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Good day all!

I have a 2010 Jeep Wrangler with the 2,8L diesel engine that has recently started running rough at idle. It has also lost some power,so I took it to a mechanic who said that it sounds like the compression is low on one cylinder. I can accept that, but how does one do a compression test on a diesel engine? Can I do it myself? Is it much different from a compression test on a petrol engine? 

Has anyone here ever done this test on any diesel engine? Any advice would be appreciated.

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It's the same, or similar. Much higher PSI's. A compression tester is around 700 AED. 

Your car seems to be imported, could be anything. 

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It's the same as doing a compression test on a petrol engine but instead of screwing the tester into the spark plug holes, you remove the injectors and use those holes instead. If it's going to be a pain to remove the injectors or it's an engine where you can't remove them like a VW PD, you can also remove the glow plugs and test through there but you need to shut off the fuel supply or disconnect the injector wiring.

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Hi Hudad,

I find it strange that your mechanic is assuming that the misfire is caused by low compression on one cylinder. The fact is that there are so few diesel engines in the UAE that many mechanics can go for several years without ever seeing even the most basic problems on diesel engines. This is off course not their fault, but let us look at the problem in a logical way;

Diesels use compression to heat up the air/fuel mixture to the point where it ignites spontaneously. This means that the compression in each cylinder is of critical importance, but a defective injector that injects too much, or too little fuel can also cause a misfire because like a petrol engine, a diesel requires an exact amount of fuel before the air/fuel mixture will ignite. In practice, too much diesel floods the cylinder, and too little diesel starves the cylinder. Much the same thing happens on petrol engines where fuel pressure or injector problems can either starve or flood the engine.

The amount of diesel injected into a diesel engine also plays a big part in the final compression, but since diesel engines are much more sensitive to compression than petrol engines are, a misfire, and especially a misfire at idle can have many causes apart from low compression due to leaking valves, rings or a cylinder head gasket. There is no way of telling without a compression test, assuming of course that the misfire is not caused by fuel delivery issues.

However, a simple compression test on either a diesel or petrol engine is not always the best way to identify a problem. A much better way to identify small compression losses on both diesel and petrol engines is to do a cylinder leak down test, where the cylinder is placed under pressure while all the valves are closed. Off course, no cylinder is 100% leak proof, but the rate at which the pressure drops is an indicator of how big the overall compression loss is.

Pressure can escape through leaking valves, past rings, or through leaks in the head gasket. In the case of a head gasket, the coolant will bubble, or spill out of the radiator, while if it escapes through valves, you can use a stethoscope to identify which valve is leaking by listening for a hissing sound from either the exhaust or the inlet manifold. If the leak is past the rings, there will be hissing sound from the oil filler cap or the dipstick, or both, depending on how fast the pressure is escaping. On blown cylinder head gaskets, the pressure drop is usually quite dramatic, especially if the leak path is between two adjacent cylinders, on one of which there will always be valves that are not fully closed.

This test works on both petrol and diesel engines, but you need some experience in doing this test to be able to interpret the results correctly. With compression tests where gauges are screwed into spark plug, or injector holes in the case of diesels, the result can be wildly inaccurate for a number of reasons, chief among which is the fact that small leaks are often not detected because the piston is compressing the air in the cylinder too fast, not allowing small amounts of air to escape fast enough to be detected.

This is true for both petrol and diesel engines, so even if the compression gauge says that the compression is fine on all cylinders, there might still be minor compression losses that the gauge does not detect, which could cause misfires on diesels at idling speed. However, on petrol engines, the engine management system often detects misfires that many of us don’t even know are present because they often present no symptoms other than a CHECK ENGINE light and a stored trouble code.

A cylinder leak down test will often identify the problem sooner than some inexperienced mechanic can by changing lots of parts in the hope of hitting on the solution by accident.

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@treks what a great knowledge you have! Respect.

I just clicked on this thread by accident, but learn too much too soon. Thanks for such exceptionally detailed answers.

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