Having contact with a friend, I was asked to have a look at this Opel Astra H, 2004 with Z18XE engine, petrol 1.8. The customer had the car for a week. Complains being engine starts and stalls immediately or starts and only idles, even on full gas.
The car has been in a garage, and also with my friend which did some basic tests. The following have been done to the car prior my arrival:
- diagnostics, no fault codes found whatsoever;
- mass air flow meter plug disconnected;
- another genuine crankshaft position sensor;
- fuel pump replaced;
- spark coil and spark plugs replaced;
- gas throttle replaced;
- compression test done with the test hose exploded.
Next suspect was the ECU, but I doubted since this model is actually reliable compared with what I see on Opels.
Coming at the car, as usual, battery voltage was really low, 11.8V, so had to put a charger on. I did a fault codes readout, nothing came on. Tried to start the engine, it run for 1-2 seconds and stalled, so I could not gain anything on the diagnostics. Tried the gas throttle test, it was opening and closing as commanded, so no problems there.
What I usually do in such a case, I want to have a look at one spark plug, they usually tell a story if you look at their color.
This one was black. So there was fuel, for sure we had air, spark well, it should of been since the spark coil was replaced with a brand new one. What is missing here?
5 minutes later I had the scope connected and the pressure transducer on cylinder 2, engine cranking only, I haven't touch the accelerator pedal.
This is how the waveform looks:
Attention went immediately to the exhaust stroke and seeing there 469 mbar .... I said, this exhaust is blocked. You can actually see that from 180 degrees further up, the pressure actually increases. While on mine Z14XE, line is almost flat at 0 mbar. Close to 360 degrees, pressure drops in two steps, which I believe are caused by:
- piston speed decreases and the amount of pressure leak through catalytic converter overcomes the cylinder increase of pressure;
- intake manifold valve starts to open and some of the exhaust gases go back in the intake.
And the pressure is really high, around 12.7 bar with almost closed throttle body!
It was clear and it was sufficient to remove the oxygen sensor 1 and see that the catalytic converter structure collapsed, thus blocking the exhaust gases passing through.
I gave it a short start and rev up to 3000 rpm, this time engine did not stall. Case closed, another catalytic converter needed and to be further followed up if the failure was caused by the oil consumption, maybe engine running lean, probably I will never find out.
What I learned from this experience:
- that I had a spark plug socket in my tools box that I had no clue about it.
- that an WPS500X was the best investment I ever had, it helps with the job massively, builds up strong evidence to be presented to the vehicle Owner.
Everyone loves a happy ending. Share your diagnostic success stories here.
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