2.4t (200bhp) misfire cylinder one.

[KBB]

There are a few thing you need to confirm but will need some decent diagnostic tools. Looking first at your compression test. The test you have done proves only one thing and that is the ability for the cylinder to create pressure at cranking speed only, the wet test confirms lower cylinder sealing capability and the dry upper cylinder sealing capability, this will not confirm the cylinders ability to “breathe”.

A dynamic compression test is the key to this, using your compression tester first remove the shrader valve from the end place it in the first cylinder leaving the rest with their coils plugs and injectors connected less the one under test, start the engine allow it to idle and record the pressure reading, next test is to literally and very quickly blip the throttle wide open and come straight off this will allow a surge of air to flood the cylinder under test before the engine races. Conduct these two tests on each cylinder in turn.

You now have 3 reading for each cylinder a cranking compression, running compression and a wide open throttle reading. We interpret the readings as follows: the idle reading should be 50% of the value of the cranking pressure, the wide open test should be 80% of the cranking value.

If the wide open value is lower than 80% this can be a valve timing issue on that cylinder due to a worn cam lobe causing a late opening and a early closing of an intake valve, excess carbon build up, worn valve guides or springs, or if fitted a stuck shut inlet manifold runner.

If the wide open test produces a higher than 80% reading this shows air is not evacuating the cylinder sufficiently, potential issues are a worn exhaust valve or cam lobe would need investigating creating a late opening and early closing of an exhaust valve, bad hydraulic lifter, if all cylinders showed high values the blocked exhaust is likley.

A vacuum gauge is also extremely useful on a running test look for a steady 17-21iHg with a steady needle, a bouncing needle indicates a fault but cannot be cylinder specific to the fault like the dynamic compression test is. Do these tests next and you can rule out a mechanical issue early in the diagnostic plan.

[Footfistart]

I’ve never done a dynamic compressi No test. Seems fairly straight forward to do. So what sort of pressure should I be getting from what I have already? And do I go against the dry compression or wet compression? My compression tester doesn’t have a shrader valve fitted from what I can see. Can’t upset see a valve inside the hose.

How do you unplug the injectors. I take it you want me to do the entire engine and not just one cylinder? Remember this happens once the engine is hot so shall I do it once the car starts to miss as that’s when I’ve been doing the compression test.

I had my car plugged in and the vacuum is steady. Gives a reading between 17.5 and 19 until you rev it.

Thanks for explaining this to me. It’s a great help

Ryan

[KBB]

If your gauge holds pressure after cranking then it will have a shrader valve, if you cannot remove it the simply hit the bleed button whilst idling and note the pressure.

I prefer a gauge in PSI for this test. You will have 3 readings after the completed test for each cylinder, a cranking pressure, a running pressure and a wide open throttle being blipped reading.

E.g Cylinder No 1 has a cranking dry pressure hot engine of 160 psi, expect a running compression reading of 50% of cranking pressure so 80 psi would be normal. The wide open throttle blip test should produce a reading of 80% of cranking pressure so expect around 128 psi in this example.

For all compression testing you must unplug the electrical connector to the injector for the cylinder under test, if you do not fuel will be entering the cylinder, this causes "bore wash" where the oil on the rings is washed away reducing cylinder pressures and creating inaccurate false readings.

As I stated earlier this test will give a very accurate assessment of the mechanical side of the engine prior to chasing ignition and fuel related issues. Make a table and post your results psi results are more accurate to work with than Bar.