Edit: I answered my own problem, the solution was the AFM (see full answer below).
This question is for the same engine and the same car as in the following question: Likely cause of poor performance at low RPM but normal at higher RPM? However, new information is available together with a new problem, which I believe is beyond what the comments should be used for in the original question.
The question is simply what is wrong with the engine? I understand the answer will be inconclusive and only statistical in nature. I'm assuming this is to be expected for a question of this nature.
Is it the AFM (Air Flow Meter)? Could it be the AFM in addition to something else?
The question is regarding a fuel injected, in-line three cylinder 1.0 litre gasoline engine, the X10XE I3 found in Opel Corsa B, from 1999.
The engine has new spark plugs, new oil filter, and new oil. The age of the engine is 17 years. It should be noted the engine has been driven at unusually low RPM for these 17 years, if this matters. It probably never saw high RPM and especially not higher RPM and WOT at the same time. Driven ~45 000 miles. Service schedule followed.
Symptoms and issues:
Please excuse the fact I don't use exact RPM figures, the car doesn't have a tachometer.
- At WOT (Wide Open Throttle) at very low RPM, the engine will almost stop, as if there is a big hole in the fuel map and it doesn't provide the correct mixture. Keeping the throttle at WOT will eventually allow it to get out of this "hole" and start accelerating normally. No check engine light.
At only slight throttle at the same very low RPM, the car accelerates without issues.
At WOT at higher RPM (I'm guessing near 80% of redline), the engine will go into what I can only assume is fuel cut mode and the car will coast. The check engine light also becomes lit at this point. After fully releasing the accelerator and then engaging it again the engine will work as before, and the check engine light turns off.
At moderate (assume 50%) throttle at higher RPM, there is no problem up to redline.
There are no bad sounds, rattling, vibrations, smoke, smells, fluid leaks, or other visible issues.
Repair attempts and speculation:
No attempts at repair has been made beyond changing the spark plugs, fuel filter and oil. OBD has not been checked (an attempt was made, but the connection failed, however this is likely a user or equipment fault).
This car has an AFM, not a MAP (Manifold Absolute Pressure) sensor. A car mechanic has noted during regular service that the AFM (Air Flow Meter) needed to be cleaned in the recent past, and that it should be replaced.
I can imagine (and please correct me if this is not possible) that the AFM provides invalid or slightly skewed readings to the ECU (Engine Control Unit) at certain RPMs due to mechanical wear and other issues. When this happens, the air/fuel mixture becomes outside of what is ideal, and perhaps outside of what is even stoichiometric. This seems plausible enough to explain the issues at low and high RPM. As a bonus question, it would be interesting to understand the failure modes of an AFM (if there are any statistically normal failure modes for such devices). How is it possible that it can work at medium RPMs but fail at higher RPMs? It seems more intuitive to imagine it will become stuck and fail at low RPMs.
A way to possibly rule out the AFM would be if the ECU for this particular car is known to not go into fuel cut if the AFM provides inaccurate data. I'm aware the engine wouldn't run properly or even well at all, however I imagine it could still get the mixture stoichiometric based on safe values. Or am I wrong? This seems perfectly reasonable to me, that the ECU can extrapolate and/or provide safe values based on lambda values and TPS (Throttle Position Sensor) readings. However, the firmware in the ECU might just as well be written such that it decides to go into fuel cut just to be safe. I don't know what is common for an ECU in this type of failure mode. Knowing this would be helpful in diagnosing the problem.
However, could it be that the AFM data will be used to provide the correct air fuel ratio, as long as the AFM data is not obviously incorrect? Hence, at low RPM, the data is only skewed, but still accurate enough not to completely stall the engine. Whereas at higher RPM, the AFM data is so inaccurate that the ECU decides to go into fuel cut?