No fault code faults

This article is a description of a technical case presented to the AECS technical support team.

Toyota Altezza 2001 2.0L petrol

We have a client’s vehicle in today for WoF repair work. Our admin team noted that the client had experienced some vehicle performance issues that could be related to engine misfiring going by the description of issue given by client. We have scanned the vehicle for fault codes and have found nothing recorded in the vehicles control units. The customer mentioned the issue does not happen often.

We would like a plan of attack to get this problem diagnosed please. Our technician has road tested the car and seemed fine. The client has been to several garages already to try address this problem and had no results. The client feels that many professional workshops these days are only interested in fitting parts and not actually diagnosing and fixing problems. This client stressed that they owned an older car but were prepared to pay for the technician’s time to come up with the fix for this problem. These types of problems are the most difficult to resolve due to the fact of having no fault codes but also the fault only occurs at random times. To top it off, it was now late on a Friday afternoon meaning time was limited, also being in Auckland the roads were now heavily congested with traffic.

For a fault code to be registered, a series of predetermined thresholds need to be exceeded known as ‘trigger levels’. These triggers can be voltage, time and plausibility. It is possible in fast- switching electronics for units to intermittently fail for a millisecond or nanosecond but not trigger a fault code because of being inside the trigger thresholds. This is what we would be dealing with here.

Toyota vehicles of this generation have a spark quality reporting system known as an ignition feedback system (IGF). So in theory, if we had an ignition system failure, we would expect a fault code.

ATS scope

The workshop that had an ATS scope had already measured the six coils power supplies, grounds and trigger pulse (coil charge time). The spark duration had been measured. This was well over one millisecond, which is a good strong spark

Road test

Next step would be road testing while having the ATS scope connected to the vehicle. If a misfire happens, we will experience a slowing of the crankshaft speed due to no energy delivery to it. So we want to measure crankshaft speed and reference this to the firing order of the engine by connecting one of our scope’s channels to, say, ignition coil 1 trigger. This allows faulting cylinders to be located by firing order. This engine is a 6-cylinder vehicle. Being short on time we decided to use the ATS scope’s G force sensor. This simply needed to be taped, the oil filler cap connected to channel 1 of our scope, channel 2 of our scope to ignition coil trigger 1, and off we go driving, waiting to capture a potential misfire. This saved finding obscured wiring with limited time. The G force sensor gives a very similar delta line as the crankshaft sensor recording does. Rocking in one direction on compression, another on power/combustion stroke. If no power to crank, no motion is measured by the sensor, which gives a flat line instead of a rocking pulse on our G force sensor

DIAGNOSTICS - AECS

After a brief road test, the technician captured the following G force vs coil trigger 1 recording.