The Leak-Down Lie & Top-End Rebuilds

Disclaimer: The content of this document relates to the Porsche 993 (911 1995 to1998) I am not a professional automotive technician and some of the opinions expressed in this document are just that, opinions, therefore, please feel free to use this document at your own risk. Many times a lot can be learned by simply watching qualified professionals work on your 993

What is a Top-End rebuild? It is where the cylinders and heads are removed and remanufactured as needed and then put back on the engine. There are lots of reasons why folks consider Top-End rebuilds. One being the perception of excessive oil consumption whose root cause could simply be overfilling the oil. Or the engine seems to be down on power or exhibiting misfire codes both having multiple potential root causes. For this reason, when a technician recommends a top-end rebuild on a street drive car one should be suspect. Generally the convincing of the need involves a leak-down test dog and pony, err...money show that even the manufacturers of the equipment used to do this test suggest this test should not be used as the definitive method to determine the need for this expensive and invasive procedure.

Leak Down Testing - Cylinder leaks can be from a poorly seated spark plug, worn rings, a valve that does not seat properly, a leak where the test gauge is attached or in a dual spark pugged engine a leak around the 2nd spark plug.

I hear lots of discussions about leak-down testing of engines as a validation of engine condition. From reading the comments on the reports I have come to believe that the conditions under which such tests are done are in many cases, inadequately controlled, Results can vary widely making such tests unreliable as an absolute test of condition. Even the manufacturers of the gauge sets used to do these tests admit the relative measurements between cylinders using a specific gauge set are of some value but the absolute reading of the gauge readings is not too accurate. Leak-down tests may be of some very very limited antidotal value in capturing variances between individual cylinders on an engine in the presence of other gross operating symptoms not attributable to other causes.

Here is a recent comment in reply to my posting questioning a potential valve leak, a common reason a top end rebuild is proposed:

My posting question:
“Thinking that there may be carbon debris in the valve seat, can one take a valve cover off and rotate the valve by its stem to possibly knock it free? Maybe put some choke cleaner or Techron in the cylinder to help with this.”

Reply:
“I've had mixed results when doing leak-downs on older motors. Sometimes I gotta tap the valve to get it to snap shut. During hand rotation, the valve will hang open on carbon. During running operation it will snap shut [work properly] due to speed of the valve closing. I would not cry just yet. Warm the motor up well and pull the suspected valve cover and see if you can pull the rocker off and tap the valve stem slightly when doing the leak down. Don't run it up to 100 PSI. Start a bit lower when tapping the valve. You should hear the leak stop.”

Another shop that was conducting a Pre-Purchase Inspection (PPI) suggested;

that a “top-end” rebuild was needed. In investigation multiple cylinder leak-down percentages in the 75% range were found, however, the dyno test of the same engine put output at 237hp at the wheels, allowing for drive train losses of 10% from the factory stated 270hp this number is quite good for a 16+-year-old car that in this case runs perfectly otherwise. If the leak-down results were to be relied upon the car should barely run, exhibit the check engine light, not be able to hold idle and should be blowing more blue smoke than Jerry Garcia did in his lifetime. Go figure?  What appears as general low engine output, OBDC issues a rough idle or tapping like sound at the exhaust can have many reasons that should be ruled out before considering cracking open the engine to address alleged internal issues!

Here is another comment concerning leak-down testing: 

"Has this car been driven recently? These motors will show terrible leak-down numbers if not driven and a garage queen." 

Apparently just driving a car that sees little use can improve its numbers.

For me the road to a top-end rebuild should look something like this:

The GROSS symptoms referred to above are:

• The engine barely runs and: 
• Has trouble holding idle and; 
• Is significantly down on power and;
• Uses copious amounts of oil while;
• Blowing blue smoke with every startup or aggressive acceleration and exhibits;
• Constant cylinder-specific or general emissions relevant misfire codes that can't be attributed to other causes.

Also, be aware:

• OBD codes related to the secondary air injection system  NEVER require a top-end rebuild to address and;
• Intermittent light valve tapping noises at the exhaust is normal;
• Nasty bucket of bolts rattling sounds at start-up is usually the result of failed lifter cartridges under the valve covers, easily addressed without opening up the engine.

The following items should be considered FIRST or ruled out in resolving the above GROSS running symptoms before contemplating a top-end rebuild:

• The oil is not being overfilled either because it is being done improperly or the oil is being topped-off resulting in excess oil being pulled into the intake and appearing as engine wear-related oil burning;
• The DME relay is good and the electrical system is running at 13.5 to 14.2 volts with the engine running;
• The air filter is fresh;
• The intake idle valve is clean and moves freely;
• The Airflow sensor is working properly, not too expensive to simply replace if purchased in a BOSCH box;
• The oxygen sensors, all 4 on the OBD-II cars, are new or proven to be working properly, again not too expensive to replace if purchased in a BOSCH box and not from the dealer;
• The fuel filter is clean, or simply replace it;
• The caps and rotors are new and the distributor belt has been tested as intact and working;
• The ignition wires look good and are not dried out or cracked;
• The engine measures as pulling and sustaining a good vacuum;
• The vacuum actuators on the intake manifold, the heating duct in the engine compartment, and the recirculation behind the CCU are not leaking and working properly.
• The fuel injectors are reasonably clean, clean them in or out of the car;
• Once all the above have been addressed and the GROSS symptoms persist replace the spark plugs if they look anything other than dry and lightly ashed and run the car for a while with the new plugs and then pull a plug or two and evaluate their condition as being dry and lightly ash-covered, not wet with oil or caked with burnt oil by-products.
• If the sparkplug presents as oily the Valve stem seals should be replaced, a process that can be done without removing the engine or taking the heads out. The valve guides only need to be replaced if lateral play is seen in a valve stem.

If you do the above for a street-driven car resolving your running issues will likely not involve a top-end rebuild.