Questions and Answers

Jeep CJ-7 PCV System Not Working Q. Vincent, First I would like to say thanks for your web site. After researching past questions and answers, I was able to determine that I was getting water condensation in my crankcase and was not getting coolant mixing with oil like I feared. That saves me a lot of time and money. Here is another problem I've had for years and cannot find an answer:

I have a 1981 Jeep CJ-7 with a Chevy 350 in it. It has power steering, no A/C. After researching the VIN number on the block, two different sources told me the engine came out of a 1980 Chevy ½-ton pickup. I'm guessing the total mileage on the Jeep is about 140,000. I don't know when the 350 was put in or how many miles are on it.

When I first bought the Jeep it had a pretty bad oil leak coming out of a breather cap on the passenger side valve cover. I ended up replacing it with a non-breathable oil cap. The rear seal leaked for years, but because oil never hit the ground, I figured it was more of a "rust prevention leak" and ignored it. Eventually it started to leak into the starter, so I replaced the rear seal and noticed the existing rear seal was put in backwards.

I started getting oil leaks in other places, and ultimately ended up replacing the head gaskets and the front seal. After replacing the front seal, I drove it probably not more than five miles when I heard a pop and hissing sound. When I inspected the newly installed front seal, it was blown out backwards.

Apparently I'm getting quite a bit of positive air pressure building up in the crankcase, due to blow by? I bought another oil filler breather cap and all the seals and gaskets appear to be fine, but once again I'm getting oil leaking out of the breather cap, back to square one.

I have done three compression tests in the past year and all cylinders get to around 150 psi on the fifth compression cycle. I know I have 305 cylinder heads on the 350, I guess to increase the compression ratio, and I've been told it sounds like there's a racing cam. I haven't gone that deep into the engine yet.

So I don't know if 150 psi is an acceptable compression for this engine or not. When I replaced the head gaskets, it looked like there was quite a bit of carbon buildup on the piston heads and the combustion chambers. Due to a misunderstanding with the machine shop that put new umbrella seals in for the valves, the combustion chambers were never decarbonized.

The lifters were inspected and were slightly concave. I was told everything was wearing well, but I probably only have 30,000 to 40,000 miles left for engine life.

What could cause so much positive air pressure in the crankcase? Do the 305 heads have anything to do with it? Or is it because of too much carbon buildup? Is it just serious blow by? I would think if the piston rings were bad, it would show up in the compression test.

I was going to try to decarbonize with products you can buy and add to either your oil or gas. Any recommended products? Another suggestion was to add another PCV valve to the passenger side valve cover. Would that one also tee into the existing one that plugs into the carburetor, four barrel Rochester Quadrajet?

The existing PCV valve in the drivers side valve cover was replaced a year ago and only has about 3,000 miles on it. I've owned the Jeep since August 1996 and have only put about 35,000 miles on it.

A. Lets understand what the PCV system and PCV valve do.

SYSTEM OPERATION:
The PCV system prevents blow-by gasses from entering the atmosphere by routing blow-by gases into the intake manifold through a vacuum controlled ventilating valve and a hose while allowing proper crankcase ventilation.

Blow-by gases mix with the air/fuel mixture and are burned during combustion. When the engine is running, fresh air is drawn into the crankcase through a tube or hose connected to the air cleaner housing.

This system is still the same as it was when it was first used in 1961.

PCV VALVE OPERATION: The PCV valve consists of needle valve, spring and housing. When the engine is off: the spring holds the needle valve closed to stop vapors from entering the intake manifold. When the engine is running: manifold vacuum unseats the needle valve allowing crankcase vapors to enter the intake manifold. In case of a backfire in the intake manifold: the valve closes, stopping the back flow and preventing ignition of fumes in the crankcase.

During certain engine conditions more blow-by gases are created than the ventilator valve can handle. The excess is returned through the air intake tube to the air cleaner and carburetor where it is burned in the engine.

A quick check of the system can he made by pulling the end of the PCV valve out of the valve cover and placing a finger over the end of the valve to block the air flow, while the engine is idling.

• A vacuum should be felt and the engine speed should drop approximately 50 rpm if the system is satisfactory and no further testing is necessary.
• If there is no change in engine speed a clogged system is indicated, proceed to the next step.

To isolate the problem, remove the PCV valve from the hose and note vacuum felt and any change in idle speed.

• If the ventilator hoses and carburetor passages are clear, a strong vacuum will be felt and the engine idle will change drastically or the engine will stall when the end of the hose is uncovered. If this occurs, the trouble is in the valve.
• If the engine continues to idle approximately as it did before the hose was uncovered, the hoses or carburetor passages are blocked.

As for the other end, the air intake side, there is little that can go wrong. As long as the hose or breather filter is not clogged, air has to enter the system.

Minimum cylinder compression is 100 psi at 200 rpm. The lowest cylinder reading should not be less than 80% of the highest. Perform compression test with engine at normal operating temperature, spark plugs removed and throttle wide open. 150 psi is good for this engine.

What I would suggest is going to a GM dealer and get a can of "Top Engine Cleaner". Run it through the engine according to the directions on the label.