Loose parts in engine bay caused the vehicle to stall

This is how a simple situation was causing a car to completely stall at intermittent times, with no apparent cause. This could happen to any car, though actual technology should has already corrected this bug in engine computers.

For some time, we were wondering about the cause of an intermittent power loss on the car of one of our customers, that seemed to have no cure, as all the engine parts and electronics we checked tested ok.

After doing tests, the problem was tracked down to the ignition system. The scanner reading showed a sudden drop in ignition timing advance, to a lower than usual level (angle) when the problem started showing.

Of course, it was logical to first check the spark plugs, ignition wiring and all other ignition related parts, but all parts tested good. We took out the ecu (engine computer) for inspection, as we were almost sure that the problem was there. To our surprise, the ecu was good too. Running out of ideas for the possible cause of the problem, we observed one more time the facts and started looking in our diagrams in search of electrical components that could be associated with the sudden drop in ignition timing under “normal” circumstances. I mention normal circumstances, because until now, we couldn’t find any damaged or faulty part.

After consulting the diagrams, we realize that the only part that could lower ignition advance in the model we were working with, was the knock sensor. When the knock sensor detects a knock, also known as a “detonation” or “engine ping”, the ECM lowers ignition timing in an attempt to get rid of the knocking. Knocking or detonation, is normally a result of a too lean air/gas mixture, where the air vs. fuel in the combustion chamber at the moment of igniting the mixture, is higher than 14.7. Too high compression or heat can also produce detonations, and so happens when there is too much of ignition timing advance. That is why the software in the ECM was programmed to lower ignition timing when knocking was detected.

So, we replaced the knock sensor convinced that we finally hit the problem. We were shocked to see that the problem came back after a few minutes of driving the car. The worst thing about such problem was that it was intermittent and we were not able to duplicate it all the time, making it more difficult to troubleshoot.

Then, it occurred to us that maybe the sensor is doing its job. maybe the engine was producing knocking and we were somehow unable to hear it or notice it. So the next step was to keep the knock sensor connected, but withdrawn from its place. Effectively, the problem went away, so the conclusion was that knocking was happening and it was unnoticeable.

The problem was that, if there is knocking being produced, how is the engine running and working so smooth without the sensor? Even the scanner charts were perfect…

After realizing that the way knock sensors have to detect knocking is by sound, then we thought that maybe there was a “false” alarm firing up the sensor. The knock sensor is a piezoelectric element that “hears” the engine as a microphone and send the collected signals to the ECM. The ECM is the one that actually “detects” the knocking after using special and complex filters to rule out common noise and other things. This is done in the “knock sensor amplifier” circuit of the ECM.

Well, in our case, we found a loose metallic part near the exhaust manifold, that apparently was creating knocking-like noise with the vibration of the engine and that sound “confused” the ECM into “thinking” that knocking was produced.

Even when the filtering process is complex enough, the vibrations of this metallic shield were either, not filtered out because of a glitch in the filter circuit design, or the generated noise was simply too similar to engine knock, fooling the ECM.

Below are examples of knock sensors. A knock sensor, being a piezoelectric part, when tested with an ohmmeter, it should measure infinite resistance or a very high value, from 500,000 Ohms to several MegOhms (1×10^6 Ohms).

Engine knock is very specific in every car or application and it will not “sound” equal in all models. For us, all engine knocking will sound the same, but for a sensitive, high speed computerized circuit, it will have different sound in different models, as each model has different environments. Each ECM computer has different programming and filtering for the model it was made for.

Well, I think that the bottom line is to keep all mechanical parts tighten, in place and clean, as the minimal thing may negatively affect the engine of your car, due to the complex computerized systems installed on them now a days. Something that looks inoffensive, may cause lots of untraceable and annoying troubles in our vehicle.

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1 Response

  1. Hosting says:

    Proper, regular maintenance and routine oil changes can eliminate the problems outlined above and help you avoid further damage to primary engine components, essentially preventing damage and leaks before they happen.  A good approach is to use a regular maintenance checklist to stay on top of this.  Even a vehicle storage checklist can help you focus on critical areas for maintaining your car.

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