Quote:
Originally Posted by Jamiewelch
If it isn't accurate it is a useless test unless something is seriously wrong. The cranking speed will slow because of compression and the load on the battery massively increasing, with each crank the battery will be weaker and weaker so will skew any potential results. A slight crack that virtually seals up when warm wouldn't be found but is still head gasket failure. You would need to make sure that power is stable and on something to deliver it to the car, using a car battery will have huge voltage dips making it useless. Only using something that can provide 14v at 400-500amps would give anything even potentially usable as a result.
Just take the glow plugs out, do a test then fit new glow plugs (they will be knackered now anyway). Also make sure its a diesel compression tester as a petrol one won't read high enough.
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I didn't say it wasn't accurate but that it won't be as accurate a gauge test and it is an official comparison test. As for the variations in cranking speed and battery load, they wouldn't affect an individual cylinder being down on compression (or two if the gasket has failed between them) because they would show up as low compression compared to the rest.
On the M47R which is what we are discussing here, head gasket failure is virtually unheard of. Stuck glowplugs aren't though. In a 20+ year old engine where they have never been touched before they can seize in place and break when you attempt to get them out and then you are in all sorts of bother.
Each to their own, and if you don't rate the test then don't do it, but I'd recommend anyone with access to a T4 to try that first before touching the glowplugs if they suspect a compression fault, and of course they can check injector compensation, any stored faults etc at the same time.
The path is System diagnostics: Engine Management: Real Time Display: Engine Roughness: Cylinder Balancing Status: and then with the engine stopped for compression test and running for idle speed variation.