I have been trying to track down this problem for the last week and it is driving me crazy. Hopefully some of you could shed some light on this situation. 2008 Ford Crown Vic Police Interceptor with 87,000 mile sand 6100 idle hours

First, some background info. The car was running fine the night I bought it. Wife took it to get smogged and registered. Car passed smog and 15 minutes later it had a blinking CEL. I read the code as a P0302. Cleared the code and 20 mins later it came back. This is the list of things I have done to diagnose the problem.

  • Moved coil to another cylinder to see if code followed. It did not.
  • Cleaned MAF with appropriate cleaner.
  • Removed and replaced spark plug. I noticed a thin coating of oil on the threads.
  • Checked fuel injector and connections to coil with noid light. They all have power and were pulsing normally.
  • Checked resistance of fuel injectors. They all read 13.3 ohms
  • Checked coolant for oil and checked oil for coolant. None present.
  • Did a compression test on cylinders 1-4. Compression was low (40 PSI) on cylinder 2. Compression on cylinders 1, 3, and 4 was between 125-130 PSI. Added a small amount of oil to see if compression was affected. It was not.
  • Performed leak down test. I heard a very faint hissing from oil cap. No bubbles in coolant, no hissing from intake or exhaust.
  • Ran advanced diagnostic with code reader. Timing was retarded by 10 degrees.
  • Car has a very noticeable shudder at 1100-1500 RPM at 40 MPH, and lack of power and acceleration.

I can smell the car running rich when it had the blinking CEL. After the oil was added the for the compression test, and the battery hooked back up, the CEL has not come back on. I am totally stumped at this point. I had several people give me several opinions on what the cause may be. So far they range from bad head gasket, bad piston rings, bent valve, and bad valvetrain components. I am to the point where I am going to get it diagnosed professionally to confirm what the root cause is. Just wanted to see if anyone had any advice on how to proceed. Thanks in advance!

  • Have you tried an engine detergent/treatment on the top end/intake? Have you tried swapping the subject injector with a "working" injector to see if the issue moves?
    – NitrusInc
    Commented Apr 15, 2018 at 11:33
  • Check all hoses on intake manifold for vacuum leaks, some cannot be seen at the lower rear of the intake. This is the main cause of that code when no other issues can be found.
    – Moab
    Commented Apr 15, 2018 at 15:43
  • 1
    I changed out the injector with a new Motorcraft one. Still the same rough idle after the new injector.
    – Jon
    Commented Apr 15, 2018 at 18:46
  • @ Moab would a vacuum leak be specific to one cylinder, or will it affect all of them?
    – Jon
    Commented Apr 15, 2018 at 19:00

3 Answers 3


its either excess carbon on the valve, broke valve spring or burnt valve ....if valve is burnt need to check cat

  • Welcome to Motor Vehicle Maintenance & Repair! Whilst this may answer the question, It would be better if you fleshed out your answer. What is it they'd need to check their cat for? How could they check their cat? How would they go about checking for a burnt valve or a broke valve spring? There's a lot which could be said to help not only the OP, but others who may come seeking advice. What you've written is good advice, it could be great advice, though. Commented Apr 15, 2018 at 13:56

The Wet test helps to exclude the Rings as an issue, in most cases

Here is a regurgitation of Material Handling Network's article on Engine Diagnostics:


Engine Diagnosis Methods. Several techniques can be used to properly diagnose your customer’s engine problems. This month’s article will cover the engine diagnosis methods including: compression tests (dry & wet) and the leak down test. These tests are straightforward yet provide valuable information useful for diagnosis.

Compression Test - This test measures the dynamic pumping pressure of the cylinder when the crankshaft is rotated. This test should be performed when the engine is warm – not cold and not hot.

Dry Test – Install the compression gauge into the spark plug hole and crank engine over 5-10 revolutions. For consistency, crank the engine over the same number of revolutions for each cylinder. If one cylinder has a lower reading this indicates the problematic cylinder. The dry test should yield readings within 10% of manufacturer’s specifications. If no compression information is available, use 100 psi as a reference test pressure. However, it does not indicate whether the cylinder or piston rings are the cause. The wet compression test is effective at determining the source of the problem.

Wet Test - With the spark plug removed, squirt about one teaspoon of 30-weight motor oil into the spark plug hole. Take a compression reading and observe the difference between the wet and dry tests.

Readings from the wet test should not increase by more than 10 percent, cylinder to cylinder.

If the compression increases with the wet test, the results identify the problem as the piston rings and/or cylinder walls. The theory behind this test is that the oil is providing a wet seal for the rings. If they are not sealing on their own they will when the oil creates a seal and an increase in compression will be observed.

If the compression stays the same, the results point to the valve train. The theory is that when the rings are sealing, the oil will have no effect on compression and therefore the valves are most likely the cause of the problem.

Leak Down Test - This test pinpoints specific leakage. This test uses a set of pressure gauges with a regulating device and can quantify the percentage of leakage. This is a static test that takes more time to perform compared to a regular compression test.

The ignition system should be disabled and/or grounded to prevent shock or fire.

The engine must be at top dead center where both intake and exhaust valves are closed (compression stroke). Before pressurizing the cylinder, be sure that you can prevent the piston from moving in the cylinder (engine from turning over). Pressurize the cylinder through the spark plug hole at around 100-115psi. You will be able to measure the percentage of pressure leakage.

  • Leak from crankcase indicates the piston rings are not sealing or a burnt piston.

  • Leak from the air intake indicates a bent or burnt intake valve.

  • Exhaust leaks from the exhaust manifold or muffler indicates a bent or burnt exhaust valve.

  • Leak from the radiator indicates a leaking gasket or cracked cylinder head or block.

  • If air is leaking from the companion cylinder it indicates a blown head gasket.

Properly functioning cylinders show a cylinder to cylinder leakage of less than 8%. A greater drop should prompt you to investigate further.

Each test provides different diagnosis information and takes a different amount of time to perform. By using either method of diagnosis you will be able to provide your customers with accurate information and continue to provide the highest level of quality service available.


it seems you have eliminated most possibilities, the fact that the compression on that cylinder is so low would cause a misfire environment, low power, jerky and sluggish in low rpm, the wet test did not raise the compression, so we should be able to rule out the rings, may be carbon build up on either intake or exhaust valve, bent valve however unlikely, worn valves, broken or damaged valve spring/s. Also another less likely thing is a compression leak from the head gasket or a hairline crack in the head

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .