on top of the cam cover is naked.

Today while the car was running I placed my thumb over the cam cover vent tube to see how much PRESSURE was there. I was dumbfounded to discover VACUUM there! It built up the longer I left my thumb on it, and when I removed my thumb I heard a 'WHOOSH!" as air ran in, and the engine backfired slightly.

With my thumb over the vent tube I raced the engine ~ the vacuum built up really high and when I removed my thumb I got a REAL backfire.

I'm fairly familiar with the L24 but this is completely baffling to me. Why would there be vacuum under the cam cover, and WHERE would it come from? The only places vacuum can originate are in the cylinders during intake stroke, and in the intake manifold while intake valves are open. There is no hose or pipe connecting the cam cover area to these sources, so now I'm worried. And I can't picture why I would get a backfire from this procedure.......

If this is familiar to anyone, please educate me. I feel there should be a slight pressure under the cam cover, if anything. Certainly there should be no cause or reason for vacuum? Something has to be wrong with this picture, no?


Frank T

PS ~ I considered a leaky intake valve guide seal, but all six plugs are clean-burned and there are no indications of burning oil.

DISREGARD! This has been answered off-screen by Bryan from Z Garage. It's normal (THANK GOODNESS!). The intake manifold's EGR hose attaches to the lower front of the block (I knew that) to pull crankcase vapors out of the block and into the carbs to be burned. The timing chain cover is OPEN at the front of the head, allowing the vacuum to reach up under the cam cover. So this is normal. Still doesn't explain the backfire, tho ~ that remains a mystery.
(like the little girl on Saturday Night Live used to say, "NEVER MIND!")

I know on my 83 or any ZX at that, it sounds normal, I'm not shure with those carbed engines, (my 240 manifolds are off so I can't see for myself,) but when you remove the oil cap on the ZX, it almost stalls, sucking air. Well, at least I hope it's normal for ya. Maybe that's why that fire retarder is in the tube coming off the valve cover? If air was sucking you wouldn't need one? Just some thoughts there. Does it run good besides that?

Yeah Paul ~ it runs OK, altho it starts hard and I have always suspected it had a vacuum leak somewhere. But the brake booster doesnt work at all so I suspected the diaphragm was ruptured, which would allow for a vacuum leak there. Saving now to replace it and I suspect it will solve the problem. Phil has volunteered to help me turn a few wrenches sometime so when I can buy a replacement vacuum booster, I'll take him up on it ~ draining the brakes and replacing the fluid is a 2-man operation if done correctly.

I think I figured out why it backfires when I cover the cam cover vent for awhile then release it.

All vacuum comes from the intake manifold. The EGR system works off that vacuum. While the engine idles, the EGR system is pulling fresh air in thru the cam cover vent, down the front of the head and block (following the timing chain opening), thru the EGR pipe and hose beneath the distributor, then up into the intake manifold and into the cylinders.

When I plug the cam cover vent, that must increase the level of vacuum moving thru the SUs, which could pick up more fuel, enriching the mixture. When I unplug the cam cover vent, the vacuum actually drops suddenly (because of the sudden supply of outside air) which lessens the airflow thru the SUs , which in turn leans out the mixture. When that sudden change of air/fuel mixture first hits the cylinders, I'd bet it doesn't burn, so the cylinders pump some unburned fuel into the exhaust system, where the hot pipes ignite it. The distributor wouldn't have time to adjust to the instant vacuum change either, so the advance would be far ahead of the new requirement for a moment. The spark would be early (or late?) and the cylinder(s) wouldn't fire properly. The compressed air/fuel mixture would be pumped out into the hot exhaust to ignite there, resulting in a backfire.

At least that's what I'm gonna tell myself ~ and I ain't gonna argue with my self about it anymore.

I feel better, thanks.