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A Drive By Wire Problem?

3 November 2011
Saskatchewan, Canada
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 else experienced this kind of behaviour and if they have determined what the problem was? The car is a 2000 model year USDM car.
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Definitely no CEL illumination. I used a generic OBDII scanner which may only support the mandatory OBDII codes and not read the extended P1241 - 1248 codes for the DBW. I looked through the SM and I can't find any indication as to whether using the service check connector would trigger pending or history codes. I also don't know whether any of the DBW error codes would have the pending or history feature. Most of the DBW DTCs look rather 'fatal' and I expect that you would know that you had a problem. The exception being the one associated with an error between the two accelerator position sensors and there is no indication as to how that error would manifest itself when you are driving the car - perhaps the ECU just ignores one of the sensors?

I had my air conditioning on at the time and I am starting to wonder whether this fluctuation might have been the idle up signal associated with the compressor cycling. I would have thought that the idle up would have been suppressed when the throttle was open; but, the very small vehicle speed change does seem like it would be consistent with the idle up. However, having it disappear after I stopped for the traffic light with the AC still running would not support this hypothesis.
Possible clutch slipping and then regaining traction? But, that would do it for every speed.
I have a 4 channel digital storage scope; but, the 120v extension cord is going to be a bit of a problem while driving :) .

I was thinking about an OBDII port data logger with on-board memory. I know that throttle position is one of the standard OBDII PIDs and should be available for capture. I did not see anything that looked like accelerator position for a DBW system. It looks like there are some idle control variables in the standard OBDII PID list; but, they look like PIDs for a PWM idle control device.

There are a lot of OBDII interface dongles which will allow you to observe the OBDII PID data stream and dump it to a laptop or mobile device for display and recording. Since the problem I am experiencing is really infrequent that kind of solution does not work since I really do not want to have to plug in a laptop every time I drive the car and wait for something to happen. Hence the OBDII port data logger with on board memory. These devices are much less common and some of them are quite expensive. CSS Electronics looks like they might have something in the almost affordable category; however, their product literature is a little unclear about whether it only works over the later ISO 15765 CAN connection or whether it supports the older ISO 9141 K line connection that the NSX ECU uses. CAN became mandatory in 2008 so new products may not bother with the older K line connection. I need to parse the CSS product info carefully or perhaps look for a different product.
Possible clutch slipping and then regaining traction? But, that would do it for every speed.

The clutch shows no signs of slippage under moderately heavy acceleration and this phenomena only occurs at light throttle steady cruising along at about 30 mi/hr. It is also very infrequent. I took the car out of storage in mid April and I have only noticed this slight speed variation two or three times. It was only on this last occassion where I happened to be driving for an extended period of time at a steady speed around 30 mi/hr that it became apparent that this was something originating in the power train as opposed to a twitchy right foot. In normal traffic I always seem to be accelerating or de accelerating and the phenomena never shows up and if it is occurring at highways speeds it is completely un detectable.