How to See the Invisible

To see the invisible. One could say that this is what modern engine diagnostics is all about.

There’s something, something preventing the engine from running normally. The diagnostician’s purpose is to find this something and then eliminate it, if possible.

The problem is that a defect most often is not visible to the naked eye. To detect it, diagnostic equipment is required, as well as knowledge and skills in working with this equipment.

Our colleague and student Magomed Beltoev told us about a very interesting case:

Recently, a Chevrolet Niva manufactured in 2004 arrived, with a BOSCH MP7 ECU installed. The client’s complaint was that the engine was misfiring.

Before we arrived, they had replaced the injectors, the ignition module, spark plugs and high-voltage wires in the car. According to the car owner, another repair shop measured the compression and told him it was fine. Eventually, after all these pointless replacements and manipulations, they delicately condemned his ECU (alleging that there might be some software failure).

The client brought with him a new Yanvar 5.1.1 unit bought at a shop for 7000 rubles. He asked to have this unit re-flashed and installed in the car. Of course, the client’s request was denied. But we offered our assistance in searching for the cause of the misfiring, especially after all that replacement of spare parts.

The MT-10 didn’t bring happy news. There were no codes; all work parameters were ok at first glance.

The secondary voltage oscillogram was also fine.

Then a Css test with a Postolovsky oscillograph was performed, followed by a Px test in the first cylinder based on its results.

Based on the Css test results, it was quite evident that there is a hardware problem in the first cylinder, and not just a problem, but a disaster. The Px test in the first cylinder confirmed this assumption: the pressure oscillogram was abnormal, the graphs of the levels looked distorted. And the script itself immediately indicated unacceptable gas loss in the cylinder, which was no less than 26%.

My partner-teacher didn’t believe me when I told him that the reason was a lack of compression (it would be probably more precise to say a “lack of pressure”) in the running engine. There was compression during starting – it was checked with a compressometer and a good result was obtained.

To make a long story short, we sent the client to return the new Yavar to the shop and then to run to a mechanic.

To tell the truth, I thought that the matter was related to a catching valve or a hydraulic adjuster that was preventing a valve from closing.

In two or three days, the client came back with the camshaft and the valve lever both replaced and a small new problem... but that’s another discussion.

Here is a photo of the faulty assemblies:

PS. It’s nice to see what others don’t :-)


Thank you, Magomed! The case is really very interesting. Why wasn’t such a gross defect detected with a compressometer? Because the valves in the cylinder were tight, and, upon turning, when the cylinder gets filled with air during slow engine rotation, there is compression in the cylinder. The compressometer would show an acceptable result.

However, gas flow is restrained due to a worn camshaft when the engine is running. Cylinder filling is poor and, accordingly, gives zero output. Please pay attention that graphs’ “tails” in the Css test converged at the very end. The problem in the first cylinder sort of disappeared. But it is while starting that we are measuring compression!

The in-cylinder pressure oscillogram is very interesting. Due to cam and rocker arm wear, normal valve opening is out of question. Therefore, the oscillogram looks as if valve timing is off.


Competent engine diagnostics must be based on application and skilful use of diagnostic instruments.

The above case is absolute proof of this.