A Drive By Wire Problem?

I have noticed in the past that when driving right around 50 km/ hr - 30 mi/hr at steady speed, I would sometimes get a very tiny speed decrease then resumption back to normal. The change was small, perhaps equivalent to 1-2 km/hr. At first I thought it might have been a brief misfire, perhaps a coil problem; but, no misfire codes and no sign of coil problems. The problem was very infrequent and in the absence of any evidence of misfires I attributed it to throttle input fluctuations - me moving my foot around on the throttle or bumpy roads causing throttle inputs. When I lifted my foot off the throttle and then reapplied pressure the problem would always completely disappear and not repeat which is definitely not consistent with a misfire problem.

Today I was out driving and the same thing occurred, except this time traffic was light and I had a good distance over which I could try and drive at a steady 50 km/hr. What I noticed is that the car would slow down by 1-2 km/hr for about 2 seconds and then speed back up to 50 km/hr and then about 5 - 10 seconds would slow down again and then speed up. This repeated about 4 - 5 times with consistent timing before I finally had to end the test because of a traffic light. When the light changed I pulled away and drove under the same conditions to see if the problem would repeat itself; but, it did not return. When the problem occurred I was very conscious about holding my foot steady. The speed change was very regular and definite, very much like when the cruise control engages.

When this occurred today the cruise control switch was on; but, the cruise control was not engaged. It had been set for 110 km/hr so clearly it was not attempting to control to that set point and as soon as I let up on the throttle or if I pressed the throttle the throttle responded as it should. At this point I have not determined whether this occurs only when the cruise control is switched on or whether it is unrelated to the operation of the cruise control. If it is unrelated to the cruise control I am wondering if the there might be a dirty spot / dead spot on one of the drive by wire throttle position sensors causing a input fluctuation even if the throttle pedal isn't moving. However, that would not really jive with the consistent speed change repetition frequency when the problem does occur. I have also only noticed the problem around 50 km/hr. It might be present at other speeds; but, I have not noticed it. Speed control with cruise control at highway speeds is smooth with no obvious fluctuations. There are no DTCs stored in the ECU associated with the DBW control.

Has anybody experienced this kind of behaviour and if they have, determined what the problem was? The car is a 2000 model year North American market version.

Mine is DBW, but never experienced that issue. You could try to put it into a different gear and see if the issue stays with the specific pedal position or the speed.
From an electronics point of view the resistors used to get the pedal position could show some wear but that's just a guess into the blue ..

If I could shift gears without lifting the throttle that might work :). As soon as I move the throttle at all the problem seems to go away and does not return.

The AC was on at the time and I was wondering whether it might be the AC idle up function associated with the compressor cycling - really reaching for causes!

VSS dropping for a few seconds with that heavy single disc dual mass flywheel…

While OBD2 data stream is too slow and timing not guaranteed for any detailed analysis, you should be able to see something on some of the parameters.
IG timing, rpm (Ne), fuel trimmer, etc. No fuel pressure data though.
Do you have OBD2 adaptor and hardware or App to carry out basic data logging?

By the way, misfire detection is surrounded by so many patents from all sorts of companies, car manufactures, etc so some misfire modes have difficulty in being detected by certain algorithm.
Depending on the local regulation, the misfire detection method can be different.
For example, JDM NSX never had 3pin IG coil but uses different method.
For the clarification, having 3pin IG coil doesn't mean OBD2 compliant.

The workshop manual doesn’t show the full picture of the IG coil equivalent circuit and the misfire detection module for the above reason.
You can only check the primary side of the IG coil and to test it properly, you'll need the IG dyno.

Already saw several OBD2 NSX with clear misfire even at idle rpm yet no CEL triggered and not even a code stored.


Kaz

The OBDII scanner I used to check for stored DTCs belonged to my Son. Its a generic low cost scanner and may not even report anything other than the mandatory OBDII emission codes and might have been incapable of reporting the DBW error codes. It certainly does not support logging. I do have a lap top based diagnostic tool that is designed specifically for Audi - VW products and supports logging. The vendor says it is OBDII compliant; but, will not guarantee that it will report anything other than the mandatory OBDII codes on non VAG vehicles. The tool has a vehicle limited license and if I enter the NSX's VIN in the license and the logging function does not work I have wasted that license provision. I think I may have to investigate one of those OBDII dongles if I can find one with internal memory for logging. This erratic operation has so far been very infrequent (or I just don't notice it most of the time) so I would have to leave the data logging function continuously active and hope to capture an event. The OBDII device needs internal memory for logging because driving around all summer with a laptop attached waiting for something to happen isn't really an option - another reason why I did not use the VAG diagnostic tool. After the last occurrence of this where it was obvious that there was a definite step change down in speed lasting about 2 seconds and then it restored and then repeated, I think the changes will be slow enough to be captured in the OBDII data stream. Out of curiosity, do you know how many of the measured and calculated parameters are available through the OBDII data stream. Is something like the accelerator position sensor voltage accessible?

Because of the definite step change in speed lasting about 2 seconds and having it repeat with the same pattern a few times, I am currently inclined to believe that this is not a misfire of any kind. Misfire tends to have a random component to it and this behaviour was, in my opinion far too periodic to be misfire.

In the absence of any obvious causes I think that data logging through the OBDII port may be the best option. Now I just need to find the appropriate tool.

I use the product from OBDLink and the android app ‘Torque’ (paid version).
I think I got it more than 10 years ago and since then, so many products and software developed that you now have all sorts of options.

Even with that ancient product, I was able to diagnose intermittent O2 sensor issue (voltage shifted from time to time) so the latest ones should have faster sampling rate and better interface.

If your phenomenon is specific to certain VSS, then I agree, not misfire related.

The fuel pump stays in low speed mode at such low load region.

A/C signal is mainly for the idle control module so once outside of it, you should only notice the A/C CL On/Off and not the rpm change.

In order for the VSS to drop for 1 – 2 sec, should be something related to multiple or all six cyl so IG Timing, EGR, etc would be interesting to monitor.

Any aftermarket modifications?


Kaz

No modifications to the car.

I resolutely live in a previous century and do not carry a cell phone so it has to be lap top based. That is why I was looking for a device that I could just plug in to the prot and leave it until something happened. There are a few OBD2 data loggers with built in memory on the market. Some of them are very expensive. CSS Electronics appears to offer one for a semi reasonable price; but, appears to only use the CAN bus - I need to confirm that. If the CSS device does not support the K bus communication then I might have to use an OBDLink or similar.

Since taking the car out of storage in mid April, there have only been 2 or 3 occasions that I have been aware of this phenomena so it is very infrequent and does not persist when it does occur. That is why I liked the idea of a plug it in, forget it and let it do its thing option. However, that may not be an option and OBDLink or similar might be the path.